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Subject:	Re: Sv: Limit for number of child processes
To:	sten DOT kristian DOT ivarsson AT gmail DOT com, cygwin AT cygwin DOT com
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Date:	Thu, 27 Aug 2020 21:42:48 -0400
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From:	Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Ken Brown <kbrown AT cornell DOT edu>
Errors-To:	cygwin-bounces AT cygwin DOT com
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On 8/27/2020 8:17 AM, Kristian Ivarsson via Cygwin wrote:
> Hi Corinna
> 
>>> Dear cygwin folks
>>>
>>> It seems like there's a limit of the number of possible child
>>> processes defined to 256 with 'NPROCS' in //winsup/cygwin/child_info.h
>>> used in 'cprocs' in //winsup/cygwin/sigproc.cc
>>>
>>> 256 is quite few possible children in an enterprise environment and
>>> perhaps the limit should be limited by the physical resources or
>> possibly Windows ?
>>
>> The info has to be kept available in the process itself so we need this
>> array of NPROCS * sizeof (pinfo).
>>
>> Of course, there's no reason to use a static array, the code could just as
>> well use a dynamically allocated array or a linked list.  It's just not
>> the way it is right now and would need a patch or rewrite.
>>
>> As for the static array, sizeof pinfo is 64, so the current size of the
>> array is just 16K.  We could easily bump it to 64K with NPROCS raised to
>> 1024 for the next Cygwin release, at least on 64 bit.
>> I don't think we should raise this limit for 32 bit Cygwin, which is kind
>> of EOL anyway, given the massive restrictions.
> 
> I don't know the exact purpose of this and how the cprocs is used, but I'd
> prefer something totally dynamic 7 days out of 7 or otherwise another limit
> would just bite you in the ass some other day instead ;-)
> 
> A linked list could be used if you wanna optimize (dynamic) memory usage but
> an (amortized) array would probably provide faster linear search but I guess
> simplicity of the code and external functionality is the most important
> demands for this choice

Any change here (aside from just increasing NPROCS) would have to be done with 
care to avoid a performance hit.  I looked at the history of changes to 
sigproc.cc, and I found commit 4ce15a49 in 2001 in which a static array 
something like cprocs was replaced by a dynamically allocated buffer in order to 
save DLL space.  This was reverted 3 days later (commit e2ea684e) because of 
performance issues.

Ken
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