dbench 15% regression with 2.6.28-rc1

[Zhang, Yanmin]

Comparing with 2.6.27, dbench result has regression with 2.6.28-rc1 on 2 machines.
1) 8-core stoakley: 15%
2) 8 core+mutl-thread new-model x86-64: 12%

Bisect located below patch.

695698500912c4479ddf4723e492de3970ff8530 is first bad commit
commit 695698500912c4479ddf4723e492de3970ff8530
Author: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Date:   Tue Sep 23 14:54:23 2008 +0200

    sched: rework wakeup preemption
    
    Rework the wakeup preemption to work on real runtime instead of
    the virtual runtime. This greatly simplifies the code.
    
    Signed-off-by: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
    Signed-off-by: Ingo Molnar <mingo@xxxxxxx>


I reverted the patch against 2.6.28-rc2 and the regression mostly disappears
on 8-core stoakley and 8-core+multiThread x86-64 machines.


ïOn other 2 machines, I see improvement instead of regression.
1) 16-core tigerton: improvement 48%
2) 8-core+hyperThreading tulsa: 10%.
I just checked it by reverting above patch to see if the patch improves it. At least
it isn't on tigerton. I'm doing a new bisect on tigerton to see what patch improves
dbench result.


I start online cpu number of dbench processes.

-yanmin


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