On 2018/7/26 7:49 PM, Stefan Priebe - Profihost AG wrote:

Am 26.07.2018 um 12:42 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.

Tested-by: Stefan Priebe <redacted>

Working fine now.

Great news, thanks !

Coly Li

quoted

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>
---
Channgelog:
v3, Do not acquire bch_register_lock in set_at_max_writeback_rate().
v2, Fix a deadlock reported by Stefan Priebe.
v1, Initial version.

 drivers/md/bcache/bcache.h    | 10 ++--
 drivers/md/bcache/request.c   | 54 ++++++++++++++++++++-
 drivers/md/bcache/super.c     |  4 ++
 drivers/md/bcache/sysfs.c     | 14 ++++--
 drivers/md/bcache/util.c      |  2 +-
 drivers/md/bcache/util.h      |  2 +-
 drivers/md/bcache/writeback.c | 91 +++++++++++++++++++++++------------
 7 files changed, 133 insertions(+), 44 deletions(-)

[snipped]