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1 | NAME |
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2 | AnyEvent::Fork - everything you wanted to use fork() for, but couldn't |
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3 | |
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4 | SYNOPSIS |
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5 | use AnyEvent::Fork; |
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6 | |
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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 |
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155 | Create a single new process, tell it to run your worker function. |
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156 | AnyEvent::Fork |
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157 | ->new |
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158 | ->require ("MyModule") |
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159 | ->run ("MyModule::worker, sub { |
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160 | my ($master_filehandle) = @_; |
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161 | |
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162 | # now $master_filehandle is connected to the |
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163 | # $slave_filehandle in the new process. |
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164 | }); |
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165 | |
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166 | "MyModule" might look like this: |
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167 | |
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168 | package MyModule; |
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169 | |
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170 | sub worker { |
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171 | my ($slave_filehandle) = @_; |
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172 | |
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173 | # now $slave_filehandle is connected to the $master_filehandle |
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174 | # in the original prorcess. have fun! |
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175 | } |
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176 | |
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177 | Create a pool of server processes all accepting on the same socket. |
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178 | # create listener socket |
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179 | my $listener = ...; |
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180 | |
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181 | # create a pool template, initialise it and give it the socket |
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182 | my $pool = AnyEvent::Fork |
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183 | ->new |
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184 | ->require ("Some::Stuff", "My::Server") |
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185 | ->send_fh ($listener); |
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186 | |
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187 | # now create 10 identical workers |
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188 | for my $id (1..10) { |
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189 | $pool |
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190 | ->fork |
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191 | ->send_arg ($id) |
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192 | ->run ("My::Server::run"); |
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193 | } |
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194 | |
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195 | # now do other things - maybe use the filehandle provided by run |
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196 | # to wait for the processes to die. or whatever. |
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197 | |
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198 | "My::Server" might look like this: |
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199 | |
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200 | package My::Server; |
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201 | |
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202 | sub run { |
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203 | my ($slave, $listener, $id) = @_; |
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204 | |
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205 | close $slave; # we do not use the socket, so close it to save resources |
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206 | |
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207 | # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
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208 | # or anything we usually couldn't do in a process forked normally. |
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209 | while (my $socket = $listener->accept) { |
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210 | # do sth. with new socket |
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211 | } |
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212 | } |
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213 | |
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214 | use AnyEvent::Fork as a faster fork+exec |
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215 | This runs "/bin/echo hi", with standard output redirected to /tmp/log |
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216 | and standard error redirected to the communications socket. It is |
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217 | usually faster than fork+exec, but still lets you prepare the |
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218 | environment. |
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219 | |
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220 | open my $output, ">/tmp/log" or die "$!"; |
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221 | |
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222 | AnyEvent::Fork |
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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 |
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264 | App::Staticperl, Urlader or PAR::Packer for example. |
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265 | |
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266 | Two modules help here - AnyEvent::Fork::Early forks a template process |
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267 | for all further calls to "new_exec", and AnyEvent::Fork::Template forks |
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268 | the main program as a template process. |
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269 | |
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270 | Here is how your main program should look like: |
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271 | |
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272 | #! perl |
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273 | |
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274 | # optional, as the very first thing. |
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275 | # in case modules want to create their own processes. |
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276 | use AnyEvent::Fork::Early; |
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277 | |
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278 | # next, load all modules you need in your template process |
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279 | use Example::My::Module |
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280 | use Example::Whatever; |
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281 | |
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282 | # next, put your run function definition and anything else you |
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283 | # need, but do not use code outside of BEGIN blocks. |
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284 | sub worker_run { |
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285 | my ($fh, @args) = @_; |
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286 | ... |
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287 | } |
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288 | |
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289 | # now preserve everything so far as AnyEvent::Fork object |
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290 | # in §TEMPLATE. |
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291 | use AnyEvent::Fork::Template; |
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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; |
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308 | |
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309 | lhead1 CONCEPTS |
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310 | |
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311 | This module can create new processes either by executing a new perl |
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312 | process, or by forking from an existing "template" process. |
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313 | |
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314 | All these processes are called "child processes" (whether they are |
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315 | direct children or not), while the process that manages them is called |
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316 | the "parent process". |
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317 | |
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318 | Each such process comes with its own file handle that can be used to |
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319 | communicate with it (it's actually a socket - one end in the new |
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320 | process, one end in the main process), and among the things you can do |
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321 | in it are load modules, fork new processes, send file handles to it, and |
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322 | execute functions. |
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323 | |
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324 | There are multiple ways to create additional processes to execute some |
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325 | jobs: |
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326 | |
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327 | fork a new process from the "default" template process, load code, run |
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328 | it |
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329 | This module has a "default" template process which it executes when |
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330 | it is needed the first time. Forking from this process shares the |
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331 | memory used for the perl interpreter with the new process, but |
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332 | loading modules takes time, and the memory is not shared with |
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333 | anything else. |
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334 | |
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335 | This is ideal for when you only need one extra process of a kind, |
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336 | with the option of starting and stopping it on demand. |
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337 | |
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338 | Example: |
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339 | |
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340 | AnyEvent::Fork |
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341 | ->new |
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342 | ->require ("Some::Module") |
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343 | ->run ("Some::Module::run", sub { |
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344 | my ($fork_fh) = @_; |
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345 | }); |
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346 | |
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347 | fork a new template process, load code, then fork processes off of it |
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348 | and run the code |
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349 | When you need to have a bunch of processes that all execute the same |
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350 | (or very similar) tasks, then a good way is to create a new template |
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351 | process for them, loading all the modules you need, and then create |
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352 | your worker processes from this new template process. |
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353 | |
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354 | This way, all code (and data structures) that can be shared (e.g. |
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355 | the modules you loaded) is shared between the processes, and each |
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356 | new process consumes relatively little memory of its own. |
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357 | |
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358 | The disadvantage of this approach is that you need to create a |
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359 | template process for the sole purpose of forking new processes from |
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360 | it, but if you only need a fixed number of processes you can create |
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361 | them, and then destroy the template process. |
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362 | |
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363 | Example: |
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364 | |
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365 | my $template = AnyEvent::Fork->new->require ("Some::Module"); |
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366 | |
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367 | for (1..10) { |
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368 | $template->fork->run ("Some::Module::run", sub { |
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369 | my ($fork_fh) = @_; |
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370 | }); |
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371 | } |
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372 | |
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373 | # at this point, you can keep $template around to fork new processes |
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374 | # later, or you can destroy it, which causes it to vanish. |
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375 | |
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376 | execute a new perl interpreter, load some code, run it |
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377 | This is relatively slow, and doesn't allow you to share memory |
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378 | between multiple processes. |
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379 | |
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380 | The only advantage is that you don't have to have a template process |
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381 | hanging around all the time to fork off some new processes, which |
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382 | might be an advantage when there are long time spans where no extra |
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383 | processes are needed. |
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384 | |
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385 | Example: |
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386 | |
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387 | AnyEvent::Fork |
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388 | ->new_exec |
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389 | ->require ("Some::Module") |
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390 | ->run ("Some::Module::run", sub { |
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391 | my ($fork_fh) = @_; |
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392 | }); |
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393 | |
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394 | THE "AnyEvent::Fork" CLASS |
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395 | This module exports nothing, and only implements a single class - |
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396 | "AnyEvent::Fork". |
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397 | |
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398 | There are two class constructors that both create new processes - "new" |
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399 | and "new_exec". The "fork" method creates a new process by forking an |
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400 | existing one and could be considered a third constructor. |
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401 | |
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402 | Most of the remaining methods deal with preparing the new process, by |
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403 | loading code, evaluating code and sending data to the new process. They |
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404 | usually return the process object, so you can chain method calls. |
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405 | |
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406 | If a process object is destroyed before calling its "run" method, then |
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407 | the process simply exits. After "run" is called, all responsibility is |
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408 | passed to the specified function. |
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409 | |
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410 | As long as there is any outstanding work to be done, process objects |
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411 | resist being destroyed, so there is no reason to store them unless you |
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412 | need them later - configure and forget works just fine. |
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413 | |
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414 | my $proc = new AnyEvent::Fork |
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415 | Create a new "empty" perl interpreter process and returns its |
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416 | process object for further manipulation. |
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417 | |
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418 | The new process is forked from a template process that is kept |
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419 | around for this purpose. When it doesn't exist yet, it is created by |
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420 | a call to "new_exec" first and then stays around for future calls. |
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421 | |
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422 | $new_proc = $proc->fork |
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423 | Forks $proc, creating a new process, and returns the process object |
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424 | of the new process. |
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425 | |
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426 | If any of the "send_" functions have been called before fork, then |
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427 | they will be cloned in the child. For example, in a pre-forked |
|
|
428 | server, you might "send_fh" the listening socket into the template |
|
|
429 | process, and then keep calling "fork" and "run". |
|
|
430 | |
|
|
431 | my $proc = new_exec AnyEvent::Fork |
|
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432 | Create a new "empty" perl interpreter process and returns its |
|
|
433 | process object for further manipulation. |
|
|
434 | |
|
|
435 | Unlike the "new" method, this method *always* spawns a new perl |
|
|
436 | process (except in some cases, see AnyEvent::Fork::Early for |
|
|
437 | details). This reduces the amount of memory sharing that is |
|
|
438 | possible, and is also slower. |
|
|
439 | |
|
|
440 | You should use "new" whenever possible, except when having a |
|
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441 | template process around is unacceptable. |
|
|
442 | |
|
|
443 | The path to the perl interpreter is divined using various methods - |
|
|
444 | first $^X is investigated to see if the path ends with something |
|
|
445 | that sounds as if it were the perl interpreter. Failing this, the |
|
|
446 | module falls back to using $Config::Config{perlpath}. |
|
|
447 | |
|
|
448 | $pid = $proc->pid |
|
|
449 | Returns the process id of the process *iff it is a direct child of |
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|
450 | the process running AnyEvent::Fork*, and "undef" otherwise. |
|
|
451 | |
|
|
452 | Normally, only processes created via "AnyEvent::Fork->new_exec" and |
|
|
453 | AnyEvent::Fork::Template are direct children, and you are |
|
|
454 | responsible to clean up their zombies when they die. |
|
|
455 | |
|
|
456 | All other processes are not direct children, and will be cleaned up |
|
|
457 | by AnyEvent::Fork itself. |
|
|
458 | |
|
|
459 | $proc = $proc->eval ($perlcode, @args) |
|
|
460 | Evaluates the given $perlcode as ... Perl code, while setting @_ to |
|
|
461 | the strings specified by @args, in the "main" package. |
|
|
462 | |
|
|
463 | This call is meant to do any custom initialisation that might be |
|
|
464 | required (for example, the "require" method uses it). It's not |
|
|
465 | supposed to be used to completely take over the process, use "run" |
|
|
466 | for that. |
|
|
467 | |
|
|
468 | The code will usually be executed after this call returns, and there |
|
|
469 | is no way to pass anything back to the calling process. Any |
|
|
470 | evaluation errors will be reported to stderr and cause the process |
|
|
471 | to exit. |
|
|
472 | |
|
|
473 | If you want to execute some code (that isn't in a module) to take |
|
|
474 | over the process, you should compile a function via "eval" first, |
|
|
475 | and then call it via "run". This also gives you access to any |
|
|
476 | arguments passed via the "send_xxx" methods, such as file handles. |
|
|
477 | See the "use AnyEvent::Fork as a faster fork+exec" example to see it |
|
|
478 | in action. |
|
|
479 | |
|
|
480 | Returns the process object for easy chaining of method calls. |
|
|
481 | |
|
|
482 | $proc = $proc->require ($module, ...) |
|
|
483 | Tries to load the given module(s) into the process |
|
|
484 | |
|
|
485 | Returns the process object for easy chaining of method calls. |
|
|
486 | |
|
|
487 | $proc = $proc->send_fh ($handle, ...) |
|
|
488 | Send one or more file handles (*not* file descriptors) to the |
|
|
489 | process, to prepare a call to "run". |
|
|
490 | |
|
|
491 | The process object keeps a reference to the handles until they have |
|
|
492 | been passed over to the process, so you must not explicitly close |
|
|
493 | the handles. This is most easily accomplished by simply not storing |
|
|
494 | the file handles anywhere after passing them to this method - when |
|
|
495 | AnyEvent::Fork is finished using them, perl will automatically close |
|
|
496 | them. |
|
|
497 | |
|
|
498 | Returns the process object for easy chaining of method calls. |
|
|
499 | |
|
|
500 | Example: pass a file handle to a process, and release it without |
|
|
501 | closing. It will be closed automatically when it is no longer used. |
|
|
502 | |
|
|
503 | $proc->send_fh ($my_fh); |
|
|
504 | undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
|
|
505 | |
|
|
506 | $proc = $proc->send_arg ($string, ...) |
|
|
507 | Send one or more argument strings to the process, to prepare a call |
|
|
508 | to "run". The strings can be any octet strings. |
|
|
509 | |
|
|
510 | The protocol is optimised to pass a moderate number of relatively |
|
|
511 | short strings - while you can pass up to 4GB of data in one go, this |
|
|
512 | is more meant to pass some ID information or other startup info, not |
|
|
513 | big chunks of data. |
|
|
514 | |
|
|
515 | Returns the process object for easy chaining of method calls. |
|
|
516 | |
|
|
517 | $proc->run ($func, $cb->($fh)) |
|
|
518 | Enter the function specified by the function name in $func in the |
|
|
519 | process. The function is called with the communication socket as |
|
|
520 | first argument, followed by all file handles and string arguments |
|
|
521 | sent earlier via "send_fh" and "send_arg" methods, in the order they |
|
|
522 | were called. |
|
|
523 | |
|
|
524 | The process object becomes unusable on return from this function - |
|
|
525 | any further method calls result in undefined behaviour. |
|
|
526 | |
|
|
527 | The function name should be fully qualified, but if it isn't, it |
|
|
528 | will be looked up in the "main" package. |
|
|
529 | |
|
|
530 | If the called function returns, doesn't exist, or any error occurs, |
|
|
531 | the process exits. |
|
|
532 | |
|
|
533 | Preparing the process is done in the background - when all commands |
|
|
534 | have been sent, the callback is invoked with the local |
|
|
535 | communications socket as argument. At this point you can start using |
|
|
536 | the socket in any way you like. |
|
|
537 | |
|
|
538 | If the communication socket isn't used, it should be closed on both |
|
|
539 | sides, to save on kernel memory. |
|
|
540 | |
|
|
541 | The socket is non-blocking in the parent, and blocking in the newly |
|
|
542 | created process. The close-on-exec flag is set in both. |
|
|
543 | |
|
|
544 | Even if not used otherwise, the socket can be a good indicator for |
|
|
545 | the existence of the process - if the other process exits, you get a |
|
|
546 | readable event on it, because exiting the process closes the socket |
|
|
547 | (if it didn't create any children using fork). |
|
|
548 | |
|
|
549 | Example: create a template for a process pool, pass a few strings, |
|
|
550 | some file handles, then fork, pass one more string, and run some |
|
|
551 | code. |
|
|
552 | |
|
|
553 | my $pool = AnyEvent::Fork |
|
|
554 | ->new |
|
|
555 | ->send_arg ("str1", "str2") |
|
|
556 | ->send_fh ($fh1, $fh2); |
|
|
557 | |
|
|
558 | for (1..2) { |
|
|
559 | $pool |
|
|
560 | ->fork |
|
|
561 | ->send_arg ("str3") |
|
|
562 | ->run ("Some::function", sub { |
|
|
563 | my ($fh) = @_; |
|
|
564 | |
|
|
565 | # fh is nonblocking, but we trust that the OS can accept these |
|
|
566 | # few octets anyway. |
|
|
567 | syswrite $fh, "hi #$_\n"; |
|
|
568 | |
|
|
569 | # $fh is being closed here, as we don't store it anywhere |
|
|
570 | }); |
|
|
571 | } |
|
|
572 | |
|
|
573 | # Some::function might look like this - all parameters passed before fork |
|
|
574 | # and after will be passed, in order, after the communications socket. |
|
|
575 | sub Some::function { |
|
|
576 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
|
|
577 | |
|
|
578 | print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order |
|
|
579 | } |
|
|
580 | |
|
|
581 | $proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED |
|
|
582 | Flushes all commands out to the process and then calls the callback |
|
|
583 | with the communications socket. |
|
|
584 | |
|
|
585 | The process object becomes unusable on return from this function - |
|
|
586 | any further method calls result in undefined behaviour. |
|
|
587 | |
|
|
588 | The point of this method is to give you a file handle thta you cna |
|
|
589 | pass to another process. In that other process, you can call |
|
|
590 | "new_from_fh AnyEvent::Fork" to create a new "AnyEvent::Fork" object |
|
|
591 | from it, thereby effectively passing a fork object to another |
|
|
592 | process. |
|
|
593 | |
|
|
594 | new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED |
|
|
595 | Takes a file handle originally rceeived by the "to_fh" method and |
|
|
596 | creates a new "AnyEvent:Fork" object. The child process itself will |
|
|
597 | not change in any way, i.e. it will keep all the modifications done |
|
|
598 | to it before calling "to_fh". |
|
|
599 | |
|
|
600 | The new object is very much like the original object, except that |
|
|
601 | the "pid" method will return "undef" even if the process is a direct |
|
|
602 | child. |
|
|
603 | |
|
|
604 | PERFORMANCE |
|
|
605 | Now for some unscientific benchmark numbers (all done on an amd64 |
|
|
606 | GNU/Linux box). These are intended to give you an idea of the relative |
|
|
607 | performance you can expect, they are not meant to be absolute |
|
|
608 | performance numbers. |
|
|
609 | |
|
|
610 | OK, so, I ran a simple benchmark that creates a socket pair, forks, |
|
|
611 | calls exit in the child and waits for the socket to close in the parent. |
|
|
612 | I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of |
|
|
613 | 5100kB. |
|
|
614 | |
|
|
615 | 2079 new processes per second, using manual socketpair + fork |
|
|
616 | |
|
|
617 | Then I did the same thing, but instead of calling fork, I called |
|
|
618 | AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the |
|
|
619 | socket from the child to close on exit. This does the same thing as |
|
|
620 | manual socket pair + fork, except that what is forked is the template |
|
|
621 | process (2440kB), and the socket needs to be passed to the server at the |
|
|
622 | other end of the socket first. |
|
|
623 | |
|
|
624 | 2307 new processes per second, using AnyEvent::Fork->new |
|
|
625 | |
|
|
626 | And finally, using "new_exec" instead "new", using vforks+execs to exec |
|
|
627 | a new perl interpreter and compile the small server each time, I get: |
|
|
628 | |
|
|
629 | 479 vfork+execs per second, using AnyEvent::Fork->new_exec |
|
|
630 | |
|
|
631 | So how can "AnyEvent->new" be faster than a standard fork, even though |
|
|
632 | it uses the same operations, but adds a lot of overhead? |
|
|
633 | |
|
|
634 | The difference is simply the process size: forking the 5MB process takes |
|
|
635 | so much longer than forking the 2.5MB template process that the extra |
|
|
636 | overhead is canceled out. |
|
|
637 | |
|
|
638 | If the benchmark process grows, the normal fork becomes even slower: |
|
|
639 | |
|
|
640 | 1340 new processes, manual fork of a 20MB process |
|
|
641 | 731 new processes, manual fork of a 200MB process |
|
|
642 | 235 new processes, manual fork of a 2000MB process |
|
|
643 | |
|
|
644 | What that means (to me) is that I can use this module without having a |
|
|
645 | bad conscience because of the extra overhead required to start new |
|
|
646 | processes. |
|
|
647 | |
|
|
648 | TYPICAL PROBLEMS |
|
|
649 | This section lists typical problems that remain. I hope by recognising |
|
|
650 | them, most can be avoided. |
|
|
651 | |
|
|
652 | leaked file descriptors for exec'ed processes |
|
|
653 | POSIX systems inherit file descriptors by default when exec'ing a |
|
|
654 | new process. While perl itself laudably sets the close-on-exec flags |
|
|
655 | on new file handles, most C libraries don't care, and even if all |
|
|
656 | cared, it's often not possible to set the flag in a race-free |
|
|
657 | manner. |
|
|
658 | |
|
|
659 | That means some file descriptors can leak through. And since it |
|
|
660 | isn't possible to know which file descriptors are "good" and |
|
|
661 | "necessary" (or even to know which file descriptors are open), there |
|
|
662 | is no good way to close the ones that might harm. |
|
|
663 | |
|
|
664 | As an example of what "harm" can be done consider a web server that |
|
|
665 | accepts connections and afterwards some module uses AnyEvent::Fork |
|
|
666 | for the first time, causing it to fork and exec a new process, which |
|
|
667 | might inherit the network socket. When the server closes the socket, |
|
|
668 | it is still open in the child (which doesn't even know that) and the |
|
|
669 | client might conclude that the connection is still fine. |
|
|
670 | |
|
|
671 | For the main program, there are multiple remedies available - |
|
|
672 | AnyEvent::Fork::Early is one, creating a process early and not using |
|
|
673 | "new_exec" is another, as in both cases, the first process can be |
|
|
674 | exec'ed well before many random file descriptors are open. |
|
|
675 | |
|
|
676 | In general, the solution for these kind of problems is to fix the |
|
|
677 | libraries or the code that leaks those file descriptors. |
|
|
678 | |
|
|
679 | Fortunately, most of these leaked descriptors do no harm, other than |
|
|
680 | sitting on some resources. |
|
|
681 | |
|
|
682 | leaked file descriptors for fork'ed processes |
|
|
683 | Normally, AnyEvent::Fork does start new processes by exec'ing them, |
|
|
684 | which closes file descriptors not marked for being inherited. |
|
|
685 | |
|
|
686 | However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
|
|
687 | way to create these processes by forking, and this leaks more file |
|
|
688 | descriptors than exec'ing them, as there is no way to mark |
|
|
689 | descriptors as "close on fork". |
|
|
690 | |
|
|
691 | An example would be modules like EV, IO::AIO or Gtk2. Both create |
|
|
692 | pipes for internal uses, and Gtk2 might open a connection to the X |
|
|
693 | server. EV and IO::AIO can deal with fork, but Gtk2 might have |
|
|
694 | trouble with a fork. |
|
|
695 | |
|
|
696 | The solution is to either not load these modules before use'ing |
|
|
697 | AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
|
|
698 | initialising them, for example, by calling "init Gtk2" manually. |
|
|
699 | |
|
|
700 | exiting calls object destructors |
|
|
701 | This only applies to users of AnyEvent::Fork:Early and |
|
|
702 | AnyEvent::Fork::Template, or when initialising code creates objects |
|
|
703 | that reference external resources. |
|
|
704 | |
|
|
705 | When a process created by AnyEvent::Fork exits, it might do so by |
|
|
706 | calling exit, or simply letting perl reach the end of the program. |
|
|
707 | At which point Perl runs all destructors. |
|
|
708 | |
|
|
709 | Not all destructors are fork-safe - for example, an object that |
|
|
710 | represents the connection to an X display might tell the X server to |
|
|
711 | free resources, which is inconvenient when the "real" object in the |
|
|
712 | parent still needs to use them. |
|
|
713 | |
|
|
714 | This is obviously not a problem for AnyEvent::Fork::Early, as you |
|
|
715 | used it as the very first thing, right? |
|
|
716 | |
|
|
717 | It is a problem for AnyEvent::Fork::Template though - and the |
|
|
718 | solution is to not create objects with nontrivial destructors that |
|
|
719 | might have an effect outside of Perl. |
|
|
720 | |
|
|
721 | PORTABILITY NOTES |
|
|
722 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
|
|
723 | nop, and ::Template is not going to work), and it cost a lot of blood |
|
|
724 | and sweat to make it so, mostly due to the bloody broken perl that |
|
|
725 | nobody seems to care about. The fork emulation is a bad joke - I have |
|
|
726 | yet to see something useful that you can do with it without running into |
|
|
727 | memory corruption issues or other braindamage. Hrrrr. |
|
|
728 | |
|
|
729 | Since fork is endlessly broken on win32 perls (it doesn't even remotely |
|
|
730 | work within it's documented limits) and quite obviously it's not getting |
|
|
731 | improved any time soon, the best way to proceed on windows would be to |
|
|
732 | always use "new_exec" and thus never rely on perl's fork "emulation". |
|
|
733 | |
|
|
734 | Cygwin perl is not supported at the moment due to some hilarious |
|
|
735 | shortcomings of its API - see IO::FDPoll for more details. If you never |
|
|
736 | use "send_fh" and always use "new_exec" to create processes, it should |
|
|
737 | work though. |
|
|
738 | |
|
|
739 | SEE ALSO |
|
|
740 | AnyEvent::Fork::Early, to avoid executing a perl interpreter at all |
|
|
741 | (part of this distribution). |
|
|
742 | |
|
|
743 | AnyEvent::Fork::Template, to create a process by forking the main |
|
|
744 | program at a convenient time (part of this distribution). |
|
|
745 | |
|
|
746 | AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN). |
|
|
747 | |
|
|
748 | AnyEvent::Fork::Pool, for simple worker process pool (on CPAN). |
|
|
749 | |
|
|
750 | AUTHOR AND CONTACT INFORMATION |
|
|
751 | Marc Lehmann <schmorp@schmorp.de> |
|
|
752 | http://software.schmorp.de/pkg/AnyEvent-Fork |
|
|
753 | |