On Wed, Dec 07, 2022 at 12:38:26AM +0800, Jinke Han wrote: > From: Jinke Han <hanjinke.666@xxxxxxxxxxxxx> > > Now we don't distinguish sync write ios from normal buffer write ios > in blk-throtl. A bio with REQ_SYNC tagged always mean it will be wait > until write completion soon after it submit. So it's reasonable for sync > io to complete as soon as possible. > > In our test, fio writes a 100g file in sequential 4k blocksize in > a container with low bps limit configured (wbps=10M). More than 1200 > ios were throttled in blk-throtl queue and the avarage throtle time > of each io is 140s. At the same time, the operation of saving a small > file by vim will be blocked amolst 140s. As a fsync will be send by vim, > the sync ios of fsync will be blocked by a huge amount of buffer write > ios ahead. This is also a priority inversion problem within one cgroup. > In the database scene, things got really bad with blk-throtle enabled > as fsync is called very often. > > This patch introduces a independent sync queue for write ios and gives > a huge priority to sync write ios. I think it's a nice respond to the > semantics of REQ_SYNC. Bios with REQ_META and REQ_PRIO gains the same > priority as they are important to fs. This may avoid some potential > priority inversion problems. I think the idea makes sense but wonder whether the implementation would be cleaner / simpler if the sq->queued[] are indexed by SYNC, ASYNC and the sync writes are queued in the sync queue together with reads. Thanks. -- tejun
