Messages in this thread | | | Date | Sat, 5 Dec 2009 09:50:51 +0100 | From | Jens Axboe <> | Subject | Re: [RFC]cfq-iosched: no dispatch limit for single queue |
| |
On Fri, Dec 04 2009, Corrado Zoccolo wrote: > Hi Shaohua, Jens, > On Thu, Dec 3, 2009 at 12:57 PM, Jens Axboe <jens.axboe@oracle.com> wrote: > > On Thu, Dec 03 2009, Shaohua Li wrote: > >> Since commit 2f5cb7381b737e24c8046fd4aeab571fb71315f5, each queue can send > >> up to 4 * 4 requests if only one queue exists. I wonder why we have such limit. > >> Device supports tag can send more requests. For example, AHCI can send 31 > >> requests. Test (direct aio randread) shows the limits reduce about 4% disk > >> thoughput. > >> On the other hand, since we send one request one time, if other queue > >> pop when current is sending more than cfq_quantum requests, current queue will > >> stop send requests soon after one request, so sounds there is no big latency. > >> > >> Signed-off-by: Shaohua Li <shaohua.li@intel.com> > >> > >> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > >> index aa1e953..e05650f 100644 > >> --- a/block/cfq-iosched.c > >> +++ b/block/cfq-iosched.c > >> @@ -1298,9 +1298,9 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq) > >> return false; > >> > >> /* > >> - * Sole queue user, allow bigger slice > >> + * Sole queue user, no limit > >> */ > >> - max_dispatch *= 4; > >> + max_dispatch = -1; > >> } > >> > >> /* > > > > As you mention, we do dispatches in bites of 1. In reality, there's > > going to be little difference when we get this far in the depth process, > > so I think the patch looks good. I have applied it, thanks. > > I think the limit should be removed only for sync queues. > For async queues, if cfq_latency is not set, removing the limit here can > cause very high latencies to sync queues (almost 100% increase), > without a noticeable throughput gain.
It's always problematic to say 'without a noticable throughput gain', as on some workloads/storage, the difference between 16 and eg 32 in depth WILL be noticeable. 16 is already high enough that if we hit that limit, it will cause a latency hit. The hope here is that larger wont make it much worse, but we'll see.
-- Jens Axboe
-- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |