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Re: AIX and Interix also do early PID recycling.

From: Chet Ramey
Subject: Re: AIX and Interix also do early PID recycling.
Date: Wed, 25 Jul 2012 13:06:42 -0400
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On 7/25/12 11:59 AM, Michael Haubenwallner wrote:
> On 07/25/2012 04:50 PM, Chet Ramey wrote:
>>> The AIX 6.1 I've debugged on has:
>>>   #define CHILD_MAX 128
>>>   #define _POSIX_CHILD_MAX 25
>>>   sysconf(_SC_CHILD_MAX) = 1024
>> Bash prefers sysconf(_SC_CHILD_MAX) and will use it over the other
>> defines (lib/sh/oslib.c:getmaxchild()).  I don't know why AIX chooses
>> to return a different value via sysconf than it defines for CHILD_MAX,
>> especially when it seems to use the CHILD_MAX value to decide when it
>> can recycle the PID space.
> Well, _SC_CHILD_MAX is documented across platforms as:
> (Linux)   "The max number of simultaneous processes per user ID."
> (HP-UX)   "Maximum number of simultaneous processes per user ID."
> (Solaris) "Max processes allowed to a UID"
> (AIX)     "Specifies the number of simultaneous processes per real user ID."
> (Interix) "Maximum number of simultaneous processes per user ID."
> Also, one Linux machine actually shows the _SC_CHILD_MAX value equal to
> kernel.pid_max (32768 here), so even Linux could see this problem in theory,
> because PIDs really are recycled before kernel.pid_max.

That's interesting, since Posix describes sysconf() as simply a way to
retrieve values from limits.h or unistd.h that one wishes to get at
run time rather than compile time.  And interesting that it establishes a
correspondence between CHILD_MAX and _SC_CHILD_MAX.

>> And I suspect that the single change of significance is to not check
>> against the childmax value when deciding whether or not to look for and
>> remove this pid from the list of saved termination status values.
> Agreed - but is this still different to defining RECYCLES_PIDS then?

It is not.  It is one of the things that happens when you define
RECYCLES_PIDS.  The question is whether or not that is the single thing
that makes a difference in this case.  If it is, there is merit in
removing the check against js.c_childmax entirely or making it dependent
on something else.


``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/

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