On 2018/7/23 9:14 PM, Kai Krakow wrote:
2018-07-22 18:13 GMT+02:00 Coly Li colyli@suse.de:
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.
Running with this patch on 4.17, I see reduced IO lags in desktop usage during heavy writes. The reduction is minor but noticeable. The small short IO lags seem to be fixed with this but I still see the stalls with long lags in desktop usage. I don't address those to bcache, tho. This seems more like an issue resulting from using bees on btrfs.
Fixes: Commit b1092c9af9ed ("bcache: allow quick writeback when backing idle") Cc: stable@vger.kernel.org #4.16+ Signed-off-by: Coly Li colyli@suse.de Cc: Michael Lyle mlyle@lyle.org
Hi Kai,
Tested-by: Kai Krakow kai@kaishome.de
Thank you for the testing and information sharing, very helpful :-)
Coly Li
drivers/md/bcache/bcache.h | 9 +--- drivers/md/bcache/request.c | 42 ++++++++++++++- drivers/md/bcache/sysfs.c | 14 +++-- drivers/md/bcache/util.c | 2 +- drivers/md/bcache/util.h | 2 +- drivers/md/bcache/writeback.c | 98 +++++++++++++++++++++++++---------- 6 files changed, 126 insertions(+), 41 deletions(-)
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