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Re: [glibc] Fix Linux sysconf(_SC_NPROCESSORS_[CONF|ONLN])performance problem

From: Roland McGrath <roland at hack dot frob dot com>
To: Linus Torvalds <torvalds at linux-foundation dot org>
Cc: Samuel Thibault <samuel dot thibault at ens-lyon dot org>, Ulrich Drepper <drepper at gmail dot com>, "David S. Miller" <davem at davemloft dot net>, Andreas Schwab <schwab at redhat dot com>, libc-alpha at sourceware dot org
Date: Thu, 16 Jun 2011 11:13:33 -0700 (PDT)
Subject: Re: [glibc] Fix Linux sysconf(_SC_NPROCESSORS_[CONF|ONLN])performance problem
References: <BANLkTi=ousV=JtYzAYEnMVqqX6fnF_1z9Q@mail.gmail.com><20110616175811.GF5583@const.bordeaux.inria.fr><BANLkTinbLEcMr6KhUghFBwB3qcGv2UZ5MA@mail.gmail.com>

> But the caching is needed, because whatever the process wants, I
> guarantee that it doesn't want "slow and crappy".

The caching is also wrong, since in a long-lived program it could have been
sampled before more processors came online or went offline.  I'm not sure
there is any solution that is both correct and performant with the current
kernel interface.  The kernel could provide some efficient means of query.


Thanks,
Roland

References:
- [glibc] Fix Linux sysconf(_SC_NPROCESSORS_[CONF|ONLN]) performance problem
  - From: Linus Torvalds
- Re: [glibc] Fix Linux sysconf(_SC_NPROCESSORS_[CONF|ONLN]) performance problem
  - From: Samuel Thibault
- Re: [glibc] Fix Linux sysconf(_SC_NPROCESSORS_[CONF|ONLN])performance problem
  - From: Linus Torvalds

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