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

[Corrado Zoccolo <czoccolo@gmail.com>]

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.

Thanks,
Corrado