On 02/24/2015 08:44 AM, Mike Snitzer wrote:
On really fast storage it can be beneficial to delay running the request_queue to allow the elevator more opportunity to merge requests. Otherwise, it has been observed that requests are being sent to q->request_fn much quicker than is ideal on IOPS-bound backends. Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx> --- drivers/md/dm.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/drivers/md/dm.c b/drivers/md/dm.c index fc92899..92091e0 100644 --- a/drivers/md/dm.c +++ b/drivers/md/dm.c @@ -1034,7 +1034,7 @@ static void rq_completed(struct mapped_device *md, int rw, bool run_queue) * queue lock again. */ if (run_queue) - blk_run_queue_async(md->queue); + blk_delay_queue(md->queue, HZ / 10);
This looks dangerous... How will this impact sync IO? Heuristics like this will always come back and bite you in the ass.
A slightly more friendly heuristic might be to delay running the queue, if you still have pending IO. That would give you a more sawtooth like queue depth management, so it would potentially slow down a bit, but the upside would be more efficient merging since it would allow some requests so sit a little bit before being dispatched.
-- Jens Axboe -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
