Commit 149321a6 authored Jun 28, 2016 by Jan Kara Committed by Jens Axboe Jun 28, 2016

cfq-iosched: Fix regression in bonnie++ rewrite performance



Commit 9a7f38c4 (cfq-iosched: Convert from jiffies to nanoseconds)
broke the condition for detecting starved sync IO in
cfq_completed_request() because rq->start_time remained in jiffies but
we compared it with nanosecond values. This manifested as a regression
in bonnie++ rewrite performance because we always ended up considering
sync IO starved and thus never increased async IO queue depth.

Since rq->start_time is used in a lot of places, converting it to ns
values would be non-trivial. So just revert the condition in CFQ to use
comparison with jiffies. This will lead to suboptimal results if
cfq_fifo_expire[1] will ever come close to 1 jiffie but so far we are
relatively far from that with the storage used with CFQ (the default
value is 128 ms).

Fixes: 9a7f38c4
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>

parent 93fdf147

block/cfq-iosched.c

+10 −1

Original line number	Diff line number	Diff line
		@@ -4243,7 +4243,16 @@ static void cfq_completed_request(struct request_queue q, struct request rq)
		cfqq_type(cfqq));

		st->ttime.last_end_request = now;
		if (!(rq->start_time + cfqd->cfq_fifo_expire[1] > now))
		/*
		* We have to do this check in jiffies since start_time is in
		* jiffies and it is not trivial to convert to ns. If
		* cfq_fifo_expire[1] ever comes close to 1 jiffie, this test
		* will become problematic but so far we are fine (the default
		* is 128 ms).
		*/
		if (!time_after(rq->start_time +
		nsecs_to_jiffies(cfqd->cfq_fifo_expire[1]),
		jiffies))
		cfqd->last_delayed_sync = now;
		}

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