On 08/18/2017 10:28 AM, Shaohua Li wrote: > On Fri, Aug 18, 2017 at 09:35:01AM -0600, Jens Axboe wrote: >> On 08/18/2017 09:13 AM, Shaohua Li wrote: >>> discard request usually is very big and easily use all bandwidth budget >>> of a cgroup. discard request size doesn't really mean the size of data >>> written, so it doesn't make sense to account it into bandwidth budget. >>> This patch ignores discard requests size. It makes sense to account >>> discard request into iops budget though. >> >> Some (most) devices to touch media for a discard operation, but the >> cost tends to be fairly constant and independent of discard size. >> Would it make sense to just treat it as a constant cost? Zero >> cost seems wrong. > > that would be hard to find the cost. Would this make sense? > > min_t(unsigned int, bio->bi_iter.bi_size, queue_max_sectors(q) << 9) It's all going to be approximations, for sure, unfortunately it isn't an exact science. Why not just use a constant small value? If we assume that a 4k and 8M discard end up writing roughly the same to media, then it would follow that just using a smaller constant value (regardless of actual discard command size) would be useful. -- Jens Axboe
