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Subject:	Re: posix thread scaling issue
Thread-Topic:	posix thread scaling issue
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Date:	Sat, 2 Sep 2023 19:30:16 +0000
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Jeff wrote:

> I have a program that is embarrassing parallel.
> On my older computer which has an epyc 7302 (16 cores,  32 threads) it
> scales very well using cygwin, and fully utilized all threads.
> On my new computer which has an epyc 7B13 (64 cores, 128 threads) it
> does not scale very well.

> According to the windows task manager, it only uses 74% of the cpu
> resources.
> The time it takes the program to run on windows is 166 seconds.
> Using the same hardware on a recent version of linux, I can get 100% cpu
> utilization and the program takes 100 seconds to run.

> I suspect there may be something in cygwin that doesn't scale well with
> lots of posix threads.
> I know this is a bit of an unusual situation, but you can buy a 128 core
> / 256 thread system now.

> Enclosed is the output of cygcheck.
> I updated my version of cygwin to be current as of today, Sep 2 2023.

> thanks,
> jeff

Hi Jeff,

Cygwin's memory allocation, and anything that uses it under the hood, such as some containers from the C++ standard library, don't scale well with many threads. I have observed even worse scalling than yours in my own massively multi-thread programs.

My advice would be to either rewrite your program to centralize memory allocation in a specialized thread that serves the other processing threads, or, if you don't need Posix things, compile with the Mingw cross-compiler, which produces code that uses M$ implementation for memory allocation, which scales better.

Regards,

- André Bleau

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