mod_perl children process

LorCo has asked for the wisdom of the Perl Monks concerning the following question:

CentOS 6.9, mod_perl 2.08, Apache 2.2, my script:

#!/usr/bin/perl

print "Content-type: text/html; charset=UTF-8\n\n" ;
 
++$i ;
print "i=$i, $ENV{MOD_PERL}, pid is $$<br>\n" ;

exit ;
[download]

When I run it repeatedly(F5) using browser( http://www.xx...x.com/cgi-bin/test.cgi )

I got:

i=1, mod_perl/2.0.8, pid is 17578
i=2, mod_perl/2.0.8, pid is 17578
i=3, mod_perl/2.0.8, pid is 17578
i=1, mod_perl/2.0.8, pid is 467
i=2, mod_perl/2.0.8, pid is 467
i=4, mod_perl/2.0.8, pid is 17578
i=1, mod_perl/2.0.8, pid is 24789
i=2, mod_perl/2.0.8, pid is 24789
i=3, mod_perl/2.0.8, pid is 24789
i=5, mod_perl/2.0.8, pid is 17578
i=3, mod_perl/2.0.8, pid is 467
i=1, mod_perl/2.0.8, pid is 24378
i=2, mod_perl/2.0.8, pid is 24378
i=1, mod_perl/2.0.8, pid is 2742
i=2, mod_perl/2.0.8, pid is 2742

The script re-compiled every time for new pid ( i=1 ) .

Is it something wrong with server/mod_perl or with my knowledge in mod_perl?
Is it Apache recompile a script for every new child/process?

Please help understand this.

Thank you, Alex

Comment on mod_perl children process Download Code

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Re: mod_perl children process by LanX (Saint) on Oct 07, 2017 at 00:39 UTC
Well it's not recompiled every time, but probably your Apache is operating with multiple sub processes handling the requests. And each such process must have its own mod-Perl version of your cgi. I'd expect that the max number of such processes is a matter of configuration. Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply]
Re^2: mod_perl children process by LorCo (Initiate) on Oct 07, 2017 at 06:40 UTC
Thank you. Any experienced people can help? Why Apache recompile script every time?	[reply]
Re^3: mod_perl children process by LanX (Saint) on Oct 07, 2017 at 07:47 UTC
http://httpd.apache.org/docs/2.4/en/mod/prefork.html Cheers Rolf _{(addicted to the Perl Programming Language and ☆☆☆☆ :) Je suis Charlie!}	[reply]
Re^4: mod_perl children process by LorCo (Initiate) on Oct 07, 2017 at 16:48 UTC
Re^5: mod_perl children process by LanX (Saint) on Oct 07, 2017 at 19:08 UTC
Re^5: mod_perl children process by LanX (Saint) on Oct 08, 2017 at 03:02 UTC
Re^3: mod_perl children process by Anonymous Monk on Oct 07, 2017 at 07:37 UTC
Every child gets its own copy as designed . there is no problems to solve	[reply]
Re: mod_perl children process by talexb (Chancellor) on Oct 07, 2017 at 19:43 UTC
Running a script under mod_perl can be a confusing world to learn about. The point of mod_perl is that it's a Perl interpreter baked into the Apache web server. Instead of having to run the script (and therefore Perl) every time there's a web request, mod_perl loads the script in a ready-to-run form, so as to improve performance. Falling back to food analogies (because they're easy for me), it's a bit like a diner making up a batch of pancake batter and having the griddle hot, so that when a customer shows up, the batter can go right onto the hot grill, and pancakes are up five minutes later. The alternative would be to make individual batches of batter for each customer who orders pancakes. I think you can imagine that's not as efficient. Your experimental results show that Apache is dispatching a request to a variety of kids (diner kitchens), identified by pid. Each request is served (from that kitchen), showing a monotonically increasing count (pancake order count). You should be aware that there's a configuration parameter in the Apache config called something like MaxRequests. When a kid has maxed out its number of requests (that kitchen ran out of batter), it's terminated and a new kid is created (the kitchen makes up a new batch of batter). Are you just seeing how it all works, or is there some issue that remains a mystery to you? mod_perl has been around for quite a while, but these days I'd guess it's now classified as old technology. :) Alex / talexb / Toronto Thanks PJ. We owe you so much. Groklaw -- RIP -- 2003 to 2013.	[reply]
Re: mod_perl children process by CountZero (Bishop) on Oct 08, 2017 at 11:44 UTC
You have seen for yourself how confusing it is to use global variables in a mod_perl script. You never can predict which instance of that global variable will be used. It is a good practice to only use lexical "my" variables or if you must use global variables, explicitly initialize them to a known value. CountZero A program should be light and agile, its subroutines connected like a string of pearls. The spirit and intent of the program should be retained throughout. There should be neither too little or too much, neither needless loops nor useless variables, neither lack of structure nor overwhelming rigidity." - The Tao of Programming, 4.1 - Geoffrey James My blog: Imperial Deltronics	[reply]
Re^2: mod_perl children process by KurtZ (Friar) on Oct 08, 2017 at 13:34 UTC
Why is a my variable in file scope any better?	[reply]
Re^3: mod_perl children process by hippo (Bishop) on Oct 08, 2017 at 13:47 UTC
It's better because ModPerl::Registry wraps it in its own scope so the lexical variables are limited.	[reply]
Re: mod_perl children process by afoken (Chancellor) on Oct 07, 2017 at 13:23 UTC
Consider using FastCGI instead of mod_perl. Update See also Re^2: Reusing Compiled Perl or Opcode for Apache Server Alexander -- Today I will gladly share my knowledge and experience, for there are no sweeter words than "I told you so". ;-)	[reply]
Re^2: mod_perl children process by Anonymous Monk on Oct 07, 2017 at 19:12 UTC
wrong! :P!	[reply]
Re: mod_perl children process by sundialsvc4 (Abbot) on Oct 08, 2017 at 20:38 UTC
It is, in fact, working as designed. Let’s sort your list by PID to see if we can thereby discern what is actually happening: i=1, mod_perl/2.0.8, pid is 17578 i=2, mod_perl/2.0.8, pid is 17578 i=3, mod_perl/2.0.8, pid is 17578 i=4, mod_perl/2.0.8, pid is 17578 i=5, mod_perl/2.0.8, pid is 17578 i=1, mod_perl/2.0.8, pid is 467 i=2, mod_perl/2.0.8, pid is 467 i=3, mod_perl/2.0.8, pid is 467 i=1, mod_perl/2.0.8, pid is 24789 i=2, mod_perl/2.0.8, pid is 24789 i=3, mod_perl/2.0.8, pid is 24789 i=1, mod_perl/2.0.8, pid is 24378 i=2, mod_perl/2.0.8, pid is 24378 i=1, mod_perl/2.0.8, pid is 2742 i=2, mod_perl/2.0.8, pid is 2742 [download] We plainly see from this that Apache is running your process using five child-processes, and that the requests are being handled by a per-process Perl instance that has not disappeared (since `$i` is incrementing), so we know that `mod_perl` is doing its job. (Your program is not “being recompiled each time.”) We also observe that each process is an independent instance, each owning its own independent Perl interpreter context, each with its own distinct and unrelated notion of what `$i` is.	[reply] [d/l]
Re^2: mod_perl children process by hippo (Bishop) on Oct 09, 2017 at 08:02 UTC
`Let�s sort your list by PID` I cannot guess the sorting mechanism which results in (17578, 467, 24789, 24378, 2742) being a sorted sequence. Perhaps you could enlighten me?	[reply]
Re^3: mod_perl children process by RonW (Parson) on Oct 10, 2017 at 00:08 UTC
It's the order of the first appearance of each PID in the OP's output. So, more like de-interleaving than sorting.	[reply]
Re^2: mod_perl children process by LorCo (Initiate) on Oct 11, 2017 at 00:04 UTC
Why Apache start compile new process if few of them already exists? I have heavy scripts where perl load many libraries and I see Apache re-load and recompile whole code every few requests.	[reply]
Re^3: mod_perl children process by huck (Prior) on Oct 11, 2017 at 02:15 UTC
From Re: mpm-worker + mod_perl2 settings meaning If all running interpreters are in use, mod_perl will clone new interpreters to handle the request, up until this number of interpreters is reached. when PerlInterpMax is reached, mod_perl will block (via COND_WAIT()) until one becomes available (signaled via COND_SIGNAL()). Default value: 5	[reply]

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