linux-kernel - IO CPU affinity test results

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Date:	Fri, 14 Mar 2008 08:14:22 -0400
From:	"Alan D. Brunelle" <Alan.Brunelle@...com>
To:	linux-kernel@...r.kernel.org
Cc:	Jens Axboe <jens.axboe@...cle.com>, npiggin@...e.de, dgc@....com
Subject: IO CPU affinity test results

Good morning Jens - 

I had two machines running the latest patches hang last night: 

o  2-way AMD64 - I inadvertently left the patched kernel running, and I was moving a ton of data (100+GB) back up over the net to this node. It hard hung (believe it or not) about 99% of the way through. Hard hang, wouldn't respond to anything.

o  4-way IA64 - I was performing a simple test: [mkfs / mount / untar linux sources / make allnoconfig / make -j 5 / umount] repeatedly switching rq_affinity to 0/1 between each run. After 22 passes it had a hard hang with rq_affinity set to 1.

Of course, there is no way of knowing if either hang had anything to do with the patches, but it seems a bit ominous as RQ=1 was set in both cases.

This same test worked fine for 30 passes on a 2-way AMD64 box, with the following results:

Part  RQ   MIN     AVG     MAX      Dev
----- --  ------  ------  ------  ------
 mkfs  0  41.656  41.862  42.086   0.141
 mkfs  1  41.618  41.909  42.270   0.192

untar  0  18.055  19.611  20.906   0.720
untar  1  18.523  19.905  21.988   0.738

 make  0  50.480  50.991  51.752   0.340
 make  1  49.819  50.442  51.000   0.292

 comb  0 110.433 112.464 114.176   0.932
 comb  1 110.694 112.256 114.683   0.948

 psys  0  10.28%  10.91%  11.29%   0.243
 psys  1  10.21%  11.05%  11.80%   0.350

All results are in seconds (as measured by Python's time.time()), except for the psys - which was the average of mpstat's %sys column over the life of the whole run. The mkfs part consisted of [mkfs -t ext2 ; sync ; sync], untar [mount; untar linux sources; umount; sync; sync], make [mount; make allnoconfig; make -j 3; umount; sync; sync], and comb is the combined times of the mkfs, untar and make parts. 

So, in a nutshell, we saw slightly better overall performance, but not conclusively, and we saw slightly elevated %system time to accomplish the task. 

On the 4-way, results were much worse: the final data shown before the system hung showed the rq=1 passes taking significantly longer, albeit at lower %system. I'm going to try the runs again, but I have a feeling that the latest "clean" patch based upon Nick's single call mechanism is a step backwards.

Alan
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