On 2018/7/23 9:14 PM, Kai Krakow wrote:
> 2018-07-22 18:13 GMT+02:00 Coly Li <colyli@xxxxxxx>:
>> 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@xxxxxxxxxxxxxxx #4.16+
>> Signed-off-by: Coly Li <colyli@xxxxxxx>
>> Cc: Michael Lyle <mlyle@xxxxxxxx>
>
Hi Kai,
> Tested-by: Kai Krakow <kai@xxxxxxxxxxx>
>
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(-)
[snip]
