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From: "fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug malloc/25817] Tune the number of malloc arenas based on the CPU
 affinity mask
Date: Tue, 14 Apr 2020 09:13:00 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D25817

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
              Flags|                            |security-
          Component|libc                        |malloc
                 CC|                            |fweimer at redhat dot com
            Summary|__get_nprocs reads a file   |Tune the number of malloc
                   |rather than calling         |arenas based on the CPU
                   |sched_getaffinity           |affinity mask

--- Comment #1 from Florian Weimer <fweimer at redhat dot com> ---
Given that more and more kernel information is available only via /sys and
/proc, the test expectations are not really valid. We have to open file
descriptors behind the scenes occasionally, and it is impossible to predict
what the future will bring in this regard.

For malloc tuning, it may indeed be more reasonable to look at the current
affinity mask. But we cannot do so by changing __get_nprocs because some
software depends on that returning 1 if the system is a true uniprocessor
system because it uses this information to elide barriers, depending on the
return value of sysconf (_SC_NPROCESSORS_ONLN). See bug 21542.

For the malloc tuning, this does not matter because the locks are there no
matter whether the system is SMP or not. (Internally, we have a different w=
ay
to elide atomics on some architectures, by parsing uname -v output. I'm sure
you didn't want to know that.)

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