Re: [PATCH] cfq-iosched: non-rot devices do not need read queue merging

[Corrado Zoccolo]

On Mon, Jan 4, 2010 at 8:04 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> Vivek Goyal <vgoyal@xxxxxxxxxx> writes:
>
>> On Mon, Jan 04, 2010 at 07:37:17PM +0100, Corrado Zoccolo wrote:
>>> On Mon, Jan 4, 2010 at 5:51 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
>>> > Corrado Zoccolo <czoccolo@xxxxxxxxx> writes:
>>> >
>>> >> Hi Vivkek,
>>> >>
>>> >> On Mon, Jan 4, 2010 at 3:47 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
>>> >>> On Wed, Dec 30, 2009 at 11:22:47PM +0100, Corrado Zoccolo wrote:
>>> >>>> Non rotational devices' performances are not affected by
>>> >>>> distance of read requests, so there is no point in having
>>> >>>> overhead to merge such queues.
>>> >>>> This doesn't apply to writes, so this patch changes the
>>> >>>> queued[] field, to be indexed by READ/WRITE instead of
>>> >>>> SYNC/ASYNC, and only compute proximity for queues with
>>> >>>> WRITE requests.
>>> >>>>
>>> >>>
>>> >>> Hi Corrado,
>>> >>>
>>> >>> What's the reason that reads don't benefit from merging queues and hence
>>> >>> merging requests and only writes do on SSD?
>>> >>
>>> >> On SSDs, reads are just limited by the maximum transfer rate, and
>>> >> larger (i.e. merged) reads will just take proportionally longer.
>>> >
>>> > This is simply not true. ÂYou can get more bandwidth from an SSD (I just
>>> > checked numbers for 2 vendors' devices) by issuing larger read requests,
>>> > no matter whether the access pattern is sequential or random.
>>> I know, but the performance increase given the size is sublinear, and
>>> the situation here is slightly different.
>>> In order for the requests to be merged, they have to be submitted concurrently.
>>> So you have to compare 2 concurrent requests of size x with one
>>> request of size 2*x (with some CPU overhead).
>>> Moreover, you always pay the CPU overhead, even if you can't do the
>>> merging, and you must be very lucky to keep merging, because it means
>>> the two processes are working in lockstep; it is not sufficient that
>>> the requests are just nearby, as for rotational disks.
>>>
>>
>> For jeff, at least "dump" utility threads were kind of working in lockstep
>> for writes and he gained significantly by merging these queues together.
>
> Actually, it was for reads.
>
>> So the argument is that CPU overhead saving in this case is more substantial
>> as compared to gains made by lockstep read threads. I think we shall have to
>> have some numbers to justify that.
>
> Agreed. ÂCorrado, I know you don't have the hardware, so I'll give this
> a run through the read-test2 program and see if it regresses at all.
Great.

Thanks a lot,
Corrado

>
> Cheers,
> Jeff
>
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