[PATCH 2/2] blk-stat: add a poll_size value to the request_queue struct

[Jens Axboe]

On 03/28/2017 01:58 PM, Stephen  Bates wrote:
>>>
>>> Thanks for the review Sagi. I’d be OK going with <=0 as the exact
>>> match would normally be for minimal IO sizes (where <= and = are the
>>> same thing). I will see what other feedback I get and aim to do a
>>> respin soon…
>>
>> No tunables for this, please. There's absolutely no reason why we should
>> need it.
> 
> Jens – by this you mean you want to only bucket IO that are exactly
> the minimum block size supported by the underlying block device? I was
> envisioning we might want to relax that in certain cases (e.g. bucket
> 4KB and below going to a 512B device).

Sorry, the above was a bit terse. I think a much better solution would
be to create a number of buckets (per data direction) and do stats on
all of them. The buckets would cover a reasonable range of request
sizes. Then when you poll for a given request, we can base our timed
number on the data direction AND size of it. You can get pretty far with
a few buckets:

512b
4k
8k
16k
32k
64k
128k

and you could even have your time estimation function turn these into
something sane. Or just use a composite of buckets, if you wish.

-- 
Jens Axboe