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

[Corrado Zoccolo <czoccolo@gmail.com>]

On Mon, Jan 11, 2010 at 5:44 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> On Mon, Jan 11, 2010 at 03:53:00PM +0100, Corrado Zoccolo wrote:
>> 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).
>
> Hi Corrado,
>
> In Jeff's test case of running read-test2, I am not even sure if any
> merging between the queues took place or not as on NCQ SSD, we are driving
> deeper queue depths and unless read-test2 is creating more than 32
> threads, there might not be any merging taking place at all.

Jeff's test was modeled after real use cases: widely used, legacy
programs like dump.
Since we often said that splitting the sequential stream in multiple
threads was not the
correct approach, and we did introduce the change in the kernel just
to support those
programs (not to encourage writing more of this league), we can assume
that if they
do not drive deeper queues, no one will. So the overhead is just
overhead, and will never
give any benefit.

I therefore want to remove it, since for SSD it matters.
>
> We also don't have any data/numbers what kind of cpu savings does this
> patch bring in.

Jeff's test showed larger bandwidth with merge disabled, so it implies
some saving is present.

Thanks,
Corrado

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