[Bug nptl/4274] New: Performance issue: pthread_cond_signal() causes three context switches instead of one

["bart dot vanassche at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

Pthread mutexes and condition variables are very often used in combination in
multithreaded software. In this combination pthread_cond_signal() is used to
wake up another thread. Optimally this wakeup would cause only one context
switch. Unfortunately pthread_cond_signal() seems to cause two context switches
on average when using linuxthreads, and three when using the NPTL.

$ uname -a
Linux pc-101 2.6.18.6 #8 Sun Feb 4 11:17:43 CET 2007 i686 athlon i386 GNU/Linux

$ /home/bart/glibc236/lib/libc.so.6
GNU C Library stable release version 2.3.6, by Roland McGrath et al.
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.1.2 20061115 (prerelease) (SUSE Linux).
Compiled on a Linux >>2.6.18.6<< system on 2007-03-17.
Available extensions:
        GNU libio by Per Bothner
        crypt add-on version 2.1 by Michael Glad and others
        linuxthreads-0.10 by Xavier Leroy
        BIND-8.2.3-T5B
        libthread_db work sponsored by Alpha Processor Inc
        NIS(YP)/NIS+ NSS modules 0.19 by Thorsten Kukuk
Thread-local storage support included.
For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.

$ /lib/libc.so.6
GNU C Library stable release version 2.5 (20061011), by Roland McGrath et al.
Copyright (C) 2006 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Configured for i686-suse-linux.
Compiled by GNU CC version 4.1.2 20061115 (prerelease) (SUSE Linux).
Compiled on a Linux 2.6.18 system on 2006-11-26.
Available extensions:
        crypt add-on version 2.1 by Michael Glad and others
        GNU Libidn by Simon Josefsson
        GNU libio by Per Bothner
        NIS(YP)/NIS+ NSS modules 0.19 by Thorsten Kukuk
        NoVersion patch for broken glibc 2.0 binaries
        Native POSIX Threads Library by Ulrich Drepper et al
        BIND-8.2.3-T5B
Thread-local storage support included.
For bug reporting instructions, please see:
<http://www.gnu.org/software/libc/bugs.html>.

$ LD_LIBRARY_PATH=/home/bart/glibc236/lib: /home/bart/glibc236/lib/ld-linux.so.2
./glibc236-condvar-perf
linuxthreads
mutex                 elapsed:     5287 us; per iteration:   52 ns / 0 context
switches.
c.v. ping-pong test   elapsed:   772543 us; per iteration: 7725 ns / 4 context
switches.
signal ping-pong test elapsed:   913529 us; per iteration: 9135 ns / 4 context
switches.

$ ./condvar-perf
NPTL
mutex                 elapsed:     4000 us; per iteration:   40 ns / 0 context
switches.
c.v. ping-pong test   elapsed:   461905 us; per iteration: 4619 ns / 6 context
switches.
signal ping-pong test elapsed:   424947 us; per iteration: 4249 ns / 4 context
switches.

-- 
           Summary: Performance issue: pthread_cond_signal() causes three
                    context switches instead of one
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: nptl
        AssignedTo: drepper at redhat dot com
        ReportedBy: bart dot vanassche at gmail dot com
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: i686-suse-linux-gnu
  GCC host triplet: i686-suse-linux-gnu
GCC target triplet: i686-suse-linux-gnu


http://sourceware.org/bugzilla/show_bug.cgi?id=4274

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