Re: [PATCH] block: check more requests for multiple_queues in blk_attempt_plug_merge

[Jens Axboe <axboe@kernel.dk>]

On 3/10/22 6:32 PM, Ming Lei wrote:
> On Thu, Mar 10, 2022 at 06:21:33PM -0700, Jens Axboe wrote:
>> On 3/10/22 6:14 PM, Ming Lei wrote:
>>> On Thu, Mar 10, 2022 at 05:36:44PM -0700, Jens Axboe wrote:
>>>> On 3/10/22 5:31 PM, Song Liu wrote:
>>>>> On Thu, Mar 10, 2022 at 4:07 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>
>>>>>> On 3/10/22 4:33 PM, Song Liu wrote:
>>>>>>> On Thu, Mar 10, 2022 at 3:02 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>>
>>>>>>>> On 3/10/22 3:37 PM, Song Liu wrote:
>>>>>>>>> On Thu, Mar 10, 2022 at 2:15 PM Jens Axboe <axboe@kernel.dk> wrote:
>>>>>>>>>>
>>>>>>>>>> On 3/8/22 11:42 PM, Song Liu wrote:
>>>>>>>>>>> RAID arrays check/repair operations benefit a lot from merging requests.
>>>>>>>>>>> If we only check the previous entry for merge attempt, many merge will be
>>>>>>>>>>> missed. As a result, significant regression is observed for RAID check
>>>>>>>>>>> and repair.
>>>>>>>>>>>
>>>>>>>>>>> Fix this by checking more than just the previous entry when
>>>>>>>>>>> plug->multiple_queues == true.
>>>>>>>>>>>
>>>>>>>>>>> This improves the check/repair speed of a 20-HDD raid6 from 19 MB/s to
>>>>>>>>>>> 103 MB/s.
>>>>>>>>>>
>>>>>>>>>> Do the underlying disks not have an IO scheduler attached? Curious why
>>>>>>>>>> the merges aren't being done there, would be trivial when the list is
>>>>>>>>>> flushed out. Because if the perf difference is that big, then other
>>>>>>>>>> workloads would be suffering they are that sensitive to being within a
>>>>>>>>>> plug worth of IO.
>>>>>>>>>
>>>>>>>>> The disks have mq-deadline by default. I also tried kyber, the result
>>>>>>>>> is the same. Raid repair work sends IOs to all the HDDs in a
>>>>>>>>> round-robin manner. If we only check the previous request, there isn't
>>>>>>>>> much opportunity for merge. I guess other workloads may have different
>>>>>>>>> behavior?
>>>>>>>>
>>>>>>>> Round robin one at the time? I feel like there's something odd or
>>>>>>>> suboptimal with the raid rebuild, if it's that sensitive to plug
>>>>>>>> merging.
>>>>>>>
>>>>>>> It is not one request at a time, but more like (for raid456):
>>>>>>>    read 4kB from HDD1, HDD2, HDD3...,
>>>>>>>    then read another 4kB from HDD1, HDD2, HDD3, ...
>>>>>>
>>>>>> Ehm, that very much looks like one-at-the-time from each drive, which is
>>>>>> pretty much the worst way to do it :-)
>>>>>>
>>>>>> Is there a reason for that? Why isn't it using 64k chunks or something
>>>>>> like that? You could still do that as a kind of read-ahead, even if
>>>>>> you're still processing in chunks of 4k.
>>>>>
>>>>> raid456 handles logic in the granularity of stripe. Each stripe is 4kB from
>>>>> every HDD in the array. AFAICT, we need some non-trivial change to
>>>>> enable the read ahead.
>>>>
>>>> Right, you'd need to stick some sort of caching in between so instead of
>>>> reading 4k directly, you ask the cache for 4k and that can manage
>>>> read-ahead.
>>>>
>>>>>>>> Plug merging is mainly meant to reduce the overhead of merging,
>>>>>>>> complement what the scheduler would do. If there's a big drop in
>>>>>>>> performance just by not getting as efficient merging on the plug side,
>>>>>>>> that points to an issue with something else.
>>>>>>>
>>>>>>> We introduced blk_plug_max_rq_count() to give md more opportunities to
>>>>>>> merge at plug side, so I guess the behavior has been like this for a
>>>>>>> long time. I will take a look at the scheduler side and see whether we
>>>>>>> can just merge later, but I am not very optimistic about it.
>>>>>>
>>>>>> Yeah I remember, and that also kind of felt like a work-around for some
>>>>>> underlying issue. Maybe there's something about how the IO is issued
>>>>>> that makes it go straight to disk and we never get any merging? Is it
>>>>>> because they are sync reads?
>>>>>>
>>>>>> In any case, just doing larger reads would likely help quite a bit, but
>>>>>> would still be nice to get to the bottom of why we're not seeing the
>>>>>> level of merging we expect.
>>>>>
>>>>> Let me look more into this. Maybe we messed something up in the
>>>>> scheduler.
>>>>
>>>> I'm assuming you have a plug setup for doing the reads, which is why you
>>>> see the big difference (or there would be none). But
>>>> blk_mq_flush_plug_list() should really take care of this when the plug
>>>> is flushed, requests should be merged at that point. And from your
>>>> description, doesn't sound like they are at all.
>>>
>>> requests are shared, when running out of request, plug list will be
>>> flushed early.
>>
>> That is true, but I don't think that's the problem here with the round
>> robin approach. Seems like it'd drive a pretty low queue depth, even
>> considering SATA.
> 
> Another one may be plug list not sorted before inserting requests to
> scheduler in blk_mq_flush_plug_list(), looks you have mentioned.

Yep, it'd probably be the first thing I'd try... The way the IO is
issued, it's pretty much guaranteed that the plug list will be fully
interleaved with different queues and we're then issuine one-by-one and
running the queue each time.

-- 
Jens Axboe