Re: [PATCH 1/1] bcache: set max writeback rate when I/O request is idle

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Sep 09, 2018 at 02:15:07PM +0200, Kai Krakow wrote:
> From: 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.
> 
> 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>
> Tested-by: Kai Krakow <kai@kaishome.de>
> Tested-by: Stefan Priebe <s.priebe@profihost.ag>
> Cc: Michael Lyle <mlyle@lyle.org>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> (cherry picked from commit ea8c5356d39048bc94bae068228f51ddbecc6b89)
> Signed-off-by: Kai Krakow <kai@kaishome.de>

Given the size of this, and the fact that no one responded, I'm going to
drop this from my review queue now, sorry.  If you get some testers,
please feel free to resend it.

thanks,

greg k-h