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2 | AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
2 | AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
3 | |
3 | |
4 | SYNOPSIS |
4 | SYNOPSIS |
5 | use AnyEvent::Fork; |
5 | use AnyEvent::Fork; |
6 | |
6 | |
7 | ################################################################## |
7 | AnyEvent::Fork |
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8 | ->new |
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9 | ->require ("MyModule") |
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10 | ->run ("MyModule::server", my $cv = AE::cv); |
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11 | |
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12 | my $fh = $cv->recv; |
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13 | |
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14 | DESCRIPTION |
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15 | This module allows you to create new processes, without actually forking |
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16 | them from your current process (avoiding the problems of forking), but |
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17 | preserving most of the advantages of fork. |
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18 | |
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19 | It can be used to create new worker processes or new independent |
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20 | subprocesses for short- and long-running jobs, process pools (e.g. for |
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21 | use in pre-forked servers) but also to spawn new external processes |
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22 | (such as CGI scripts from a web server), which can be faster (and more |
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23 | well behaved) than using fork+exec in big processes. |
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24 | |
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25 | Special care has been taken to make this module useful from other |
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26 | modules, while still supporting specialised environments such as |
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27 | App::Staticperl or PAR::Packer. |
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28 | |
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29 | WHAT THIS MODULE IS NOT |
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30 | This module only creates processes and lets you pass file handles and |
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31 | strings to it, and run perl code. It does not implement any kind of RPC |
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32 | - there is no back channel from the process back to you, and there is no |
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33 | RPC or message passing going on. |
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34 | |
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35 | If you need some form of RPC, you could use the AnyEvent::Fork::RPC |
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36 | companion module, which adds simple RPC/job queueing to a process |
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37 | created by this module. |
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38 | |
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39 | And if you need some automatic process pool management on top of |
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40 | AnyEvent::Fork::RPC, you can look at the AnyEvent::Fork::Pool companion |
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41 | module. |
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42 | |
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43 | Or you can implement it yourself in whatever way you like: use some |
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44 | message-passing module such as AnyEvent::MP, some pipe such as |
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45 | AnyEvent::ZeroMQ, use AnyEvent::Handle on both sides to send e.g. JSON |
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46 | or Storable messages, and so on. |
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47 | |
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48 | COMPARISON TO OTHER MODULES |
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49 | There is an abundance of modules on CPAN that do "something fork", such |
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50 | as Parallel::ForkManager, AnyEvent::ForkManager, AnyEvent::Worker or |
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51 | AnyEvent::Subprocess. There are modules that implement their own process |
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52 | management, such as AnyEvent::DBI. |
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53 | |
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54 | The problems that all these modules try to solve are real, however, none |
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55 | of them (from what I have seen) tackle the very real problems of |
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56 | unwanted memory sharing, efficiency, not being able to use event |
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57 | processing or similar modules in the processes they create. |
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58 | |
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59 | This module doesn't try to replace any of them - instead it tries to |
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60 | solve the problem of creating processes with a minimum of fuss and |
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61 | overhead (and also luxury). Ideally, most of these would use |
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62 | AnyEvent::Fork internally, except they were written before AnyEvent:Fork |
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63 | was available, so obviously had to roll their own. |
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64 | |
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65 | PROBLEM STATEMENT |
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66 | There are two traditional ways to implement parallel processing on UNIX |
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67 | like operating systems - fork and process, and fork+exec and process. |
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68 | They have different advantages and disadvantages that I describe below, |
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69 | together with how this module tries to mitigate the disadvantages. |
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70 | |
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71 | Forking from a big process can be very slow. |
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72 | A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box. |
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73 | This overhead is often shared with exec (because you have to fork |
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74 | first), but in some circumstances (e.g. when vfork is used), |
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75 | fork+exec can be much faster. |
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76 | |
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77 | This module can help here by telling a small(er) helper process to |
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78 | fork, which is faster then forking the main process, and also uses |
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79 | vfork where possible. This gives the speed of vfork, with the |
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80 | flexibility of fork. |
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81 | |
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82 | Forking usually creates a copy-on-write copy of the parent process. |
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83 | For example, modules or data files that are loaded will not use |
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84 | additional memory after a fork. When exec'ing a new process, modules |
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85 | and data files might need to be loaded again, at extra CPU and |
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86 | memory cost. But when forking, literally all data structures are |
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87 | copied - if the program frees them and replaces them by new data, |
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88 | the child processes will retain the old version even if it isn't |
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89 | used, which can suddenly and unexpectedly increase memory usage when |
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90 | freeing memory. |
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91 | |
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92 | The trade-off is between more sharing with fork (which can be good |
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93 | or bad), and no sharing with exec. |
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94 | |
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95 | This module allows the main program to do a controlled fork, and |
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96 | allows modules to exec processes safely at any time. When creating a |
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97 | custom process pool you can take advantage of data sharing via fork |
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98 | without risking to share large dynamic data structures that will |
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99 | blow up child memory usage. |
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100 | |
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101 | In other words, this module puts you into control over what is being |
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102 | shared and what isn't, at all times. |
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103 | |
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104 | Exec'ing a new perl process might be difficult. |
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105 | For example, it is not easy to find the correct path to the perl |
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106 | interpreter - $^X might not be a perl interpreter at all. |
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107 | |
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108 | This module tries hard to identify the correct path to the perl |
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109 | interpreter. With a cooperative main program, exec'ing the |
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110 | interpreter might not even be necessary, but even without help from |
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111 | the main program, it will still work when used from a module. |
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112 | |
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113 | Exec'ing a new perl process might be slow, as all necessary modules have |
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114 | to be loaded from disk again, with no guarantees of success. |
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115 | Long running processes might run into problems when perl is upgraded |
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116 | and modules are no longer loadable because they refer to a different |
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117 | perl version, or parts of a distribution are newer than the ones |
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118 | already loaded. |
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119 | |
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120 | This module supports creating pre-initialised perl processes to be |
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121 | used as a template for new processes. |
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122 | |
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123 | Forking might be impossible when a program is running. |
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124 | For example, POSIX makes it almost impossible to fork from a |
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125 | multi-threaded program while doing anything useful in the child - in |
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126 | fact, if your perl program uses POSIX threads (even indirectly via |
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127 | e.g. IO::AIO or threads), you cannot call fork on the perl level |
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128 | anymore without risking corruption issues on a number of operating |
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129 | systems. |
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130 | |
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131 | This module can safely fork helper processes at any time, by calling |
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132 | fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn). |
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133 | |
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134 | Parallel processing with fork might be inconvenient or difficult to |
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135 | implement. Modules might not work in both parent and child. |
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136 | For example, when a program uses an event loop and creates watchers |
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137 | it becomes very hard to use the event loop from a child program, as |
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138 | the watchers already exist but are only meaningful in the parent. |
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139 | Worse, a module might want to use such a module, not knowing whether |
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140 | another module or the main program also does, leading to problems. |
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141 | |
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142 | Apart from event loops, graphical toolkits also commonly fall into |
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143 | the "unsafe module" category, or just about anything that |
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144 | communicates with the external world, such as network libraries and |
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145 | file I/O modules, which usually don't like being copied and then |
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146 | allowed to continue in two processes. |
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147 | |
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148 | With this module only the main program is allowed to create new |
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149 | processes by forking (because only the main program can know when it |
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150 | is still safe to do so) - all other processes are created via |
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151 | fork+exec, which makes it possible to use modules such as event |
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152 | loops or window interfaces safely. |
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153 | |
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154 | EXAMPLES |
8 | # create a single new process, tell it to run your worker function |
155 | Create a single new process, tell it to run your worker function. |
9 | |
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10 | AnyEvent::Fork |
156 | AnyEvent::Fork |
11 | ->new |
157 | ->new |
12 | ->require ("MyModule") |
158 | ->require ("MyModule") |
13 | ->run ("MyModule::worker, sub { |
159 | ->run ("MyModule::worker, sub { |
14 | my ($master_filehandle) = @_; |
160 | my ($master_filehandle) = @_; |
15 | |
161 | |
16 | # now $master_filehandle is connected to the |
162 | # now $master_filehandle is connected to the |
17 | # $slave_filehandle in the new process. |
163 | # $slave_filehandle in the new process. |
18 | }); |
164 | }); |
19 | |
165 | |
20 | # MyModule::worker might look like this |
166 | "MyModule" might look like this: |
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167 | |
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168 | package MyModule; |
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169 | |
21 | sub MyModule::worker { |
170 | sub worker { |
22 | my ($slave_filehandle) = @_; |
171 | my ($slave_filehandle) = @_; |
23 | |
172 | |
24 | # now $slave_filehandle is connected to the $master_filehandle |
173 | # now $slave_filehandle is connected to the $master_filehandle |
25 | # in the original prorcess. have fun! |
174 | # in the original prorcess. have fun! |
26 | } |
175 | } |
27 | |
176 | |
28 | ################################################################## |
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29 | # create a pool of server processes all accepting on the same socket |
177 | Create a pool of server processes all accepting on the same socket. |
30 | |
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31 | # create listener socket |
178 | # create listener socket |
32 | my $listener = ...; |
179 | my $listener = ...; |
33 | |
180 | |
34 | # create a pool template, initialise it and give it the socket |
181 | # create a pool template, initialise it and give it the socket |
35 | my $pool = AnyEvent::Fork |
182 | my $pool = AnyEvent::Fork |
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46 | } |
193 | } |
47 | |
194 | |
48 | # now do other things - maybe use the filehandle provided by run |
195 | # now do other things - maybe use the filehandle provided by run |
49 | # to wait for the processes to die. or whatever. |
196 | # to wait for the processes to die. or whatever. |
50 | |
197 | |
51 | # My::Server::run might look like this |
198 | "My::Server" might look like this: |
52 | sub My::Server::run { |
199 | |
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200 | package My::Server; |
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201 | |
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202 | sub run { |
53 | my ($slave, $listener, $id) = @_; |
203 | my ($slave, $listener, $id) = @_; |
54 | |
204 | |
55 | close $slave; # we do not use the socket, so close it to save resources |
205 | close $slave; # we do not use the socket, so close it to save resources |
56 | |
206 | |
57 | # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
207 | # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
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59 | while (my $socket = $listener->accept) { |
209 | while (my $socket = $listener->accept) { |
60 | # do sth. with new socket |
210 | # do sth. with new socket |
61 | } |
211 | } |
62 | } |
212 | } |
63 | |
213 | |
64 | DESCRIPTION |
214 | use AnyEvent::Fork as a faster fork+exec |
65 | This module allows you to create new processes, without actually forking |
215 | This runs "/bin/echo hi", with standard output redirected to /tmp/log |
66 | them from your current process (avoiding the problems of forking), but |
216 | and standard error redirected to the communications socket. It is |
67 | preserving most of the advantages of fork. |
217 | usually faster than fork+exec, but still lets you prepare the |
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218 | environment. |
68 | |
219 | |
69 | It can be used to create new worker processes or new independent |
220 | open my $output, ">/tmp/log" or die "$!"; |
70 | subprocesses for short- and long-running jobs, process pools (e.g. for |
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71 | use in pre-forked servers) but also to spawn new external processes |
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72 | (such as CGI scripts from a webserver), which can be faster (and more |
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73 | well behaved) than using fork+exec in big processes. |
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74 | |
221 | |
75 | Special care has been taken to make this module useful from other |
222 | AnyEvent::Fork |
76 | modules, while still supporting specialised environments such as |
223 | ->new |
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224 | ->eval (' |
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225 | # compile a helper function for later use |
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226 | sub run { |
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227 | my ($fh, $output, @cmd) = @_; |
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228 | |
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229 | # perl will clear close-on-exec on STDOUT/STDERR |
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230 | open STDOUT, ">&", $output or die; |
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231 | open STDERR, ">&", $fh or die; |
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232 | |
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233 | exec @cmd; |
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234 | } |
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235 | ') |
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236 | ->send_fh ($output) |
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237 | ->send_arg ("/bin/echo", "hi") |
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238 | ->run ("run", my $cv = AE::cv); |
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239 | |
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240 | my $stderr = $cv->recv; |
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241 | |
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242 | For stingy users: put the worker code into a "DATA" section. |
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243 | When you want to be stingy with files, you cna put your code into the |
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244 | "DATA" section of your module (or program): |
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245 | |
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246 | use AnyEvent::Fork; |
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247 | |
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248 | AnyEvent::Fork |
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249 | ->new |
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250 | ->eval (do { local $/; <DATA> }) |
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251 | ->run ("doit", sub { ... }); |
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252 | |
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253 | __DATA__ |
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254 | |
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255 | sub doit { |
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256 | ... do something! |
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257 | } |
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258 | |
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259 | For stingy standalone programs: do not rely on external files at |
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260 | all. |
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261 | For single-file scripts it can be inconvenient to rely on external files |
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262 | - even when using < "DATA" section, you still need to "exec" an external |
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263 | perl interpreter, which might not be available when using |
77 | App::Staticperl or PAR::Packer. |
264 | App::Staticperl, Urlader or PAR::Packer for example. |
78 | |
265 | |
79 | PROBLEM STATEMENT |
266 | Two modules help here - AnyEvent::Fork::Early forks a template process |
80 | There are two ways to implement parallel processing on UNIX like |
267 | for all further calls to "new_exec", and AnyEvent::Fork::Template forks |
81 | operating systems - fork and process, and fork+exec and process. They |
268 | the main program as a template process. |
82 | have different advantages and disadvantages that I describe below, |
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83 | together with how this module tries to mitigate the disadvantages. |
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84 | |
269 | |
85 | Forking from a big process can be very slow (a 5GB process needs 0.05s |
270 | Here is how your main program should look like: |
86 | to fork on my 3.6GHz amd64 GNU/Linux box for example). This overhead is |
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87 | often shared with exec (because you have to fork first), but in some |
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88 | circumstances (e.g. when vfork is used), fork+exec can be much faster. |
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89 | This module can help here by telling a small(er) helper process to |
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90 | fork, or fork+exec instead. |
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91 | |
271 | |
92 | Forking usually creates a copy-on-write copy of the parent process. |
272 | #! perl |
93 | Memory (for example, modules or data files that have been will not take |
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94 | additional memory). When exec'ing a new process, modules and data files |
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95 | might need to be loaded again, at extra cpu and memory cost. Likewise |
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96 | when forking, all data structures are copied as well - if the program |
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97 | frees them and replaces them by new data, the child processes will |
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98 | retain the memory even if it isn't used. |
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99 | This module allows the main program to do a controlled fork, and |
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100 | allows modules to exec processes safely at any time. When creating a |
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101 | custom process pool you can take advantage of data sharing via fork |
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102 | without risking to share large dynamic data structures that will |
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103 | blow up child memory usage. |
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104 | |
273 | |
105 | Exec'ing a new perl process might be difficult and slow. For example, it |
274 | # optional, as the very first thing. |
106 | is not easy to find the correct path to the perl interpreter, and all |
275 | # in case modules want to create their own processes. |
107 | modules have to be loaded from disk again. Long running processes might |
276 | use AnyEvent::Fork::Early; |
108 | run into problems when perl is upgraded for example. |
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109 | This module supports creating pre-initialised perl processes to be |
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110 | used as template, and also tries hard to identify the correct path |
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111 | to the perl interpreter. With a cooperative main program, exec'ing |
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112 | the interpreter might not even be necessary. |
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113 | |
277 | |
114 | Forking might be impossible when a program is running. For example, |
278 | # next, load all modules you need in your template process |
115 | POSIX makes it almost impossible to fork from a multithreaded program |
279 | use Example::My::Module |
116 | and do anything useful in the child - strictly speaking, if your perl |
280 | use Example::Whatever; |
117 | program uses posix threads (even indirectly via e.g. IO::AIO or |
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118 | threads), you cannot call fork on the perl level anymore, at all. |
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119 | This module can safely fork helper processes at any time, by caling |
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120 | fork+exec in C, in a POSIX-compatible way. |
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121 | |
281 | |
122 | Parallel processing with fork might be inconvenient or difficult to |
282 | # next, put your run function definition and anything else you |
123 | implement. For example, when a program uses an event loop and creates |
283 | # need, but do not use code outside of BEGIN blocks. |
124 | watchers it becomes very hard to use the event loop from a child |
284 | sub worker_run { |
125 | program, as the watchers already exist but are only meaningful in the |
285 | my ($fh, @args) = @_; |
126 | parent. Worse, a module might want to use such a system, not knowing |
286 | ... |
127 | whether another module or the main program also does, leading to |
287 | } |
128 | problems. |
288 | |
129 | This module only lets the main program create pools by forking |
289 | # now preserve everything so far as AnyEvent::Fork object |
130 | (because only the main program can know when it is still safe to do |
290 | # in §TEMPLATE. |
131 | so) - all other pools are created by fork+exec, after which such |
291 | use AnyEvent::Fork::Template; |
132 | modules can again be loaded. |
292 | |
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293 | # do not put code outside of BEGIN blocks until here |
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294 | |
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295 | # now use the $TEMPLATE process in any way you like |
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296 | |
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297 | # for example: create 10 worker processes |
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298 | my @worker; |
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299 | my $cv = AE::cv; |
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300 | for (1..10) { |
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301 | $cv->begin; |
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302 | $TEMPLATE->fork->send_arg ($_)->run ("worker_run", sub { |
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303 | push @worker, shift; |
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304 | $cv->end; |
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305 | }); |
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306 | } |
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307 | $cv->recv; |
133 | |
308 | |
134 | CONCEPTS |
309 | CONCEPTS |
135 | This module can create new processes either by executing a new perl |
310 | This module can create new processes either by executing a new perl |
136 | process, or by forking from an existing "template" process. |
311 | process, or by forking from an existing "template" process. |
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312 | |
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313 | All these processes are called "child processes" (whether they are |
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314 | direct children or not), while the process that manages them is called |
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315 | the "parent process". |
137 | |
316 | |
138 | Each such process comes with its own file handle that can be used to |
317 | Each such process comes with its own file handle that can be used to |
139 | communicate with it (it's actually a socket - one end in the new |
318 | communicate with it (it's actually a socket - one end in the new |
140 | process, one end in the main process), and among the things you can do |
319 | process, one end in the main process), and among the things you can do |
141 | in it are load modules, fork new processes, send file handles to it, and |
320 | in it are load modules, fork new processes, send file handles to it, and |
… | |
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151 | memory used for the perl interpreter with the new process, but |
330 | memory used for the perl interpreter with the new process, but |
152 | loading modules takes time, and the memory is not shared with |
331 | loading modules takes time, and the memory is not shared with |
153 | anything else. |
332 | anything else. |
154 | |
333 | |
155 | This is ideal for when you only need one extra process of a kind, |
334 | This is ideal for when you only need one extra process of a kind, |
156 | with the option of starting and stipping it on demand. |
335 | with the option of starting and stopping it on demand. |
157 | |
336 | |
158 | Example: |
337 | Example: |
159 | |
338 | |
160 | AnyEvent::Fork |
339 | AnyEvent::Fork |
161 | ->new |
340 | ->new |
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175 | the modules you loaded) is shared between the processes, and each |
354 | the modules you loaded) is shared between the processes, and each |
176 | new process consumes relatively little memory of its own. |
355 | new process consumes relatively little memory of its own. |
177 | |
356 | |
178 | The disadvantage of this approach is that you need to create a |
357 | The disadvantage of this approach is that you need to create a |
179 | template process for the sole purpose of forking new processes from |
358 | template process for the sole purpose of forking new processes from |
180 | it, but if you only need a fixed number of proceses you can create |
359 | it, but if you only need a fixed number of processes you can create |
181 | them, and then destroy the template process. |
360 | them, and then destroy the template process. |
182 | |
361 | |
183 | Example: |
362 | Example: |
184 | |
363 | |
185 | my $template = AnyEvent::Fork->new->require ("Some::Module"); |
364 | my $template = AnyEvent::Fork->new->require ("Some::Module"); |
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209 | ->require ("Some::Module") |
388 | ->require ("Some::Module") |
210 | ->run ("Some::Module::run", sub { |
389 | ->run ("Some::Module::run", sub { |
211 | my ($fork_fh) = @_; |
390 | my ($fork_fh) = @_; |
212 | }); |
391 | }); |
213 | |
392 | |
214 | FUNCTIONS |
393 | THE "AnyEvent::Fork" CLASS |
215 | my $pool = new AnyEvent::Fork key => value... |
394 | This module exports nothing, and only implements a single class - |
216 | Create a new process pool. The following named parameters are |
395 | "AnyEvent::Fork". |
217 | supported: |
396 | |
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397 | There are two class constructors that both create new processes - "new" |
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398 | and "new_exec". The "fork" method creates a new process by forking an |
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399 | existing one and could be considered a third constructor. |
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400 | |
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401 | Most of the remaining methods deal with preparing the new process, by |
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402 | loading code, evaluating code and sending data to the new process. They |
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403 | usually return the process object, so you can chain method calls. |
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404 | |
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405 | If a process object is destroyed before calling its "run" method, then |
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406 | the process simply exits. After "run" is called, all responsibility is |
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407 | passed to the specified function. |
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408 | |
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409 | As long as there is any outstanding work to be done, process objects |
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410 | resist being destroyed, so there is no reason to store them unless you |
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411 | need them later - configure and forget works just fine. |
218 | |
412 | |
219 | my $proc = new AnyEvent::Fork |
413 | my $proc = new AnyEvent::Fork |
220 | Create a new "empty" perl interpreter process and returns its |
414 | Create a new "empty" perl interpreter process and returns its |
221 | process object for further manipulation. |
415 | process object for further manipulation. |
222 | |
416 | |
223 | The new process is forked from a template process that is kept |
417 | The new process is forked from a template process that is kept |
224 | around for this purpose. When it doesn't exist yet, it is created by |
418 | around for this purpose. When it doesn't exist yet, it is created by |
225 | a call to "new_exec" and kept around for future calls. |
419 | a call to "new_exec" first and then stays around for future calls. |
226 | |
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227 | When the process object is destroyed, it will release the file |
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228 | handle that connects it with the new process. When the new process |
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229 | has not yet called "run", then the process will exit. Otherwise, |
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230 | what happens depends entirely on the code that is executed. |
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231 | |
420 | |
232 | $new_proc = $proc->fork |
421 | $new_proc = $proc->fork |
233 | Forks $proc, creating a new process, and returns the process object |
422 | Forks $proc, creating a new process, and returns the process object |
234 | of the new process. |
423 | of the new process. |
235 | |
424 | |
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… | |
248 | possible, and is also slower. |
437 | possible, and is also slower. |
249 | |
438 | |
250 | You should use "new" whenever possible, except when having a |
439 | You should use "new" whenever possible, except when having a |
251 | template process around is unacceptable. |
440 | template process around is unacceptable. |
252 | |
441 | |
253 | The path to the perl interpreter is divined usign various methods - |
442 | The path to the perl interpreter is divined using various methods - |
254 | first $^X is investigated to see if the path ends with something |
443 | first $^X is investigated to see if the path ends with something |
255 | that sounds as if it were the perl interpreter. Failing this, the |
444 | that looks as if it were the perl interpreter. Failing this, the |
256 | module falls back to using $Config::Config{perlpath}. |
445 | module falls back to using $Config::Config{perlpath}. |
257 | |
446 | |
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447 | The path to perl can also be overriden by setting the global |
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448 | variable $AnyEvent::Fork::PERL - it's value will be used for all |
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449 | subsequent invocations. |
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450 | |
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451 | $pid = $proc->pid |
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452 | Returns the process id of the process *iff it is a direct child of |
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453 | the process running AnyEvent::Fork*, and "undef" otherwise. As a |
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454 | general rule (that you cannot rely upon), processes created via |
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455 | "new_exec", AnyEvent::Fork::Early or AnyEvent::Fork::Template are |
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456 | direct children, while all other processes are not. |
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457 | |
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458 | Or in other words, you do not normally have to take care of zombies |
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459 | for processes created via "new", but when in doubt, or zombies are a |
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460 | problem, you need to check whether a process is a diretc child by |
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461 | calling this method, and possibly creating a child watcher or reap |
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462 | it manually. |
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463 | |
258 | $proc = $proc->eval ($perlcode, @args) |
464 | $proc = $proc->eval ($perlcode, @args) |
259 | Evaluates the given $perlcode as ... perl code, while setting @_ to |
465 | Evaluates the given $perlcode as ... Perl code, while setting @_ to |
260 | the strings specified by @args. |
466 | the strings specified by @args, in the "main" package. |
261 | |
467 | |
262 | This call is meant to do any custom initialisation that might be |
468 | This call is meant to do any custom initialisation that might be |
263 | required (for example, the "require" method uses it). It's not |
469 | required (for example, the "require" method uses it). It's not |
264 | supposed to be used to completely take over the process, use "run" |
470 | supposed to be used to completely take over the process, use "run" |
265 | for that. |
471 | for that. |
… | |
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267 | The code will usually be executed after this call returns, and there |
473 | The code will usually be executed after this call returns, and there |
268 | is no way to pass anything back to the calling process. Any |
474 | is no way to pass anything back to the calling process. Any |
269 | evaluation errors will be reported to stderr and cause the process |
475 | evaluation errors will be reported to stderr and cause the process |
270 | to exit. |
476 | to exit. |
271 | |
477 | |
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478 | If you want to execute some code (that isn't in a module) to take |
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479 | over the process, you should compile a function via "eval" first, |
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480 | and then call it via "run". This also gives you access to any |
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481 | arguments passed via the "send_xxx" methods, such as file handles. |
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482 | See the "use AnyEvent::Fork as a faster fork+exec" example to see it |
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483 | in action. |
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484 | |
272 | Returns the process object for easy chaining of method calls. |
485 | Returns the process object for easy chaining of method calls. |
273 | |
486 | |
274 | $proc = $proc->require ($module, ...) |
487 | $proc = $proc->require ($module, ...) |
275 | Tries to load the given module(s) into the process |
488 | Tries to load the given module(s) into the process |
276 | |
489 | |
… | |
… | |
278 | |
491 | |
279 | $proc = $proc->send_fh ($handle, ...) |
492 | $proc = $proc->send_fh ($handle, ...) |
280 | Send one or more file handles (*not* file descriptors) to the |
493 | Send one or more file handles (*not* file descriptors) to the |
281 | process, to prepare a call to "run". |
494 | process, to prepare a call to "run". |
282 | |
495 | |
283 | The process object keeps a reference to the handles until this is |
496 | The process object keeps a reference to the handles until they have |
284 | done, so you must not explicitly close the handles. This is most |
497 | been passed over to the process, so you must not explicitly close |
285 | easily accomplished by simply not storing the file handles anywhere |
498 | the handles. This is most easily accomplished by simply not storing |
286 | after passing them to this method. |
499 | the file handles anywhere after passing them to this method - when |
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500 | AnyEvent::Fork is finished using them, perl will automatically close |
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501 | them. |
287 | |
502 | |
288 | Returns the process object for easy chaining of method calls. |
503 | Returns the process object for easy chaining of method calls. |
289 | |
504 | |
290 | Example: pass an fh to a process, and release it without closing. it |
505 | Example: pass a file handle to a process, and release it without |
291 | will be closed automatically when it is no longer used. |
506 | closing. It will be closed automatically when it is no longer used. |
292 | |
507 | |
293 | $proc->send_fh ($my_fh); |
508 | $proc->send_fh ($my_fh); |
294 | undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
509 | undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
295 | |
510 | |
296 | $proc = $proc->send_arg ($string, ...) |
511 | $proc = $proc->send_arg ($string, ...) |
297 | Send one or more argument strings to the process, to prepare a call |
512 | Send one or more argument strings to the process, to prepare a call |
298 | to "run". The strings can be any octet string. |
513 | to "run". The strings can be any octet strings. |
299 | |
514 | |
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515 | The protocol is optimised to pass a moderate number of relatively |
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516 | short strings - while you can pass up to 4GB of data in one go, this |
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517 | is more meant to pass some ID information or other startup info, not |
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518 | big chunks of data. |
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519 | |
300 | Returns the process object for easy chaining of emthod calls. |
520 | Returns the process object for easy chaining of method calls. |
301 | |
521 | |
302 | $proc->run ($func, $cb->($fh)) |
522 | $proc->run ($func, $cb->($fh)) |
303 | Enter the function specified by the fully qualified name in $func in |
523 | Enter the function specified by the function name in $func in the |
304 | the process. The function is called with the communication socket as |
524 | process. The function is called with the communication socket as |
305 | first argument, followed by all file handles and string arguments |
525 | first argument, followed by all file handles and string arguments |
306 | sent earlier via "send_fh" and "send_arg" methods, in the order they |
526 | sent earlier via "send_fh" and "send_arg" methods, in the order they |
307 | were called. |
527 | were called. |
308 | |
528 | |
309 | If the called function returns, the process exits. |
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310 | |
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311 | Preparing the process can take time - when the process is ready, the |
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312 | callback is invoked with the local communications socket as |
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313 | argument. |
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314 | |
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315 | The process object becomes unusable on return from this function. |
529 | The process object becomes unusable on return from this function - |
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530 | any further method calls result in undefined behaviour. |
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531 | |
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532 | The function name should be fully qualified, but if it isn't, it |
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533 | will be looked up in the "main" package. |
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534 | |
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535 | If the called function returns, doesn't exist, or any error occurs, |
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536 | the process exits. |
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537 | |
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538 | Preparing the process is done in the background - when all commands |
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539 | have been sent, the callback is invoked with the local |
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540 | communications socket as argument. At this point you can start using |
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541 | the socket in any way you like. |
316 | |
542 | |
317 | If the communication socket isn't used, it should be closed on both |
543 | If the communication socket isn't used, it should be closed on both |
318 | sides, to save on kernel memory. |
544 | sides, to save on kernel memory. |
319 | |
545 | |
320 | The socket is non-blocking in the parent, and blocking in the newly |
546 | The socket is non-blocking in the parent, and blocking in the newly |
321 | created process. The close-on-exec flag is set on both. Even if not |
547 | created process. The close-on-exec flag is set in both. |
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548 | |
322 | used otherwise, the socket can be a good indicator for the existance |
549 | Even if not used otherwise, the socket can be a good indicator for |
323 | of the process - if the other process exits, you get a readable |
550 | the existence of the process - if the other process exits, you get a |
324 | event on it, because exiting the process closes the socket (if it |
551 | readable event on it, because exiting the process closes the socket |
325 | didn't create any children using fork). |
552 | (if it didn't create any children using fork). |
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553 | |
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554 | Compatibility to AnyEvent::Fork::Remote |
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555 | If you want to write code that works with both this module and |
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556 | AnyEvent::Fork::Remote, you need to write your code so that it |
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557 | assumes there are two file handles for communications, which |
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558 | might not be unix domain sockets. The "run" function should |
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559 | start like this: |
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560 | |
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561 | sub run { |
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562 | my ($rfh, @args) = @_; # @args is your normal arguments |
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563 | my $wfh = fileno $rfh ? $rfh : *STDOUT; |
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564 | |
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565 | # now use $rfh for reading and $wfh for writing |
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566 | } |
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567 | |
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568 | This checks whether the passed file handle is, in fact, the |
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569 | process "STDIN" handle. If it is, then the function was invoked |
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570 | visa AnyEvent::Fork::Remote, so STDIN should be used for reading |
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571 | and "STDOUT" should be used for writing. |
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572 | |
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573 | In all other cases, the function was called via this module, and |
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574 | there is only one file handle that should be sued for reading |
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575 | and writing. |
326 | |
576 | |
327 | Example: create a template for a process pool, pass a few strings, |
577 | Example: create a template for a process pool, pass a few strings, |
328 | some file handles, then fork, pass one more string, and run some |
578 | some file handles, then fork, pass one more string, and run some |
329 | code. |
579 | code. |
330 | |
580 | |
… | |
… | |
339 | ->send_arg ("str3") |
589 | ->send_arg ("str3") |
340 | ->run ("Some::function", sub { |
590 | ->run ("Some::function", sub { |
341 | my ($fh) = @_; |
591 | my ($fh) = @_; |
342 | |
592 | |
343 | # fh is nonblocking, but we trust that the OS can accept these |
593 | # fh is nonblocking, but we trust that the OS can accept these |
344 | # extra 3 octets anyway. |
594 | # few octets anyway. |
345 | syswrite $fh, "hi #$_\n"; |
595 | syswrite $fh, "hi #$_\n"; |
346 | |
596 | |
347 | # $fh is being closed here, as we don't store it anywhere |
597 | # $fh is being closed here, as we don't store it anywhere |
348 | }); |
598 | }); |
349 | } |
599 | } |
… | |
… | |
351 | # Some::function might look like this - all parameters passed before fork |
601 | # Some::function might look like this - all parameters passed before fork |
352 | # and after will be passed, in order, after the communications socket. |
602 | # and after will be passed, in order, after the communications socket. |
353 | sub Some::function { |
603 | sub Some::function { |
354 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
604 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
355 | |
605 | |
356 | print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" |
606 | print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order |
357 | } |
607 | } |
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608 | |
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609 | EXPERIMENTAL METHODS |
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610 | These methods might go away completely or change behaviour, at any time. |
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611 | |
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612 | $proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED |
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613 | Flushes all commands out to the process and then calls the callback |
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614 | with the communications socket. |
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615 | |
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616 | The process object becomes unusable on return from this function - |
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617 | any further method calls result in undefined behaviour. |
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618 | |
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619 | The point of this method is to give you a file handle that you can |
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620 | pass to another process. In that other process, you can call |
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621 | "new_from_fh AnyEvent::Fork $fh" to create a new "AnyEvent::Fork" |
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622 | object from it, thereby effectively passing a fork object to another |
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623 | process. |
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624 | |
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625 | new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED |
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626 | Takes a file handle originally rceeived by the "to_fh" method and |
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627 | creates a new "AnyEvent:Fork" object. The child process itself will |
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628 | not change in any way, i.e. it will keep all the modifications done |
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629 | to it before calling "to_fh". |
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630 | |
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631 | The new object is very much like the original object, except that |
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632 | the "pid" method will return "undef" even if the process is a direct |
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633 | child. |
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634 | |
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635 | PERFORMANCE |
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636 | Now for some unscientific benchmark numbers (all done on an amd64 |
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637 | GNU/Linux box). These are intended to give you an idea of the relative |
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638 | performance you can expect, they are not meant to be absolute |
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639 | performance numbers. |
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640 | |
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641 | OK, so, I ran a simple benchmark that creates a socket pair, forks, |
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642 | calls exit in the child and waits for the socket to close in the parent. |
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643 | I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of |
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644 | 5100kB. |
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645 | |
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646 | 2079 new processes per second, using manual socketpair + fork |
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647 | |
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648 | Then I did the same thing, but instead of calling fork, I called |
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649 | AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the |
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650 | socket from the child to close on exit. This does the same thing as |
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651 | manual socket pair + fork, except that what is forked is the template |
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652 | process (2440kB), and the socket needs to be passed to the server at the |
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653 | other end of the socket first. |
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654 | |
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655 | 2307 new processes per second, using AnyEvent::Fork->new |
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656 | |
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657 | And finally, using "new_exec" instead "new", using vforks+execs to exec |
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658 | a new perl interpreter and compile the small server each time, I get: |
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659 | |
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660 | 479 vfork+execs per second, using AnyEvent::Fork->new_exec |
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661 | |
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662 | So how can "AnyEvent->new" be faster than a standard fork, even though |
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663 | it uses the same operations, but adds a lot of overhead? |
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664 | |
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665 | The difference is simply the process size: forking the 5MB process takes |
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666 | so much longer than forking the 2.5MB template process that the extra |
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667 | overhead is canceled out. |
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668 | |
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669 | If the benchmark process grows, the normal fork becomes even slower: |
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670 | |
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671 | 1340 new processes, manual fork of a 20MB process |
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672 | 731 new processes, manual fork of a 200MB process |
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673 | 235 new processes, manual fork of a 2000MB process |
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674 | |
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675 | What that means (to me) is that I can use this module without having a |
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676 | bad conscience because of the extra overhead required to start new |
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677 | processes. |
358 | |
678 | |
359 | TYPICAL PROBLEMS |
679 | TYPICAL PROBLEMS |
360 | This section lists typical problems that remain. I hope by recognising |
680 | This section lists typical problems that remain. I hope by recognising |
361 | them, most can be avoided. |
681 | them, most can be avoided. |
362 | |
682 | |
363 | "leaked" file descriptors for exec'ed processes |
683 | leaked file descriptors for exec'ed processes |
364 | POSIX systems inherit file descriptors by default when exec'ing a |
684 | POSIX systems inherit file descriptors by default when exec'ing a |
365 | new process. While perl itself laudably sets the close-on-exec flags |
685 | new process. While perl itself laudably sets the close-on-exec flags |
366 | on new file handles, most C libraries don't care, and even if all |
686 | on new file handles, most C libraries don't care, and even if all |
367 | cared, it's often not possible to set the flag in a race-free |
687 | cared, it's often not possible to set the flag in a race-free |
368 | manner. |
688 | manner. |
369 | |
689 | |
370 | That means some file descriptors can leak through. And since it |
690 | That means some file descriptors can leak through. And since it |
371 | isn't possible to know which file descriptors are "good" and |
691 | isn't possible to know which file descriptors are "good" and |
372 | "neccessary" (or even to know which file descreiptors are open), |
692 | "necessary" (or even to know which file descriptors are open), there |
373 | there is no good way to close the ones that might harm. |
693 | is no good way to close the ones that might harm. |
374 | |
694 | |
375 | As an example of what "harm" can be done consider a web server that |
695 | As an example of what "harm" can be done consider a web server that |
376 | accepts connections and afterwards some module uses AnyEvent::Fork |
696 | accepts connections and afterwards some module uses AnyEvent::Fork |
377 | for the first time, causing it to fork and exec a new process, which |
697 | for the first time, causing it to fork and exec a new process, which |
378 | might inherit the network socket. When the server closes the socket, |
698 | might inherit the network socket. When the server closes the socket, |
… | |
… | |
385 | exec'ed well before many random file descriptors are open. |
705 | exec'ed well before many random file descriptors are open. |
386 | |
706 | |
387 | In general, the solution for these kind of problems is to fix the |
707 | In general, the solution for these kind of problems is to fix the |
388 | libraries or the code that leaks those file descriptors. |
708 | libraries or the code that leaks those file descriptors. |
389 | |
709 | |
390 | Fortunately, most of these lekaed descriptors do no harm, other than |
710 | Fortunately, most of these leaked descriptors do no harm, other than |
391 | sitting on some resources. |
711 | sitting on some resources. |
392 | |
712 | |
393 | "leaked" file descriptors for fork'ed processes |
713 | leaked file descriptors for fork'ed processes |
394 | Normally, AnyEvent::Fork does start new processes by exec'ing them, |
714 | Normally, AnyEvent::Fork does start new processes by exec'ing them, |
395 | which closes file descriptors not marked for being inherited. |
715 | which closes file descriptors not marked for being inherited. |
396 | |
716 | |
397 | However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
717 | However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
398 | way to create these processes by forking, and this leaks more file |
718 | way to create these processes by forking, and this leaks more file |
… | |
… | |
405 | trouble with a fork. |
725 | trouble with a fork. |
406 | |
726 | |
407 | The solution is to either not load these modules before use'ing |
727 | The solution is to either not load these modules before use'ing |
408 | AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
728 | AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
409 | initialising them, for example, by calling "init Gtk2" manually. |
729 | initialising them, for example, by calling "init Gtk2" manually. |
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730 | |
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731 | exiting calls object destructors |
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732 | This only applies to users of AnyEvent::Fork:Early and |
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733 | AnyEvent::Fork::Template, or when initialising code creates objects |
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734 | that reference external resources. |
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735 | |
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736 | When a process created by AnyEvent::Fork exits, it might do so by |
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737 | calling exit, or simply letting perl reach the end of the program. |
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738 | At which point Perl runs all destructors. |
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739 | |
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740 | Not all destructors are fork-safe - for example, an object that |
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741 | represents the connection to an X display might tell the X server to |
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742 | free resources, which is inconvenient when the "real" object in the |
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743 | parent still needs to use them. |
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744 | |
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745 | This is obviously not a problem for AnyEvent::Fork::Early, as you |
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746 | used it as the very first thing, right? |
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747 | |
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748 | It is a problem for AnyEvent::Fork::Template though - and the |
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749 | solution is to not create objects with nontrivial destructors that |
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750 | might have an effect outside of Perl. |
410 | |
751 | |
411 | PORTABILITY NOTES |
752 | PORTABILITY NOTES |
412 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
753 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
413 | nop, and ::Template is not going to work), and it cost a lot of blood |
754 | nop, and ::Template is not going to work), and it cost a lot of blood |
414 | and sweat to make it so, mostly due to the bloody broken perl that |
755 | and sweat to make it so, mostly due to the bloody broken perl that |
415 | nobody seems to care about. The fork emulation is a bad joke - I have |
756 | nobody seems to care about. The fork emulation is a bad joke - I have |
416 | yet to see something useful that you cna do with it without running into |
757 | yet to see something useful that you can do with it without running into |
417 | memory corruption issues or other braindamage. Hrrrr. |
758 | memory corruption issues or other braindamage. Hrrrr. |
418 | |
759 | |
419 | Cygwin perl is not supported at the moment, as it should implement fd |
760 | Since fork is endlessly broken on win32 perls (it doesn't even remotely |
420 | passing, but doesn't, and rolling my own is hard, as cygwin doesn't |
761 | work within it's documented limits) and quite obviously it's not getting |
421 | support enough functionality to do it. |
762 | improved any time soon, the best way to proceed on windows would be to |
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763 | always use "new_exec" and thus never rely on perl's fork "emulation". |
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764 | |
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765 | Cygwin perl is not supported at the moment due to some hilarious |
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766 | shortcomings of its API - see IO::FDPoll for more details. If you never |
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767 | use "send_fh" and always use "new_exec" to create processes, it should |
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768 | work though. |
422 | |
769 | |
423 | SEE ALSO |
770 | SEE ALSO |
424 | AnyEvent::Fork::Early (to avoid executing a perl interpreter), |
771 | AnyEvent::Fork::Early, to avoid executing a perl interpreter at all |
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772 | (part of this distribution). |
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773 | |
425 | AnyEvent::Fork::Template (to create a process by forking the main |
774 | AnyEvent::Fork::Template, to create a process by forking the main |
426 | program at a convenient time). |
775 | program at a convenient time (part of this distribution). |
427 | |
776 | |
428 | AUTHOR |
777 | AnyEvent::Fork::Remote, for another way to create processes that is |
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778 | mostly compatible to this module and modules building on top of it, but |
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779 | works better with remote processes. |
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780 | |
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781 | AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN). |
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782 | |
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783 | AnyEvent::Fork::Pool, for simple worker process pool (on CPAN). |
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784 | |
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785 | AUTHOR AND CONTACT INFORMATION |
429 | Marc Lehmann <schmorp@schmorp.de> |
786 | Marc Lehmann <schmorp@schmorp.de> |
430 | http://home.schmorp.de/ |
787 | http://software.schmorp.de/pkg/AnyEvent-Fork |
431 | |
788 | |