• Peter Zijlstra's avatar
    llist: Remove cpu_relax() usage in cmpxchg loops · f0f1d32f
    Peter Zijlstra authored
    
    
    Initial benchmarks show they're a net loss:
    
     $ for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor ; do echo performance > $i; done
     $ echo 4096 32000 64 128 > /proc/sys/kernel/sem
     $ ./sembench -t 2048 -w 1900 -o 0
    
    Pre:
    
     run time 30 seconds 778936 worker burns per second
     run time 30 seconds 912190 worker burns per second
     run time 30 seconds 817506 worker burns per second
     run time 30 seconds 830870 worker burns per second
     run time 30 seconds 845056 worker burns per second
    
    Post:
    
     run time 30 seconds 905920 worker burns per second
     run time 30 seconds 849046 worker burns per second
     run time 30 seconds 886286 worker burns per second
     run time 30 seconds 822320 worker burns per second
     run time 30 seconds 900283 worker burns per second
    
    So about 4% faster. (!)
    
    cpu_relax() stalls the pipeline, therefore, when used in a tight loop
    it has the following benefits:
    
     - allows SMT siblings to have a go;
     - reduces pressure on the CPU interconnect.
    
    However, cmpxchg loops are unfair and thus have unbounded completion
    time, therefore we should avoid getting in such heavily contended
    situations where the above benefits make any difference.
    
    A typical cmpxchg loop should not go round more than a handfull of
    times at worst, therefore adding extra delays just slows things down.
    
    Since the llist primitives are new, there aren't any bad users yet,
    and we should avoid growing them. Heavily contended sites should
    generally be better off using the ticket locks for serialization since
    they provide bounded completion times (fifo-fair over the cpus).
    Signed-off-by: default avatarPeter Zijlstra <a.p.zijlstra@chello.nl>
    Cc: Huang Ying <ying.huang@intel.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Link: http://lkml.kernel.org/r/1315836358.26517.43.camel@twins
    
    Signed-off-by: default avatarIngo Molnar <mingo@elte.hu>
    f0f1d32f
llist.h 6.06 KB