Re: tiobench read 50% regression with 2.6.30-rc1

[Jens Axboe]

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@xxxxxxxxxx>
> > > 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.

Can you try with this patch?

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index a4809de..66f00e5 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1905,10 +1905,17 @@ cfq_rq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		 * Remember that we saw a request from this process, but
 		 * don't start queuing just yet. Otherwise we risk seeing lots
 		 * of tiny requests, because we disrupt the normal plugging
-		 * and merging.
+		 * and merging. If the request is already larger than a single
+		 * page, let it rip immediately. For that case we assume that
+		 * merging is already done.
 		 */
-		if (cfq_cfqq_wait_request(cfqq))
+		if (cfq_cfqq_wait_request(cfqq)) {
+			if (blk_rq_bytes(rq) > PAGE_CACHE_SIZE) {
+				del_timer(&cfqd->idle_slice_timer);
+				blk_start_queueing(cfqd->queue);
+			}
 			cfq_mark_cfqq_must_dispatch(cfqq);
+		}
 	} else if (cfq_should_preempt(cfqd, cfqq, rq)) {
 		/*
 		 * not the active queue - expire current slice if it is

-- 
Jens Axboe

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