Re: [PATCH] cfq-iosched: NCQ SSDs do not need read queue merging

[Corrado Zoccolo <czoccolo@gmail.com>]

On Mon, Jan 11, 2010 at 2:18 PM, Jeff Garzik <jeff@garzik.org> wrote:
> On 01/11/2010 08:13 AM, Jens Axboe wrote:
>>
>> On Mon, Jan 11 2010, Corrado Zoccolo wrote:
>>>
>>> On Mon, Jan 11, 2010 at 12:25 PM, Jeff Garzik<jeff@garzik.org>  wrote:
>>>>
>>>> On 01/10/2010 04:04 PM, Corrado Zoccolo wrote:
>>>>>
>>>>> NCQ SSDs' performances are not affected by
>>>>> distance of read requests, so there is no point in having
>>>>> overhead to merge such queues.
>>>>>
>>>>> Non-NCQ SSDs showed regression in some special cases, so
>>>>> they are ruled out by this patch.
>>>>>
>>>>> This patch intentionally doesn't affect writes, so
>>>>> it changes the queued[] field, to be indexed by
>>>>> READ/WRITE instead of SYNC/ASYNC, and only compute proximity
>>>>> for queues with WRITE requests.
>>>>>
>>>>> Signed-off-by: Corrado Zoccolo<czoccolo@gmail.com>
>>>>
>>>> That's not really true.  Overhead always increases as the total number
>>>> of
>>>> ATA commands issued increases.
>>>
>>> Jeff Moyer tested the patch on the workload that mostly benefit of
>>> queue merging, and found that
>>> the performance was improved by the patch.
>>> So removing the CPU overhead helps much more than the marginal gain
>>> given by merging on this hardware.
>>
>> It's not always going to be true. On SATA the command overhead is fairly
>> low, but on other hardware that may not be the case. Unless you are CPU
>> bound by your IO device, then merging will always be beneficial. I'm a
>> little behind on emails after my vacation, Jeff what numbers did you
>> generate and on what hardware?
>
>  ...and on what workload?   "the workload that mostly benefit of queue
> merging" is highly subjective, and likely does not cover most workloads SSDs
> will see in the field.
Hi Jeff,
exactly.
The workloads that benefits from queue merging are the ones in which a
sequential
read is actually splitt, and carried out by different processes in
different I/O context, each
sending requests with strides. This is clearly not the best way of
doing sequential access
(I would happily declare those programs as buggy).
CFQ has code that merges queues in this case. I'm disabling the READ
part for NCQ SSDs,
since, as Jeff measured, the code overhead outweight the gain from
merging (if any).

As you said, most workloads don't benefit from queue merging. On those
ones, the patch
just removes an overhead.

Thanks,
Corrado

>        Jeff
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