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

[Jeff Moyer <jmoyer@redhat.com>]

Vivek Goyal <vgoyal@redhat.com> 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@redhat.com> wrote:
>> > Corrado Zoccolo <czoccolo@gmail.com> writes:
>> >
>> >> Hi Vivkek,
>> >>
>> >> On Mon, Jan 4, 2010 at 3:47 PM, Vivek Goyal <vgoyal@redhat.com> 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.  You 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.  Corrado, 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.

Cheers,
Jeff