Messages in this thread | | | Date | Fri, 10 Apr 2009 09:26:01 +0200 | From | Jens Axboe <> | Subject | Re: tiobench read 50% regression with 2.6.30-rc1 |
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On Fri, Apr 10 2009, Zhang, Yanmin wrote: > On Thu, 2009-04-09 at 11:57 +0200, Jens Axboe wrote: > > On Thu, Apr 09 2009, Zhang, Yanmin wrote: > > > Comparing with 2.6.29's result, tiobench (read) has about 50% regression > > > with 2.6.30-rc1 on all my machines. Bisect down to below patch. > > > > > > b029195dda0129b427c6e579a3bb3ae752da3a93 is first bad commit > > > commit b029195dda0129b427c6e579a3bb3ae752da3a93 > > > Author: Jens Axboe <jens.axboe@oracle.com> > > > Date: Tue Apr 7 11:38:31 2009 +0200 > > > > > > cfq-iosched: don't let idling interfere with plugging > > > > > > When CFQ is waiting for a new request from a process, currently it'll > > > immediately restart queuing when it sees such a request. This doesn't > > > work very well with streamed IO, since we then end up splitting IO > > > that would otherwise have been merged nicely. For a simple dd test, > > > this causes 10x as many requests to be issued as we should have. > > > Normally this goes unnoticed due to the low overhead of requests > > > at the device side, but some hardware is very sensitive to request > > > sizes and there it can cause big slow downs. > > > > > > > > > > > > Command to start the testing: > > > #tiotest -k0 -k1 -k3 -f 80 -t 32 > > > > > > It's a multi-threaded program and starts 32 threads. Every thread does I/O > > > on its own 80MB file. > The files should be created before the testing and pls. drop page caches > by "echo 3 >/proc/sys/vm/drop_caches" before testing. > > > > > It's not a huge surprise that we regressed there. I'll get this fixed up > > next week. Can you I talk you into trying to change the 'quantum' sysfs > > variable for the drive? It's in /sys/block/xxx/queue/iosched where xxx > > is your drive(s). It's set to 4, if you could try progressively larger > > settings and retest, that would help get things started. > I tried 4,8,16,64,128 and didn't find result difference.
OK, that's good. I'm away from the source atm, but I think I know what is going on. We need to kick the queue for anything but the first queued request. I'll get it fixed up next week.
-- Jens Axboe
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