Re: [PATCH 6/8] block: add scalable completion tracking of requests

[Jens Axboe <axboe@kernel.dk>]

On 11/09/2016 09:09 AM, Jens Axboe wrote:
> On 11/09/2016 02:01 AM, Jan Kara wrote:
>> On Tue 08-11-16 08:25:52, Jens Axboe wrote:
>>> On 11/08/2016 06:30 AM, Jan Kara wrote:
>>>> On Tue 01-11-16 15:08:49, Jens Axboe wrote:
>>>>> For legacy block, we simply track them in the request queue. For
>>>>> blk-mq, we track them on a per-sw queue basis, which we can then
>>>>> sum up through the hardware queues and finally to a per device
>>>>> state.
>>>>>
>>>>> The stats are tracked in, roughly, 0.1s interval windows.
>>>>>
>>>>> Add sysfs files to display the stats.
>>>>>
>>>>> Signed-off-by: Jens Axboe <axboe@fb.com>
>>>>
>>>> This patch looks mostly good to me but I have one concern: You track
>>>> statistics in a fixed 134ms window, stats get cleared at the
>>>> beginning of
>>>> each window. Now this can interact with the writeback window and
>>>> latency
>>>> settings which are dynamic and settable from userspace - so if the
>>>> writeback code observation window gets set larger than the stats
>>>> window,
>>>> things become strange since you'll likely miss quite some observations
>>>> about read latencies. So I think you need to make sure stats window is
>>>> always larger than writeback window. Or actually, why do you have
>>>> something
>>>> like stats window and don't leave clearing of statistics completely
>>>> to the
>>>> writeback tracking code?
>>>
>>> That's a good point, and there actually used to be a comment to that
>>> effect in the code. I think the best solution here would be to make the
>>> stats code mask available somewhere, and allow a consumer of the stats
>>> to request a larger window.
>>>
>>> Similarly, we could make the stat window be driven by the consumer, as
>>> you suggest.
>>>
>>> Currently there are two pending submissions that depend on the stats
>>> code. One is this writeback series, and the other one is the hybrid
>>> polling code. The latter does not really care about the window size as
>>> such, since it has no monitoring window of its own, and it wants the
>>> auto-clearing as well.
>>>
>>> I don't mind working on additions for this, but I'd prefer if we could
>>> layer them on top of the existing series instead of respinning it.
>>> There's considerable test time on the existing patchset. Would that work
>>> for you? Especially collapsing the stats and wbt windows would require
>>> some re-architecting.
>>
>> OK, that works for me. Actually, when thinking about this, I have one
>> more
>> suggestion: Do we really want to expose the wbt window as a sysfs
>> tunable?
>> I guess it is good for initial experiments but longer term having the wbt
>> window length be a function of target read latency might be better.
>> Generally you want the window length to be considerably larger than the
>> target latency but OTOH not too large so that the algorithm can react
>> reasonably quickly so that suggests it could really be autotuned (and we
>> scale the window anyway to adapt it to current situation).
>
> That's not a bad idea, I have thought about that as well before. We
> don't need the window tunable, and you are right, it can be a function
> of the desired latency.
>
> I'll hardwire the 100msec latency window for now and get rid of the
> exposed tunable. It's harder to remove sysfs files once they have made
> it into the kernel...

Killed the sysfs variable, so for now it'll be a 100msec window by
default.

-- 
Jens Axboe