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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 | ################################################################## |
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8 | # create a single new process, tell it to run your worker function |
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9 | |
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10 | AnyEvent::Fork |
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11 | ->new |
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12 | ->require ("MyModule") |
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13 | ->run ("MyModule::worker, sub { |
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14 | my ($master_filehandle) = @_; |
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15 | |
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16 | # now $master_filehandle is connected to the |
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17 | # $slave_filehandle in the new process. |
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18 | }); |
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19 | |
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20 | # MyModule::worker might look like this |
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21 | sub MyModule::worker { |
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22 | my ($slave_filehandle) = @_; |
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23 | |
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24 | # now $slave_filehandle is connected to the $master_filehandle |
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25 | # in the original prorcess. have fun! |
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26 | } |
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27 | |
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28 | ################################################################## |
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29 | # create a pool of server processes all accepting on the same socket |
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30 | |
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31 | # create listener socket |
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32 | my $listener = ...; |
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33 | |
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34 | # create a pool template, initialise it and give it the socket |
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35 | my $pool = AnyEvent::Fork |
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36 | ->new |
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37 | ->require ("Some::Stuff", "My::Server") |
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38 | ->send_fh ($listener); |
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39 | |
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40 | # now create 10 identical workers |
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41 | for my $id (1..10) { |
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42 | $pool |
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43 | ->fork |
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44 | ->send_arg ($id) |
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45 | ->run ("My::Server::run"); |
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46 | } |
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47 | |
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48 | # now do other things - maybe use the filehandle provided by run |
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49 | # to wait for the processes to die. or whatever. |
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50 | |
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51 | # My::Server::run might look like this |
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52 | sub My::Server::run { |
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53 | my ($slave, $listener, $id) = @_; |
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54 | |
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55 | close $slave; # we do not use the socket, so close it to save resources |
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56 | |
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57 | # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
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58 | # or anything we usually couldn't do in a process forked normally. |
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59 | while (my $socket = $listener->accept) { |
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60 | # do sth. with new socket |
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61 | } |
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62 | } |
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63 | |
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64 | DESCRIPTION |
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65 | This module allows you to create new processes, without actually forking |
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66 | them from your current process (avoiding the problems of forking), but |
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67 | preserving most of the advantages of fork. |
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68 | |
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69 | It can be used to create new worker processes or new independent |
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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 | |
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75 | Special care has been taken to make this module useful from other |
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76 | modules, while still supporting specialised environments such as |
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77 | App::Staticperl or PAR::Packer. |
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78 | |
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79 | PROBLEM STATEMENT |
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80 | There are two ways to implement parallel processing on UNIX like |
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81 | operating systems - fork and process, and fork+exec and process. They |
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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 | |
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85 | Forking from a big process can be very slow (a 5GB process needs 0.05s |
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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 | |
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92 | Forking usually creates a copy-on-write copy of the parent process. |
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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 | |
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105 | Exec'ing a new perl process might be difficult and slow. For example, it |
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106 | is not easy to find the correct path to the perl interpreter, and all |
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107 | modules have to be loaded from disk again. Long running processes might |
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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 | |
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114 | Forking might be impossible when a program is running. For example, |
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115 | POSIX makes it almost impossible to fork from a multithreaded program |
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116 | and do anything useful in the child - strictly speaking, if your perl |
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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 | |
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122 | Parallel processing with fork might be inconvenient or difficult to |
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123 | implement. For example, when a program uses an event loop and creates |
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124 | watchers it becomes very hard to use the event loop from a child |
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125 | program, as the watchers already exist but are only meaningful in the |
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126 | parent. Worse, a module might want to use such a system, not knowing |
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127 | whether another module or the main program also does, leading to |
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128 | problems. |
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129 | This module only lets the main program create pools by forking |
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130 | (because only the main program can know when it is still safe to do |
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131 | so) - all other pools are created by fork+exec, after which such |
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132 | modules can again be loaded. |
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133 | |
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134 | CONCEPTS |
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135 | This module can create new processes either by executing a new perl |
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136 | process, or by forking from an existing "template" process. |
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137 | |
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138 | Each such process comes with its own file handle that can be used to |
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139 | communicate with it (it's actually a socket - one end in the new |
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140 | process, one end in the main process), and among the things you can do |
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141 | in it are load modules, fork new processes, send file handles to it, and |
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142 | execute functions. |
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143 | |
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144 | There are multiple ways to create additional processes to execute some |
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145 | jobs: |
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146 | |
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147 | fork a new process from the "default" template process, load code, run |
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148 | it |
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149 | This module has a "default" template process which it executes when |
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150 | it is needed the first time. Forking from this process shares the |
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151 | memory used for the perl interpreter with the new process, but |
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152 | loading modules takes time, and the memory is not shared with |
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153 | anything else. |
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154 | |
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155 | This is ideal for when you only need one extra process of a kind, |
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156 | with the option of starting and stipping it on demand. |
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157 | |
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158 | Example: |
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159 | |
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160 | AnyEvent::Fork |
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161 | ->new |
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162 | ->require ("Some::Module") |
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163 | ->run ("Some::Module::run", sub { |
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164 | my ($fork_fh) = @_; |
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165 | }); |
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166 | |
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167 | fork a new template process, load code, then fork processes off of it |
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168 | and run the code |
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169 | When you need to have a bunch of processes that all execute the same |
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170 | (or very similar) tasks, then a good way is to create a new template |
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171 | process for them, loading all the modules you need, and then create |
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172 | your worker processes from this new template process. |
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173 | |
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174 | This way, all code (and data structures) that can be shared (e.g. |
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175 | the modules you loaded) is shared between the processes, and each |
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176 | new process consumes relatively little memory of its own. |
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177 | |
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178 | The disadvantage of this approach is that you need to create a |
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179 | template process for the sole purpose of forking new processes from |
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180 | it, but if you only need a fixed number of proceses you can create |
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181 | them, and then destroy the template process. |
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182 | |
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183 | Example: |
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184 | |
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185 | my $template = AnyEvent::Fork->new->require ("Some::Module"); |
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186 | |
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187 | for (1..10) { |
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188 | $template->fork->run ("Some::Module::run", sub { |
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189 | my ($fork_fh) = @_; |
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190 | }); |
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191 | } |
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192 | |
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193 | # at this point, you can keep $template around to fork new processes |
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194 | # later, or you can destroy it, which causes it to vanish. |
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195 | |
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196 | execute a new perl interpreter, load some code, run it |
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197 | This is relatively slow, and doesn't allow you to share memory |
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198 | between multiple processes. |
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199 | |
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200 | The only advantage is that you don't have to have a template process |
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201 | hanging around all the time to fork off some new processes, which |
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202 | might be an advantage when there are long time spans where no extra |
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203 | processes are needed. |
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204 | |
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205 | Example: |
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206 | |
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207 | AnyEvent::Fork |
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208 | ->new_exec |
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209 | ->require ("Some::Module") |
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210 | ->run ("Some::Module::run", sub { |
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211 | my ($fork_fh) = @_; |
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212 | }); |
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213 | |
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214 | FUNCTIONS |
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215 | my $pool = new AnyEvent::Fork key => value... |
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216 | Create a new process pool. The following named parameters are |
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217 | supported: |
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218 | |
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219 | my $proc = new AnyEvent::Fork |
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220 | Create a new "empty" perl interpreter process and returns its |
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221 | process object for further manipulation. |
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222 | |
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223 | The new process is forked from a template process that is kept |
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224 | around for this purpose. When it doesn't exist yet, it is created by |
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225 | a call to "new_exec" and kept around for future calls. |
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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 | |
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232 | $new_proc = $proc->fork |
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233 | Forks $proc, creating a new process, and returns the process object |
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234 | of the new process. |
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235 | |
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236 | If any of the "send_" functions have been called before fork, then |
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237 | they will be cloned in the child. For example, in a pre-forked |
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238 | server, you might "send_fh" the listening socket into the template |
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239 | process, and then keep calling "fork" and "run". |
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240 | |
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241 | my $proc = new_exec AnyEvent::Fork |
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242 | Create a new "empty" perl interpreter process and returns its |
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243 | process object for further manipulation. |
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244 | |
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245 | Unlike the "new" method, this method *always* spawns a new perl |
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246 | process (except in some cases, see AnyEvent::Fork::Early for |
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247 | details). This reduces the amount of memory sharing that is |
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248 | possible, and is also slower. |
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249 | |
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250 | You should use "new" whenever possible, except when having a |
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251 | template process around is unacceptable. |
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252 | |
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253 | The path to the perl interpreter is divined usign various methods - |
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254 | first $^X is investigated to see if the path ends with something |
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255 | that sounds as if it were the perl interpreter. Failing this, the |
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256 | module falls back to using $Config::Config{perlpath}. |
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257 | |
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258 | $proc = $proc->eval ($perlcode, @args) |
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259 | Evaluates the given $perlcode as ... perl code, while setting @_ to |
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260 | the strings specified by @args. |
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261 | |
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262 | This call is meant to do any custom initialisation that might be |
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263 | required (for example, the "require" method uses it). It's not |
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264 | supposed to be used to completely take over the process, use "run" |
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265 | for that. |
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266 | |
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267 | The code will usually be executed after this call returns, and there |
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268 | is no way to pass anything back to the calling process. Any |
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269 | evaluation errors will be reported to stderr and cause the process |
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270 | to exit. |
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271 | |
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272 | Returns the process object for easy chaining of method calls. |
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273 | |
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274 | $proc = $proc->require ($module, ...) |
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275 | Tries to load the given module(s) into the process |
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276 | |
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277 | Returns the process object for easy chaining of method calls. |
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278 | |
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279 | $proc = $proc->send_fh ($handle, ...) |
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280 | Send one or more file handles (*not* file descriptors) to the |
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281 | process, to prepare a call to "run". |
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282 | |
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283 | The process object keeps a reference to the handles until this is |
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284 | done, so you must not explicitly close the handles. This is most |
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285 | easily accomplished by simply not storing the file handles anywhere |
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286 | after passing them to this method. |
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287 | |
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288 | Returns the process object for easy chaining of method calls. |
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289 | |
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290 | Example: pass an fh to a process, and release it without closing. it |
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291 | will be closed automatically when it is no longer used. |
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292 | |
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293 | $proc->send_fh ($my_fh); |
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294 | undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
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295 | |
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296 | $proc = $proc->send_arg ($string, ...) |
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297 | Send one or more argument strings to the process, to prepare a call |
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298 | to "run". The strings can be any octet string. |
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299 | |
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300 | Returns the process object for easy chaining of emthod calls. |
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301 | |
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302 | $proc->run ($func, $cb->($fh)) |
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303 | Enter the function specified by the fully qualified name in $func in |
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304 | the process. The function is called with the communication socket as |
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305 | first argument, followed by all file handles and string arguments |
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306 | sent earlier via "send_fh" and "send_arg" methods, in the order they |
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307 | were called. |
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308 | |
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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. |
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316 | |
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317 | If the communication socket isn't used, it should be closed on both |
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318 | sides, to save on kernel memory. |
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319 | |
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320 | The socket is non-blocking in the parent, and blocking in the newly |
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321 | created process. The close-on-exec flag is set on both. Even if not |
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322 | used otherwise, the socket can be a good indicator for the existance |
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323 | of the process - if the other process exits, you get a readable |
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324 | event on it, because exiting the process closes the socket (if it |
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325 | didn't create any children using fork). |
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326 | |
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327 | Example: create a template for a process pool, pass a few strings, |
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328 | some file handles, then fork, pass one more string, and run some |
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329 | code. |
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330 | |
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331 | my $pool = AnyEvent::Fork |
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332 | ->new |
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333 | ->send_arg ("str1", "str2") |
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334 | ->send_fh ($fh1, $fh2); |
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335 | |
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336 | for (1..2) { |
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337 | $pool |
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338 | ->fork |
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339 | ->send_arg ("str3") |
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340 | ->run ("Some::function", sub { |
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341 | my ($fh) = @_; |
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342 | |
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343 | # fh is nonblocking, but we trust that the OS can accept these |
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344 | # extra 3 octets anyway. |
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345 | syswrite $fh, "hi #$_\n"; |
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346 | |
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347 | # $fh is being closed here, as we don't store it anywhere |
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348 | }); |
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349 | } |
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350 | |
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351 | # Some::function might look like this - all parameters passed before fork |
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352 | # and after will be passed, in order, after the communications socket. |
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353 | sub Some::function { |
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354 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
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355 | |
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356 | print scalar <$fh>; # prints "hi 1\n" and "hi 2\n" |
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357 | } |
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358 | |
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359 | TYPICAL PROBLEMS |
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360 | This section lists typical problems that remain. I hope by recognising |
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361 | them, most can be avoided. |
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362 | |
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363 | "leaked" file descriptors for exec'ed processes |
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364 | POSIX systems inherit file descriptors by default when exec'ing a |
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365 | new process. While perl itself laudably sets the close-on-exec flags |
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366 | on new file handles, most C libraries don't care, and even if all |
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367 | cared, it's often not possible to set the flag in a race-free |
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368 | manner. |
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369 | |
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370 | That means some file descriptors can leak through. And since it |
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371 | isn't possible to know which file descriptors are "good" and |
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372 | "neccessary" (or even to know which file descreiptors are open), |
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373 | there is no good way to close the ones that might harm. |
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374 | |
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375 | As an example of what "harm" can be done consider a web server that |
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376 | accepts connections and afterwards some module uses AnyEvent::Fork |
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377 | for the first time, causing it to fork and exec a new process, which |
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378 | might inherit the network socket. When the server closes the socket, |
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379 | it is still open in the child (which doesn't even know that) and the |
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380 | client might conclude that the connection is still fine. |
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381 | |
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382 | For the main program, there are multiple remedies available - |
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383 | AnyEvent::Fork::Early is one, creating a process early and not using |
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384 | "new_exec" is another, as in both cases, the first process can be |
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385 | exec'ed well before many random file descriptors are open. |
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386 | |
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387 | In general, the solution for these kind of problems is to fix the |
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388 | libraries or the code that leaks those file descriptors. |
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389 | |
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390 | Fortunately, most of these lekaed descriptors do no harm, other than |
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391 | sitting on some resources. |
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392 | |
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393 | "leaked" file descriptors for fork'ed processes |
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394 | Normally, AnyEvent::Fork does start new processes by exec'ing them, |
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395 | which closes file descriptors not marked for being inherited. |
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396 | |
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397 | However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
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398 | way to create these processes by forking, and this leaks more file |
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399 | descriptors than exec'ing them, as there is no way to mark |
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400 | descriptors as "close on fork". |
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401 | |
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402 | An example would be modules like EV, IO::AIO or Gtk2. Both create |
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403 | pipes for internal uses, and Gtk2 might open a connection to the X |
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404 | server. EV and IO::AIO can deal with fork, but Gtk2 might have |
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405 | trouble with a fork. |
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406 | |
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407 | The solution is to either not load these modules before use'ing |
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408 | AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
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409 | initialising them, for example, by calling "init Gtk2" manually. |
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410 | |
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411 | PORTABILITY NOTES |
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412 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
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413 | nop, and ::Template is not going to work), and it cost a lot of blood |
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414 | and sweat to make it so, mostly due to the bloody broken perl that |
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415 | nobody seems to care about. The fork emulation is a bad joke - I have |
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416 | yet to see something useful that you cna do with it without running into |
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417 | memory corruption issues or other braindamage. Hrrrr. |
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418 | |
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419 | Cygwin perl is not supported at the moment, as it should implement fd |
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420 | passing, but doesn't, and rolling my own is hard, as cygwin doesn't |
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421 | support enough functionality to do it. |
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422 | |
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423 | SEE ALSO |
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424 | AnyEvent::Fork::Early (to avoid executing a perl interpreter), |
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425 | AnyEvent::Fork::Template (to create a process by forking the main |
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426 | program at a convenient time). |
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427 | |
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428 | AUTHOR |
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429 | Marc Lehmann <schmorp@schmorp.de> |
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430 | http://home.schmorp.de/ |
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431 | |