On 11/03/2022 01:35, Jens Axboe wrote:
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@xxxxxxxxx> wrote:
On 3/10/22 4:33 PM, Song Liu wrote:
On Thu, Mar 10, 2022 at 3:02 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
On 3/10/22 3:37 PM, Song Liu wrote:
On Thu, Mar 10, 2022 at 2:15 PM Jens Axboe <axboe@xxxxxxxxx> 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.
Naive question, but can you make the flush flush the first one, then
scan the queue for all bios for the same device, then go back and start
again? Simple approach if it'll work, at the expense of scanning the
queue once per device.
Cheers,
Wol
