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Re: AIX and Interix also do early PID recycling.

From: Michael Haubenwallner
Subject: Re: AIX and Interix also do early PID recycling.
Date: Wed, 25 Jul 2012 16:52:54 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:10.0.3) Gecko/20120327 Thunderbird/10.0.3

On 07/25/2012 03:20 PM, Michael Haubenwallner wrote:
> On 07/25/2012 09:59 AM, Michael Haubenwallner wrote:
>> On 07/25/2012 03:05 AM, Chet Ramey wrote:
>>> Bash holds on to the status of all terminated processes, not just
>>> background ones, and only checks for the presence of a newly-forked PID
>>> in that list if the list size exceeds CHILD_MAX.
>> The AIX 6.1 I've debugged on has:
>>   #define CHILD_MAX 128
>>   #define _POSIX_CHILD_MAX 25
>>   sysconf(_SC_CHILD_MAX) = 1024

> Tracing shows:
> The minimum fork count (including grand-childs to any depth) before PID 
> recycling starts
> looks like 255 (once), but usually 256 and more.
> However, one process does see a PID recycled after *at least* 128 forks,
> that is exactly the value of CHILD_MAX.

Got it: The value used for js.c_childmax isn't 128, but 1024.

In lib/sh/oslib.c, getmaxchild() prefers sysconf(_SC_CHILD_MAX) over CHILD_MAX 

But sysconf(_SC_CHILD_MAX) does return the number of "processes per real user 
id" (similar to
ulimit -u), rather than the number of CHILD_MAX (whenever defined).

For Interix, things are different though:
There is no CHILD_MAX nor MAXUPRC defined, and sysconf(_SC_CHILD_MAX) does 
return 512,
but PIDs start to be recycled at ~120 already...

Any idea about the "correct" fix for getmaxchild() across platforms?


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