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

From: Jens Axboe
Date: Mon Mar 01 2010 - 03:02:42 EST

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.

Jens Axboe

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