linux-kernel - Performance regression in 2.6.30-rc1

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Date:	Tue, 02 Jun 2009 16:30:19 +0530
From:	poornima nayak <mpnayak@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	venkatesh.pallipadi@...el.com, svaidy@...ux.vnet.ibm.com,
	davej@...hat.com, ego@...ibm.com
Subject: Performance regression in 2.6.30-rc1

Hi 

By executing kernbench on 2.6.30-rc1 we observed there is a performance
regression in 2.6.30-rc1. Then git-bisect was done between v2.6.29 and
v2.6.30-rc5, after 13 iterations identified the attached patch is
causing regression.

Performance data of 2.6.29 without applying the attached patch.
param-version
testname
elapsed-avg
elapsed-std
2.6.29'
pm_kernbench.Version-none-threads=2-sched_mc=2
             221.1
              0.81
2.6.29'
pm_kernbench.Version-none-threads=4-sched_mc=0
            115.09
               0.6
2.6.29'
pm_kernbench.Version-none-threads=4-sched_mc=2
            109.05
              0.25
2.6.29'
pm_kernbench.Version-none-threads=8-sched_mc=2
              60.4
              0.38
2.6.29'
pm_kernbench.Version-none-threads=8-sched_mc=0
             65.23
              0.34
2.6.29'
pm_kernbench.Version-none-threads=2-sched_mc=0
            231.61
              0.59

Performance data of 2.6.29 after applying the attached patch.
param-version
testname
elapsed-avg
elapsed-std
2.6.29'
pm_kernbench.Version-thir-bisect-threads=2-sched_mc=0
            203.77
              0.48
2.6.29'
pm_kernbench.Version-thir-bisect-threads=8-sched_mc=0
             64.38
              0.25
2.6.29'
pm_kernbench.Version-thir-bisect-threads=4-sched_mc=0
            102.46
               0.1
2.6.29'
pm_kernbench.Version-thir-bisect-threads=8-sched_mc=2
             59.94
              0.46
2.6.29'
pm_kernbench.Version-thir-bisect-threads=4-sched_mc=2
            106.84
              0.28
2.6.29'
pm_kernbench.Version-thir-bisect-threads=2-sched_mc=2
            199.44
              0.44

Performance issue here is when sched_mc_power_savings is set 2 and
kernbench is triggered with 4 threads the value of 'elapsed time' is
more then sched_mc_power_savings is set to 0. Expectation is elapsed
time should be less when sched_mc_power_savings set 2 compared to
sched_mc_power_savings set to 0.

Regds
Poornima

View attachment "performance_reg.patch" of type "text/x-patch" (901 bytes)