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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 or not being able to use event |
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57 | processing, GUI toolkits or similar modules in the processes they |
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58 | create. |
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59 | |
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60 | This module doesn't try to replace any of them - instead it tries to |
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61 | solve the problem of creating processes with a minimum of fuss and |
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62 | overhead (and also luxury). Ideally, most of these would use |
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63 | AnyEvent::Fork internally, except they were written before AnyEvent:Fork |
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64 | was available, so obviously had to roll their own. |
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65 | |
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66 | PROBLEM STATEMENT |
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67 | There are two traditional ways to implement parallel processing on UNIX |
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68 | like operating systems - fork and process, and fork+exec and process. |
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69 | They have different advantages and disadvantages that I describe below, |
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70 | together with how this module tries to mitigate the disadvantages. |
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71 | |
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72 | Forking from a big process can be very slow. |
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73 | A 5GB process needs 0.05s to fork on my 3.6GHz amd64 GNU/Linux box. |
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74 | This overhead is often shared with exec (because you have to fork |
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75 | first), but in some circumstances (e.g. when vfork is used), |
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76 | fork+exec can be much faster. |
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77 | |
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78 | This module can help here by telling a small(er) helper process to |
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79 | fork, which is faster then forking the main process, and also uses |
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80 | vfork where possible. This gives the speed of vfork, with the |
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81 | flexibility of fork. |
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82 | |
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83 | Forking usually creates a copy-on-write copy of the parent process. |
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84 | For example, modules or data files that are loaded will not use |
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85 | additional memory after a fork. Exec'ing a new process, in contrast, |
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86 | means modules and data files might need to be loaded again, at extra |
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87 | CPU and memory cost. |
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88 | |
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89 | But when forking, you still create a copy of your data structures - |
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90 | if the program frees them and replaces them by new data, the child |
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91 | processes will retain the old version even if it isn't used, which |
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92 | can suddenly and unexpectedly increase memory usage when freeing |
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93 | memory. |
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94 | |
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95 | For example, Gtk2::CV is an image viewer optimised for large |
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96 | directories (millions of pictures). It also forks subprocesses for |
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97 | thumbnail generation, which inherit the data structure that stores |
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98 | all file information. If the user changes the directory, it gets |
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99 | freed in the main process, leaving a copy in the thumbnailer |
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100 | processes. This can lead to many times the memory usage that would |
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101 | actually be required. The solution is to fork early (and being |
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102 | unable to dynamically generate more subprocesses or do this from a |
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103 | module)... or to use <AnyEvent:Fork>. |
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104 | |
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105 | There is a trade-off between more sharing with fork (which can be |
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106 | good or bad), and no sharing with exec. |
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107 | |
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108 | This module allows the main program to do a controlled fork, and |
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109 | allows modules to exec processes safely at any time. When creating a |
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110 | custom process pool you can take advantage of data sharing via fork |
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111 | without risking to share large dynamic data structures that will |
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112 | blow up child memory usage. |
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113 | |
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114 | In other words, this module puts you into control over what is being |
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115 | shared and what isn't, at all times. |
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116 | |
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117 | Exec'ing a new perl process might be difficult. |
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118 | For example, it is not easy to find the correct path to the perl |
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119 | interpreter - $^X might not be a perl interpreter at all. Worse, |
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120 | there might not even be a perl binary installed on the system. |
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121 | |
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122 | This module tries hard to identify the correct path to the perl |
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123 | interpreter. With a cooperative main program, exec'ing the |
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124 | interpreter might not even be necessary, but even without help from |
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125 | the main program, it will still work when used from a module. |
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126 | |
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127 | Exec'ing a new perl process might be slow, as all necessary modules have |
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128 | to be loaded from disk again, with no guarantees of success. |
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129 | Long running processes might run into problems when perl is upgraded |
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130 | and modules are no longer loadable because they refer to a different |
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131 | perl version, or parts of a distribution are newer than the ones |
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132 | already loaded. |
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133 | |
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134 | This module supports creating pre-initialised perl processes to be |
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135 | used as a template for new processes at a later time, e.g. for use |
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136 | in a process pool. |
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137 | |
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138 | Forking might be impossible when a program is running. |
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139 | For example, POSIX makes it almost impossible to fork from a |
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140 | multi-threaded program while doing anything useful in the child - in |
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141 | fact, if your perl program uses POSIX threads (even indirectly via |
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142 | e.g. IO::AIO or threads), you cannot call fork on the perl level |
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143 | anymore without risking memory corruption or worse on a number of |
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144 | operating systems. |
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145 | |
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146 | This module can safely fork helper processes at any time, by calling |
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147 | fork+exec in C, in a POSIX-compatible way (via Proc::FastSpawn). |
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148 | |
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149 | Parallel processing with fork might be inconvenient or difficult to |
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150 | implement. Modules might not work in both parent and child. |
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151 | For example, when a program uses an event loop and creates watchers |
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152 | it becomes very hard to use the event loop from a child program, as |
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153 | the watchers already exist but are only meaningful in the parent. |
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154 | Worse, a module might want to use such a module, not knowing whether |
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155 | another module or the main program also does, leading to problems. |
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156 | |
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157 | Apart from event loops, graphical toolkits also commonly fall into |
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158 | the "unsafe module" category, or just about anything that |
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159 | communicates with the external world, such as network libraries and |
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160 | file I/O modules, which usually don't like being copied and then |
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161 | allowed to continue in two processes. |
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162 | |
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163 | With this module only the main program is allowed to create new |
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164 | processes by forking (because only the main program can know when it |
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165 | is still safe to do so) - all other processes are created via |
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166 | fork+exec, which makes it possible to use modules such as event |
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167 | loops or window interfaces safely. |
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168 | |
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169 | EXAMPLES |
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170 | This is where the wall of text ends and code speaks. |
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171 | |
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172 | Create a single new process, tell it to run your worker function. |
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173 | AnyEvent::Fork |
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174 | ->new |
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175 | ->require ("MyModule") |
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176 | ->run ("MyModule::worker, sub { |
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177 | my ($master_filehandle) = @_; |
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178 | |
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179 | # now $master_filehandle is connected to the |
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180 | # $slave_filehandle in the new process. |
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181 | }); |
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182 | |
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183 | "MyModule" might look like this: |
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184 | |
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185 | package MyModule; |
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186 | |
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187 | sub worker { |
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188 | my ($slave_filehandle) = @_; |
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189 | |
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190 | # now $slave_filehandle is connected to the $master_filehandle |
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191 | # in the original process. have fun! |
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192 | } |
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193 | |
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194 | Create a pool of server processes all accepting on the same socket. |
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195 | # create listener socket |
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196 | my $listener = ...; |
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197 | |
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198 | # create a pool template, initialise it and give it the socket |
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199 | my $pool = AnyEvent::Fork |
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200 | ->new |
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201 | ->require ("Some::Stuff", "My::Server") |
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202 | ->send_fh ($listener); |
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203 | |
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204 | # now create 10 identical workers |
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205 | for my $id (1..10) { |
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206 | $pool |
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207 | ->fork |
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208 | ->send_arg ($id) |
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209 | ->run ("My::Server::run"); |
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210 | } |
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211 | |
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212 | # now do other things - maybe use the filehandle provided by run |
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213 | # to wait for the processes to die. or whatever. |
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214 | |
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215 | "My::Server" might look like this: |
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216 | |
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217 | package My::Server; |
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218 | |
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219 | sub run { |
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220 | my ($slave, $listener, $id) = @_; |
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221 | |
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222 | close $slave; # we do not use the socket, so close it to save resources |
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223 | |
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224 | # we could go ballistic and use e.g. AnyEvent here, or IO::AIO, |
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225 | # or anything we usually couldn't do in a process forked normally. |
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226 | while (my $socket = $listener->accept) { |
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227 | # do sth. with new socket |
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228 | } |
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229 | } |
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230 | |
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231 | use AnyEvent::Fork as a faster fork+exec |
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232 | This runs "/bin/echo hi", with standard output redirected to /tmp/log |
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233 | and standard error redirected to the communications socket. It is |
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234 | usually faster than fork+exec, but still lets you prepare the |
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235 | environment. |
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236 | |
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237 | open my $output, ">/tmp/log" or die "$!"; |
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238 | |
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239 | AnyEvent::Fork |
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240 | ->new |
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241 | ->eval (' |
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242 | # compile a helper function for later use |
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243 | sub run { |
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244 | my ($fh, $output, @cmd) = @_; |
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245 | |
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246 | # perl will clear close-on-exec on STDOUT/STDERR |
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247 | open STDOUT, ">&", $output or die; |
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248 | open STDERR, ">&", $fh or die; |
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249 | |
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250 | exec @cmd; |
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251 | } |
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252 | ') |
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253 | ->send_fh ($output) |
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254 | ->send_arg ("/bin/echo", "hi") |
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255 | ->run ("run", my $cv = AE::cv); |
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256 | |
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257 | my $stderr = $cv->recv; |
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258 | |
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259 | For stingy users: put the worker code into a "DATA" section. |
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260 | When you want to be stingy with files, you can put your code into the |
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261 | "DATA" section of your module (or program): |
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262 | |
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263 | use AnyEvent::Fork; |
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264 | |
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265 | AnyEvent::Fork |
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266 | ->new |
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267 | ->eval (do { local $/; <DATA> }) |
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268 | ->run ("doit", sub { ... }); |
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269 | |
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270 | __DATA__ |
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271 | |
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272 | sub doit { |
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273 | ... do something! |
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274 | } |
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275 | |
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276 | For stingy standalone programs: do not rely on external files at |
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277 | all. |
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278 | For single-file scripts it can be inconvenient to rely on external files |
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279 | - even when using a "DATA" section, you still need to "exec" an external |
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280 | perl interpreter, which might not be available when using |
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281 | App::Staticperl, Urlader or PAR::Packer for example. |
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282 | |
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283 | Two modules help here - AnyEvent::Fork::Early forks a template process |
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284 | for all further calls to "new_exec", and AnyEvent::Fork::Template forks |
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285 | the main program as a template process. |
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286 | |
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287 | Here is how your main program should look like: |
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288 | |
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289 | #! perl |
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290 | |
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291 | # optional, as the very first thing. |
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292 | # in case modules want to create their own processes. |
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293 | use AnyEvent::Fork::Early; |
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294 | |
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295 | # next, load all modules you need in your template process |
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296 | use Example::My::Module |
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297 | use Example::Whatever; |
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298 | |
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299 | # next, put your run function definition and anything else you |
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300 | # need, but do not use code outside of BEGIN blocks. |
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301 | sub worker_run { |
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302 | my ($fh, @args) = @_; |
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303 | ... |
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304 | } |
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305 | |
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306 | # now preserve everything so far as AnyEvent::Fork object |
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307 | # in $TEMPLATE. |
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308 | use AnyEvent::Fork::Template; |
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309 | |
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310 | # do not put code outside of BEGIN blocks until here |
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311 | |
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312 | # now use the $TEMPLATE process in any way you like |
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313 | |
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314 | # for example: create 10 worker processes |
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315 | my @worker; |
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316 | my $cv = AE::cv; |
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317 | for (1..10) { |
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318 | $cv->begin; |
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319 | $TEMPLATE->fork->send_arg ($_)->run ("worker_run", sub { |
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320 | push @worker, shift; |
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321 | $cv->end; |
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322 | }); |
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323 | } |
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324 | $cv->recv; |
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325 | |
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326 | CONCEPTS |
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327 | This module can create new processes either by executing a new perl |
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328 | process, or by forking from an existing "template" process. |
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329 | |
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330 | All these processes are called "child processes" (whether they are |
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331 | direct children or not), while the process that manages them is called |
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332 | the "parent process". |
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333 | |
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334 | Each such process comes with its own file handle that can be used to |
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335 | communicate with it (it's actually a socket - one end in the new |
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336 | process, one end in the main process), and among the things you can do |
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337 | in it are load modules, fork new processes, send file handles to it, and |
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338 | execute functions. |
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339 | |
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340 | There are multiple ways to create additional processes to execute some |
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341 | jobs: |
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342 | |
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343 | fork a new process from the "default" template process, load code, run |
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344 | it |
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345 | This module has a "default" template process which it executes when |
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346 | it is needed the first time. Forking from this process shares the |
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347 | memory used for the perl interpreter with the new process, but |
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348 | loading modules takes time, and the memory is not shared with |
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349 | anything else. |
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350 | |
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351 | This is ideal for when you only need one extra process of a kind, |
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352 | with the option of starting and stopping it on demand. |
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353 | |
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354 | Example: |
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355 | |
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356 | AnyEvent::Fork |
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357 | ->new |
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358 | ->require ("Some::Module") |
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359 | ->run ("Some::Module::run", sub { |
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360 | my ($fork_fh) = @_; |
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361 | }); |
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362 | |
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363 | fork a new template process, load code, then fork processes off of it |
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364 | and run the code |
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365 | When you need to have a bunch of processes that all execute the same |
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366 | (or very similar) tasks, then a good way is to create a new template |
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367 | process for them, loading all the modules you need, and then create |
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368 | your worker processes from this new template process. |
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369 | |
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370 | This way, all code (and data structures) that can be shared (e.g. |
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371 | the modules you loaded) is shared between the processes, and each |
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372 | new process consumes relatively little memory of its own. |
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373 | |
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374 | The disadvantage of this approach is that you need to create a |
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375 | template process for the sole purpose of forking new processes from |
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376 | it, but if you only need a fixed number of processes you can create |
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377 | them, and then destroy the template process. |
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378 | |
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379 | Example: |
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380 | |
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381 | my $template = AnyEvent::Fork->new->require ("Some::Module"); |
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382 | |
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383 | for (1..10) { |
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384 | $template->fork->run ("Some::Module::run", sub { |
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385 | my ($fork_fh) = @_; |
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386 | }); |
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387 | } |
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388 | |
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389 | # at this point, you can keep $template around to fork new processes |
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390 | # later, or you can destroy it, which causes it to vanish. |
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391 | |
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392 | execute a new perl interpreter, load some code, run it |
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393 | This is relatively slow, and doesn't allow you to share memory |
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394 | between multiple processes. |
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395 | |
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396 | The only advantage is that you don't have to have a template process |
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397 | hanging around all the time to fork off some new processes, which |
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398 | might be an advantage when there are long time spans where no extra |
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399 | processes are needed. |
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400 | |
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401 | Example: |
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402 | |
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403 | AnyEvent::Fork |
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404 | ->new_exec |
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405 | ->require ("Some::Module") |
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406 | ->run ("Some::Module::run", sub { |
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407 | my ($fork_fh) = @_; |
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408 | }); |
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409 | |
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410 | THE "AnyEvent::Fork" CLASS |
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411 | This module exports nothing, and only implements a single class - |
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412 | "AnyEvent::Fork". |
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413 | |
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414 | There are two class constructors that both create new processes - "new" |
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415 | and "new_exec". The "fork" method creates a new process by forking an |
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416 | existing one and could be considered a third constructor. |
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417 | |
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418 | Most of the remaining methods deal with preparing the new process, by |
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419 | loading code, evaluating code and sending data to the new process. They |
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420 | usually return the process object, so you can chain method calls. |
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421 | |
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422 | If a process object is destroyed before calling its "run" method, then |
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423 | the process simply exits. After "run" is called, all responsibility is |
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424 | passed to the specified function. |
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425 | |
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426 | As long as there is any outstanding work to be done, process objects |
|
|
427 | resist being destroyed, so there is no reason to store them unless you |
|
|
428 | need them later - configure and forget works just fine. |
|
|
429 | |
|
|
430 | my $proc = new AnyEvent::Fork |
|
|
431 | Create a new "empty" perl interpreter process and returns its |
|
|
432 | process object for further manipulation. |
|
|
433 | |
|
|
434 | The new process is forked from a template process that is kept |
|
|
435 | around for this purpose. When it doesn't exist yet, it is created by |
|
|
436 | a call to "new_exec" first and then stays around for future calls. |
|
|
437 | |
|
|
438 | $new_proc = $proc->fork |
|
|
439 | Forks $proc, creating a new process, and returns the process object |
|
|
440 | of the new process. |
|
|
441 | |
|
|
442 | If any of the "send_" functions have been called before fork, then |
|
|
443 | they will be cloned in the child. For example, in a pre-forked |
|
|
444 | server, you might "send_fh" the listening socket into the template |
|
|
445 | process, and then keep calling "fork" and "run". |
|
|
446 | |
|
|
447 | my $proc = new_exec AnyEvent::Fork |
|
|
448 | Create a new "empty" perl interpreter process and returns its |
|
|
449 | process object for further manipulation. |
|
|
450 | |
|
|
451 | Unlike the "new" method, this method *always* spawns a new perl |
|
|
452 | process (except in some cases, see AnyEvent::Fork::Early for |
|
|
453 | details). This reduces the amount of memory sharing that is |
|
|
454 | possible, and is also slower. |
|
|
455 | |
|
|
456 | You should use "new" whenever possible, except when having a |
|
|
457 | template process around is unacceptable. |
|
|
458 | |
|
|
459 | The path to the perl interpreter is divined using various methods - |
|
|
460 | first $^X is investigated to see if the path ends with something |
|
|
461 | that looks as if it were the perl interpreter. Failing this, the |
|
|
462 | module falls back to using $Config::Config{perlpath}. |
|
|
463 | |
|
|
464 | The path to perl can also be overridden by setting the global |
|
|
465 | variable $AnyEvent::Fork::PERL - it's value will be used for all |
|
|
466 | subsequent invocations. |
|
|
467 | |
|
|
468 | $pid = $proc->pid |
|
|
469 | Returns the process id of the process *iff it is a direct child of |
|
|
470 | the process running AnyEvent::Fork*, and "undef" otherwise. As a |
|
|
471 | general rule (that you cannot rely upon), processes created via |
|
|
472 | "new_exec", AnyEvent::Fork::Early or AnyEvent::Fork::Template are |
|
|
473 | direct children, while all other processes are not. |
|
|
474 | |
|
|
475 | Or in other words, you do not normally have to take care of zombies |
|
|
476 | for processes created via "new", but when in doubt, or zombies are a |
|
|
477 | problem, you need to check whether a process is a diretc child by |
|
|
478 | calling this method, and possibly creating a child watcher or reap |
|
|
479 | it manually. |
|
|
480 | |
|
|
481 | $proc = $proc->eval ($perlcode, @args) |
|
|
482 | Evaluates the given $perlcode as ... Perl code, while setting @_ to |
|
|
483 | the strings specified by @args, in the "main" package (so you can |
|
|
484 | access the args using $_[0] and so on, but not using implicit "shit" |
|
|
485 | as the latter works on @ARGV). |
|
|
486 | |
|
|
487 | This call is meant to do any custom initialisation that might be |
|
|
488 | required (for example, the "require" method uses it). It's not |
|
|
489 | supposed to be used to completely take over the process, use "run" |
|
|
490 | for that. |
|
|
491 | |
|
|
492 | The code will usually be executed after this call returns, and there |
|
|
493 | is no way to pass anything back to the calling process. Any |
|
|
494 | evaluation errors will be reported to stderr and cause the process |
|
|
495 | to exit. |
|
|
496 | |
|
|
497 | If you want to execute some code (that isn't in a module) to take |
|
|
498 | over the process, you should compile a function via "eval" first, |
|
|
499 | and then call it via "run". This also gives you access to any |
|
|
500 | arguments passed via the "send_xxx" methods, such as file handles. |
|
|
501 | See the "use AnyEvent::Fork as a faster fork+exec" example to see it |
|
|
502 | in action. |
|
|
503 | |
|
|
504 | Returns the process object for easy chaining of method calls. |
|
|
505 | |
|
|
506 | It's common to want to call an iniitalisation function with some |
|
|
507 | arguments. Make sure you actually pass @_ to that function (for |
|
|
508 | example by using &name syntax), and do not just specify a function |
|
|
509 | name: |
|
|
510 | |
|
|
511 | $proc->eval ('&MyModule::init', $string1, $string2); |
|
|
512 | |
|
|
513 | $proc = $proc->require ($module, ...) |
|
|
514 | Tries to load the given module(s) into the process |
|
|
515 | |
|
|
516 | Returns the process object for easy chaining of method calls. |
|
|
517 | |
|
|
518 | $proc = $proc->send_fh ($handle, ...) |
|
|
519 | Send one or more file handles (*not* file descriptors) to the |
|
|
520 | process, to prepare a call to "run". |
|
|
521 | |
|
|
522 | The process object keeps a reference to the handles until they have |
|
|
523 | been passed over to the process, so you must not explicitly close |
|
|
524 | the handles. This is most easily accomplished by simply not storing |
|
|
525 | the file handles anywhere after passing them to this method - when |
|
|
526 | AnyEvent::Fork is finished using them, perl will automatically close |
|
|
527 | them. |
|
|
528 | |
|
|
529 | Returns the process object for easy chaining of method calls. |
|
|
530 | |
|
|
531 | Example: pass a file handle to a process, and release it without |
|
|
532 | closing. It will be closed automatically when it is no longer used. |
|
|
533 | |
|
|
534 | $proc->send_fh ($my_fh); |
|
|
535 | undef $my_fh; # free the reference if you want, but DO NOT CLOSE IT |
|
|
536 | |
|
|
537 | $proc = $proc->send_arg ($string, ...) |
|
|
538 | Send one or more argument strings to the process, to prepare a call |
|
|
539 | to "run". The strings can be any octet strings. |
|
|
540 | |
|
|
541 | The protocol is optimised to pass a moderate number of relatively |
|
|
542 | short strings - while you can pass up to 4GB of data in one go, this |
|
|
543 | is more meant to pass some ID information or other startup info, not |
|
|
544 | big chunks of data. |
|
|
545 | |
|
|
546 | Returns the process object for easy chaining of method calls. |
|
|
547 | |
|
|
548 | $proc->run ($func, $cb->($fh)) |
|
|
549 | Enter the function specified by the function name in $func in the |
|
|
550 | process. The function is called with the communication socket as |
|
|
551 | first argument, followed by all file handles and string arguments |
|
|
552 | sent earlier via "send_fh" and "send_arg" methods, in the order they |
|
|
553 | were called. |
|
|
554 | |
|
|
555 | The process object becomes unusable on return from this function - |
|
|
556 | any further method calls result in undefined behaviour. |
|
|
557 | |
|
|
558 | The function name should be fully qualified, but if it isn't, it |
|
|
559 | will be looked up in the "main" package. |
|
|
560 | |
|
|
561 | If the called function returns, doesn't exist, or any error occurs, |
|
|
562 | the process exits. |
|
|
563 | |
|
|
564 | Preparing the process is done in the background - when all commands |
|
|
565 | have been sent, the callback is invoked with the local |
|
|
566 | communications socket as argument. At this point you can start using |
|
|
567 | the socket in any way you like. |
|
|
568 | |
|
|
569 | If the communication socket isn't used, it should be closed on both |
|
|
570 | sides, to save on kernel memory. |
|
|
571 | |
|
|
572 | The socket is non-blocking in the parent, and blocking in the newly |
|
|
573 | created process. The close-on-exec flag is set in both. |
|
|
574 | |
|
|
575 | Even if not used otherwise, the socket can be a good indicator for |
|
|
576 | the existence of the process - if the other process exits, you get a |
|
|
577 | readable event on it, because exiting the process closes the socket |
|
|
578 | (if it didn't create any children using fork). |
|
|
579 | |
|
|
580 | Compatibility to AnyEvent::Fork::Remote |
|
|
581 | If you want to write code that works with both this module and |
|
|
582 | AnyEvent::Fork::Remote, you need to write your code so that it |
|
|
583 | assumes there are two file handles for communications, which |
|
|
584 | might not be unix domain sockets. The "run" function should |
|
|
585 | start like this: |
|
|
586 | |
|
|
587 | sub run { |
|
|
588 | my ($rfh, @args) = @_; # @args is your normal arguments |
|
|
589 | my $wfh = fileno $rfh ? $rfh : *STDOUT; |
|
|
590 | |
|
|
591 | # now use $rfh for reading and $wfh for writing |
|
|
592 | } |
|
|
593 | |
|
|
594 | This checks whether the passed file handle is, in fact, the |
|
|
595 | process "STDIN" handle. If it is, then the function was invoked |
|
|
596 | visa AnyEvent::Fork::Remote, so STDIN should be used for reading |
|
|
597 | and "STDOUT" should be used for writing. |
|
|
598 | |
|
|
599 | In all other cases, the function was called via this module, and |
|
|
600 | there is only one file handle that should be sued for reading |
|
|
601 | and writing. |
|
|
602 | |
|
|
603 | Example: create a template for a process pool, pass a few strings, |
|
|
604 | some file handles, then fork, pass one more string, and run some |
|
|
605 | code. |
|
|
606 | |
|
|
607 | my $pool = AnyEvent::Fork |
|
|
608 | ->new |
|
|
609 | ->send_arg ("str1", "str2") |
|
|
610 | ->send_fh ($fh1, $fh2); |
|
|
611 | |
|
|
612 | for (1..2) { |
|
|
613 | $pool |
|
|
614 | ->fork |
|
|
615 | ->send_arg ("str3") |
|
|
616 | ->run ("Some::function", sub { |
|
|
617 | my ($fh) = @_; |
|
|
618 | |
|
|
619 | # fh is nonblocking, but we trust that the OS can accept these |
|
|
620 | # few octets anyway. |
|
|
621 | syswrite $fh, "hi #$_\n"; |
|
|
622 | |
|
|
623 | # $fh is being closed here, as we don't store it anywhere |
|
|
624 | }); |
|
|
625 | } |
|
|
626 | |
|
|
627 | # Some::function might look like this - all parameters passed before fork |
|
|
628 | # and after will be passed, in order, after the communications socket. |
|
|
629 | sub Some::function { |
|
|
630 | my ($fh, $str1, $str2, $fh1, $fh2, $str3) = @_; |
|
|
631 | |
|
|
632 | print scalar <$fh>; # prints "hi #1\n" and "hi #2\n" in any order |
|
|
633 | } |
|
|
634 | |
|
|
635 | CHILD PROCESS INTERFACE |
|
|
636 | This module has a limited API for use in child processes. |
|
|
637 | |
|
|
638 | @args = AnyEvent::Fork::Serve::run_args |
|
|
639 | This function, which only exists before the "run" method is called, |
|
|
640 | returns the arguments that would be passed to the run function, and |
|
|
641 | clears them. |
|
|
642 | |
|
|
643 | This is mainly useful to get any file handles passed via "send_fh", |
|
|
644 | but works for any arguments passed via "send_*xxx*" methods. |
|
|
645 | |
|
|
646 | EXPERIMENTAL METHODS |
|
|
647 | These methods might go away completely or change behaviour, at any time. |
|
|
648 | |
|
|
649 | $proc->to_fh ($cb->($fh)) # EXPERIMENTAL, MIGHT BE REMOVED |
|
|
650 | Flushes all commands out to the process and then calls the callback |
|
|
651 | with the communications socket. |
|
|
652 | |
|
|
653 | The process object becomes unusable on return from this function - |
|
|
654 | any further method calls result in undefined behaviour. |
|
|
655 | |
|
|
656 | The point of this method is to give you a file handle that you can |
|
|
657 | pass to another process. In that other process, you can call |
|
|
658 | "new_from_fh AnyEvent::Fork $fh" to create a new "AnyEvent::Fork" |
|
|
659 | object from it, thereby effectively passing a fork object to another |
|
|
660 | process. |
|
|
661 | |
|
|
662 | new_from_fh AnyEvent::Fork $fh # EXPERIMENTAL, MIGHT BE REMOVED |
|
|
663 | Takes a file handle originally rceeived by the "to_fh" method and |
|
|
664 | creates a new "AnyEvent:Fork" object. The child process itself will |
|
|
665 | not change in any way, i.e. it will keep all the modifications done |
|
|
666 | to it before calling "to_fh". |
|
|
667 | |
|
|
668 | The new object is very much like the original object, except that |
|
|
669 | the "pid" method will return "undef" even if the process is a direct |
|
|
670 | child. |
|
|
671 | |
|
|
672 | PERFORMANCE |
|
|
673 | Now for some unscientific benchmark numbers (all done on an amd64 |
|
|
674 | GNU/Linux box). These are intended to give you an idea of the relative |
|
|
675 | performance you can expect, they are not meant to be absolute |
|
|
676 | performance numbers. |
|
|
677 | |
|
|
678 | OK, so, I ran a simple benchmark that creates a socket pair, forks, |
|
|
679 | calls exit in the child and waits for the socket to close in the parent. |
|
|
680 | I did load AnyEvent, EV and AnyEvent::Fork, for a total process size of |
|
|
681 | 5100kB. |
|
|
682 | |
|
|
683 | 2079 new processes per second, using manual socketpair + fork |
|
|
684 | |
|
|
685 | Then I did the same thing, but instead of calling fork, I called |
|
|
686 | AnyEvent::Fork->new->run ("CORE::exit") and then again waited for the |
|
|
687 | socket from the child to close on exit. This does the same thing as |
|
|
688 | manual socket pair + fork, except that what is forked is the template |
|
|
689 | process (2440kB), and the socket needs to be passed to the server at the |
|
|
690 | other end of the socket first. |
|
|
691 | |
|
|
692 | 2307 new processes per second, using AnyEvent::Fork->new |
|
|
693 | |
|
|
694 | And finally, using "new_exec" instead "new", using vforks+execs to exec |
|
|
695 | a new perl interpreter and compile the small server each time, I get: |
|
|
696 | |
|
|
697 | 479 vfork+execs per second, using AnyEvent::Fork->new_exec |
|
|
698 | |
|
|
699 | So how can "AnyEvent->new" be faster than a standard fork, even though |
|
|
700 | it uses the same operations, but adds a lot of overhead? |
|
|
701 | |
|
|
702 | The difference is simply the process size: forking the 5MB process takes |
|
|
703 | so much longer than forking the 2.5MB template process that the extra |
|
|
704 | overhead is canceled out. |
|
|
705 | |
|
|
706 | If the benchmark process grows, the normal fork becomes even slower: |
|
|
707 | |
|
|
708 | 1340 new processes, manual fork of a 20MB process |
|
|
709 | 731 new processes, manual fork of a 200MB process |
|
|
710 | 235 new processes, manual fork of a 2000MB process |
|
|
711 | |
|
|
712 | What that means (to me) is that I can use this module without having a |
|
|
713 | bad conscience because of the extra overhead required to start new |
|
|
714 | processes. |
|
|
715 | |
|
|
716 | TYPICAL PROBLEMS |
|
|
717 | This section lists typical problems that remain. I hope by recognising |
|
|
718 | them, most can be avoided. |
|
|
719 | |
|
|
720 | leaked file descriptors for exec'ed processes |
|
|
721 | POSIX systems inherit file descriptors by default when exec'ing a |
|
|
722 | new process. While perl itself laudably sets the close-on-exec flags |
|
|
723 | on new file handles, most C libraries don't care, and even if all |
|
|
724 | cared, it's often not possible to set the flag in a race-free |
|
|
725 | manner. |
|
|
726 | |
|
|
727 | That means some file descriptors can leak through. And since it |
|
|
728 | isn't possible to know which file descriptors are "good" and |
|
|
729 | "necessary" (or even to know which file descriptors are open), there |
|
|
730 | is no good way to close the ones that might harm. |
|
|
731 | |
|
|
732 | As an example of what "harm" can be done consider a web server that |
|
|
733 | accepts connections and afterwards some module uses AnyEvent::Fork |
|
|
734 | for the first time, causing it to fork and exec a new process, which |
|
|
735 | might inherit the network socket. When the server closes the socket, |
|
|
736 | it is still open in the child (which doesn't even know that) and the |
|
|
737 | client might conclude that the connection is still fine. |
|
|
738 | |
|
|
739 | For the main program, there are multiple remedies available - |
|
|
740 | AnyEvent::Fork::Early is one, creating a process early and not using |
|
|
741 | "new_exec" is another, as in both cases, the first process can be |
|
|
742 | exec'ed well before many random file descriptors are open. |
|
|
743 | |
|
|
744 | In general, the solution for these kind of problems is to fix the |
|
|
745 | libraries or the code that leaks those file descriptors. |
|
|
746 | |
|
|
747 | Fortunately, most of these leaked descriptors do no harm, other than |
|
|
748 | sitting on some resources. |
|
|
749 | |
|
|
750 | leaked file descriptors for fork'ed processes |
|
|
751 | Normally, AnyEvent::Fork does start new processes by exec'ing them, |
|
|
752 | which closes file descriptors not marked for being inherited. |
|
|
753 | |
|
|
754 | However, AnyEvent::Fork::Early and AnyEvent::Fork::Template offer a |
|
|
755 | way to create these processes by forking, and this leaks more file |
|
|
756 | descriptors than exec'ing them, as there is no way to mark |
|
|
757 | descriptors as "close on fork". |
|
|
758 | |
|
|
759 | An example would be modules like EV, IO::AIO or Gtk2. Both create |
|
|
760 | pipes for internal uses, and Gtk2 might open a connection to the X |
|
|
761 | server. EV and IO::AIO can deal with fork, but Gtk2 might have |
|
|
762 | trouble with a fork. |
|
|
763 | |
|
|
764 | The solution is to either not load these modules before use'ing |
|
|
765 | AnyEvent::Fork::Early or AnyEvent::Fork::Template, or to delay |
|
|
766 | initialising them, for example, by calling "init Gtk2" manually. |
|
|
767 | |
|
|
768 | exiting calls object destructors |
|
|
769 | This only applies to users of AnyEvent::Fork:Early and |
|
|
770 | AnyEvent::Fork::Template, or when initialising code creates objects |
|
|
771 | that reference external resources. |
|
|
772 | |
|
|
773 | When a process created by AnyEvent::Fork exits, it might do so by |
|
|
774 | calling exit, or simply letting perl reach the end of the program. |
|
|
775 | At which point Perl runs all destructors. |
|
|
776 | |
|
|
777 | Not all destructors are fork-safe - for example, an object that |
|
|
778 | represents the connection to an X display might tell the X server to |
|
|
779 | free resources, which is inconvenient when the "real" object in the |
|
|
780 | parent still needs to use them. |
|
|
781 | |
|
|
782 | This is obviously not a problem for AnyEvent::Fork::Early, as you |
|
|
783 | used it as the very first thing, right? |
|
|
784 | |
|
|
785 | It is a problem for AnyEvent::Fork::Template though - and the |
|
|
786 | solution is to not create objects with nontrivial destructors that |
|
|
787 | might have an effect outside of Perl. |
|
|
788 | |
|
|
789 | PORTABILITY NOTES |
|
|
790 | Native win32 perls are somewhat supported (AnyEvent::Fork::Early is a |
|
|
791 | nop, and ::Template is not going to work), and it cost a lot of blood |
|
|
792 | and sweat to make it so, mostly due to the bloody broken perl that |
|
|
793 | nobody seems to care about. The fork emulation is a bad joke - I have |
|
|
794 | yet to see something useful that you can do with it without running into |
|
|
795 | memory corruption issues or other braindamage. Hrrrr. |
|
|
796 | |
|
|
797 | Since fork is endlessly broken on win32 perls (it doesn't even remotely |
|
|
798 | work within it's documented limits) and quite obviously it's not getting |
|
|
799 | improved any time soon, the best way to proceed on windows would be to |
|
|
800 | always use "new_exec" and thus never rely on perl's fork "emulation". |
|
|
801 | |
|
|
802 | Cygwin perl is not supported at the moment due to some hilarious |
|
|
803 | shortcomings of its API - see IO::FDPoll for more details. If you never |
|
|
804 | use "send_fh" and always use "new_exec" to create processes, it should |
|
|
805 | work though. |
|
|
806 | |
|
|
807 | USING AnyEvent::Fork IN SUBPROCESSES |
|
|
808 | AnyEvent::Fork itself cannot generally be used in subprocesses. As long |
|
|
809 | as only one process ever forks new processes, sharing the template |
|
|
810 | processes is possible (you could use a pipe as a lock by writing a byte |
|
|
811 | into it to unlock, and reading the byte to lock for example) |
|
|
812 | |
|
|
813 | To make concurrent calls possible after fork, you should get rid of the |
|
|
814 | template and early fork processes. AnyEvent::Fork will create a new |
|
|
815 | template process as needed. |
|
|
816 | |
|
|
817 | undef $AnyEvent::Fork::EARLY; |
|
|
818 | undef $AnyEvent::Fork::TEMPLATE; |
|
|
819 | |
|
|
820 | It doesn't matter whether you get rid of them in the parent or child |
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821 | after a fork. |
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822 | |
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823 | SEE ALSO |
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824 | AnyEvent::Fork::Early, to avoid executing a perl interpreter at all |
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825 | (part of this distribution). |
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826 | |
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827 | AnyEvent::Fork::Template, to create a process by forking the main |
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828 | program at a convenient time (part of this distribution). |
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829 | |
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830 | AnyEvent::Fork::Remote, for another way to create processes that is |
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831 | mostly compatible to this module and modules building on top of it, but |
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832 | works better with remote processes. |
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833 | |
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834 | AnyEvent::Fork::RPC, for simple RPC to child processes (on CPAN). |
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835 | |
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836 | AnyEvent::Fork::Pool, for simple worker process pool (on CPAN). |
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837 | |
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838 | AUTHOR AND CONTACT INFORMATION |
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839 | Marc Lehmann <schmorp@schmorp.de> |
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840 | http://software.schmorp.de/pkg/AnyEvent-Fork |
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841 | |