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SubjectRe: [RFC]cfq-iosched: no dispatch limit for single queue
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.

--
Jens Axboe



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