On Thu, 2022-03-10 at 14:37 -0800, Song Liu wrote:

On Thu, Mar 10, 2022 at 2:15 PM Jens Axboe [off-list ref] wrote:

quoted

On 3/8/22 11:42 PM, Song Liu wrote:

quoted

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?

quoted

Between your two approaches, I do greatly prefer the first one
though.

I also like the first one better. But I am not sure whether it will
slow down
other workloads. We can probably also make the second one cleaner
with a new variation of blk_start_plug.

As a matter of note and purely anecdotal: Before the raid "check" slow
down/regression my system would be responsive but delayed (opening a
program or opening the xface application menu or switching a file in
VLC would take longer than normal, fractions of seconds to a second but
slugish and notacable) and with the regression that slow down went from
annoying to unbearable. 

The slowdowns (in programs and menus and file changes) also *seems* to
get worse (in both pre & post regression) the longer the check has been
running and the slower a run naturally gets (I assume as the check
moves from the outer portion of the disk to the inner portion?) and the
lower the KB's reported in cat /proc/mdstat/.

In the post regression situation it wasn't just that the check was
taking much longer and was much slower it was also that it slowed down
everything else to the point that it was painful to try and use the
computer as it was so much less responsive (multiple seconds for
anything to load/run/swtch; even web pages). A laggy annoyance had
become an actual hindrance. 

I have no idea why the speed of the "check" would seemingly affect the
apparent responsiveness of the computer and why it would appear that
the slower the check the slower the responsiveness. 

Thanks,
Song