linux-kernel - Re: CFS Performance Issues

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Message-ID: <4A1EB272.3050902@davidnewall.com>
Date:	Fri, 29 May 2009 01:19:06 +0930
From:	David Newall <davidn@...idnewall.com>
To:	Olaf Kirch <okir@...e.de>
CC:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	Andreas Gruenbacher <agruen@...e.de>
Subject: Re: CFS Performance Issues

Olaf Kirch wrote:
> As you probably know, we've been chasing a variety of performance issues
> ...
> I see this:
>
> ./slice 16
>     avg slice:  1.12 utime: 216263.187500
> ...
> Any insight you can offer here is greatly appreciated!
>   

About that: avg slice is in nsec, not msec (the old, off-by-one-million
bug), and utime, also an average, is in usec.

The first result indicates 1.12 nsec per context switch,  193 context
switches and 346% CPU utilisation.  You must have at least four CPU
cores.  Here's your table, extended* per this interpretation:

./slice 16
    avg slice:  1.12 utime: 216263.187500:  1.12 nsec/csw,  193 csw, 346 CPU%
    avg slice:  0.25 utime: 125507.687500:  0.25 nsec/csw,  502 csw, 200 CPU%
    avg slice:  0.31 utime: 125257.937500:  0.31 nsec/csw,  404 csw, 200 CPU%
    avg slice:  0.31 utime: 125507.812500:  0.31 nsec/csw,  404 csw, 200 CPU%
    avg slice:  0.12 utime: 124507.875000:  0.12 nsec/csw, 1037 csw, 199 CPU%
    avg slice:  0.38 utime: 124757.687500:  0.38 nsec/csw,  328 csw, 199 CPU%
    avg slice:  0.31 utime: 125508.000000:  0.31 nsec/csw,  404 csw, 200 CPU%
    avg slice:  0.44 utime: 125757.750000:  0.44 nsec/csw,  285 csw, 201 CPU%
    avg slice:  2.00 utime: 128258.000000:  2.00 nsec/csw,   64 csw, 205 CPU%
 ------ here I turned off new_fair_sleepers ----
    avg slice: 10.25 utime: 137008.500000: 10.25 nsec/csw,   13 csw, 219 CPU%
    avg slice:  9.31 utime: 139008.875000:  9.31 nsec/csw,   14 csw, 222 CPU%
    avg slice: 10.50 utime: 141508.687500: 10.50 nsec/csw,   13 csw, 226 CPU%
    avg slice:  9.44 utime: 139258.750000:  9.44 nsec/csw,   14 csw, 222 CPU%
    avg slice: 10.31 utime: 140008.687500: 10.31 nsec/csw,   13 csw, 224 CPU%
    avg slice:  9.19 utime: 139008.625000:  9.19 nsec/csw,   15 csw, 222 CPU%
    avg slice: 10.00 utime: 137258.625000: 10.00 nsec/csw,   13 csw, 219 CPU%
    avg slice: 10.06 utime: 135258.562500: 10.06 nsec/csw,   13 csw, 216 CPU%
    avg slice:  9.62 utime: 138758.562500:  9.62 nsec/csw,   14 csw, 222 CPU%


You don't seem to be getting good CPU utilisation.

*awk '{printf "%s: %5.2f nsec/csw, %4d csw, %3d CPU%%\n", $0, $3, $5/$3/1000, $5*16/10000}'

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