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SubjectRe: IO CPU affinity test results
On Fri, Mar 14 2008, Alan D. Brunelle wrote:
> Alan D. Brunelle wrote:
> > 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
>
> I was able to go back and capture the results after 17 passes on the
> 4-way IA64 box (before it hung), and with rq=1 it shows a huge
> increase in time needed to do the combined tests - almost 18% longer,
> however with a reduction of about 24% less system time.
>
> Part RQ MIN AVG MAX Dev
> ----- -- ------ ------ ------ ------
> mkfs 0 18.543 19.055 19.514 0.285
> mkfs 1 18.730 19.217 19.812 0.316
>
> untar 0 17.119 21.396 43.868 8.025
> untar 1 16.987 28.155 44.637 10.175
>
> make 0 23.105 23.866 24.487 0.359
> make 1 24.015 28.384 37.829 3.598
>
> comb 0 59.610 64.317 86.733 7.896
> comb 1 63.181 75.755 94.079 10.489
>
> psys 0 10.35% 14.16% 16.28% 1.375
> psys 1 6.89% 10.73% 14.30% 2.368
>
> I'll try to snag some profile data to see what's up.

That definitely looks like a bug. One thing to try would be to drop the
test for whether to send the IPI or not, and just unconditionally do it.
For ia64, that would be in
arch/ia64/kernel/smp.c:__smp_call_function_single(), just change

if (ipi)
send_IPI_single(cpu, IPI_CALL_FUNC_SINGLE);

to

send_IPI_single(cpu, IPI_CALL_FUNC_SINGLE);

so that it always gets notified.

--
Jens Axboe



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