Re: Thread-safe & vars scope clarification

If you want to try it yourself and report back, I'm sure we'd all be
interested in what you find out. The main thing I'm aware of is that
copy-on-write works very well for preforking and threads usually can't
match it, but maybe your application is different.

Regarding your thread-safety questions, chdir() is global to the process,
so not thread-safe. I think the issue with readdir() is that open directory
handles don't get copied when threads are spawned, but maybe there's more
to it. The special Perl vars are not a problem with threads, since each
thread is a separate interpreter.

You can read about the limitations of threads here:
http://perldoc.perl.org/threads.html#BUGS-AND-LIMITATIONS

- Perrin

On Mon, Apr 4, 2016 at 5:03 PM, Ben RUBSON <[email protected]> wrote:

> Hi Perrin,
>
> Thank you for your answer.
>
> Well, for the moment I only use the prefork (non-threaded) MPM.
> But I would have liked to test performance of the threaded MPM, so I would
> have liked my code to be thread-ready.
>
> According to this :
>
> https://perl.apache.org/docs/2.0/user/performance/mpm.html#toc_Memory_Requirements_in_Threaded_MPM
> Sounds like threads could have an interest, but according to your message,
> should not be a general case.
>
> Thank you,
>
> Ben
>
> Le 4 avr. 2016 à 22:49, Perrin Harkins <[email protected]> a écrit :
>
> Hi Ben.
>
> Before you get too far into the details of using threads, can I ask why
> you're considering it? The memory footprint and performance of using forked
> processes with Perl is generally going to be better than that of threads,
> due to copy-on-write.
>
> - Perrin
>
> On Mon, Apr 4, 2016 at 4:44 PM, Ben RUBSON <[email protected]> wrote:
>
>> Hello,
>>
>> I have some questions regarding mod_perl, threads, thread-safe functions,
>> special vars scope...
>>
>> From
>> https://perl.apache.org/docs/2.0/user/coding/coding.html#toc_Thread_environment_Issues
>> :
>> "if you chdir() in one thread, all other thread now see the current
>> working directory of that thread that chdir()'ed to that directory"
>> Do we speak about threads from threaded MPM ?
>> (so the specific Apache process which owns the thread running this Perl
>> script, and all its (the process) threads, will chdir()).
>>
>> From the same page :
>> "Other usually problematic (non thread-safe) functions include readdir(),
>> srand(), etc."
>> Why readdir() would not be thread-safe ? I can't see why. Perhaps
>> readdir() chdir() to read a directory content ?
>> How could we then read dir content being thread-safe ?
>>
>> From
>> https://perl.apache.org/docs/general/perl_reference/perl_reference.html#toc_The_Scope_of_the_Special_Perl_Variables
>> :
>> "In the mod_perl situation where the child server doesn't usually exit,
>> if in one of your scripts you modify a global variable it will be changed
>> for the rest of the process' life and will affect all the scripts executed
>> by the same process."
>> So in a threaded MPM, if one of the threads modifies one of the special
>> Perl variables, it will instantly affect all other threads of this process ?
>>
>> What about variables like $? ? (return code of system() calls)
>> And ${^CHILD_ERROR_NATIVE} ? (return code too)
>> Does their value stay local to the thread ? / Does each thread have these
>> variables with their own proper value ?
>> Or is it unsafe to read them because they could be modified by another
>> thread ?
>>
>> Thank you very much for your clarification,
>>
>> Best regards,
>>
>> Ben
>>
>>
>
>