Re: [PATCH v2] bcache: set max writeback rate when I/O request is idle

Am 24.07.2018 um 10:28 schrieb Coly Li:

On 2018/7/24 3:16 PM, Stefan Priebe - Profihost AG wrote:

quoted

Am 24.07.2018 um 06:03 schrieb Coly Li:

quoted

Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
allows the writeback rate to be faster if there is no I/O request on a
bcache device. It works well if there is only one bcache device attached
to the cache set. If there are many bcache devices attached to a cache
set, it may introduce performance regression because multiple faster
writeback threads of the idle bcache devices will compete the btree level
locks with the bcache device who have I/O requests coming.

This patch fixes the above issue by only permitting fast writebac when
all bcache devices attached on the cache set are idle. And if one of the
bcache devices has new I/O request coming, minimized all writeback
throughput immediately and let PI controller __update_writeback_rate()
to decide the upcoming writeback rate for each bcache device.

Also when all bcache devices are idle, limited wrieback rate to a small
number is wast of thoughput, especially when backing devices are slower
non-rotation devices (e.g. SATA SSD). This patch sets a max writeback
rate for each backing device if the whole cache set is idle. A faster
writeback rate in idle time means new I/Os may have more available space
for dirty data, and people may observe a better write performance then.

Please note bcache may change its cache mode in run time, and this patch
still works if the cache mode is switched from writeback mode and there
is still dirty data on cache.

Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle")
Cc: stable@vger.kernel.org #4.16+
Signed-off-by: Coly Li <redacted>
Tested-by: Kai Krakow <redacted>
Cc: Michael Lyle <redacted>
Cc: Stefan Priebe <redacted>

Hi Coly,

i'm still experiencing the same bug as yesterday while rebooting every
two times i get a deadlock in cache_set_free.

Sadly it's so late ion the shutdown process that netconsole is already
unloaded or at least i get no messages from while it happens. The traces
look the same like yesterday.

Hi Stefan,

Do you use the v2 patch on latest SLE15 kernel source?

Yes - i use the kernel code from here:
https://github.com/openSUSE/kernel-source/commits/SLE15

Let me try to
reproduce it on my machine. I assume when you reboot, the cache is still
dirty and not clean up, am I right ?

Yes with around 15GB of dirty data - ceph is running on top of it and
generated many random writes.

And which file system do you mount
on top of the bcache device ?

XFS

Thanks.

Coly Li

Greets,
Stefan