Re: [PATCH] cfq-iosched: quantum check tweak --resend

From: Shaohua Li
Date: Mon Mar 01 2010 - 03:15:33 EST

On Mon, Mar 01, 2010 at 04:02:34PM +0800, Jens Axboe wrote:
> On Mon, Mar 01 2010, Shaohua Li wrote:
> > Currently a queue can only dispatch up to 4 requests if there are other queues.
> > This isn't optimal, device can handle more requests, for example, AHCI can
> > handle 31 requests. I can understand the limit is for fairness, but we could
> > do a tweak: if the queue still has a lot of slice left, sounds we could
> > ignore the limit. Test shows this boost my workload (two thread randread of
> > a SSD) from 78m/s to 100m/s.
> > Thanks for suggestions from Corrado and Vivek for the patch.
> As mentioned before, I think we definitely want to ensure that we drive
> the full queue depth whenever possible. I think your patch is a bit
> dangerous, though. The problematic workload here is a buffered write,
> interleaved with the occasional sync reader. If the sync reader has to
> endure 32 requests every time, latency rises dramatically for him.
the patch still matains a hardlimit for dispatched request. For a async,
the limit is cfq_slice_async/cfq_slice_idle = 5. For sync, the limit is 8.
And we only pipe out such number of requests at the begining of a slice.
For the workload you mentioned here, we only dispatch 1 extra request.

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