On Tue, Jan 10, 2023 at 11:59:09PM +0800, hanjinke wrote: > > > 在 2023/1/10 上午4:40, Tejun Heo 写道: > > On Sat, Jan 07, 2023 at 09:07:38PM +0800, Jinke Han wrote: > > > + * Assumed that there were only bios queued in ASYNC queue and the SYNC > > > + * queue was empty. The ASYNC bio was selected to dispatch and the > > > + * disp_sync_cnt was set to 0 after each dispatching. If a ASYNC bio > > > + * can't be dispatched because of overlimit in current slice, the process > > > + * of dispatch should give up and the spin lock of the request queue > > > + * may be released. A new SYNC bio may be queued in the SYNC queue then. > > > + * When it's time to dispatch this tg, the SYNC bio was selected and pop > > > + * to dispatch as the disp_sync_cnt is 0 and the SYNC queue is no-empty. > > > + * If the dispatched bio is smaller than the waiting bio, the bandwidth > > > + * may be hard to satisfied as the slice may be trimed after each dispatch. > > > > I still can't make a good sense of this scenario. Can you give concrete > > example scenarios with IOs and why it would matter? > > > > Thanks. > > > > Assumed that there are many buffer write bios queued in ASYNC queue and the > SYNC queue is empty. The buffer write bios are all 1M in size and the bps > limit is 1M/s. The throtl_slice is 100ms. > > Assumed that the start/end_slice is [jiffies1, jiffies1+100] and > bytes_disp[w] = 0. The next ASYNC bio can't dispatch because of overlimit > within this slice. The wait time is 900ms and the slice will be extended to > [jiffies1, jiffies1 + 1000] in tg_may_dispatch. > > During the waiting of the ASYNC bio, a SYNC 4k bio be queued in SYNC queue. > After 900ms, it's time to dispatch the ASYNC io, but the SYNC 4k bio be > selected to be dispatched. Now the slice is [jiffies1, jiffies1+1000] and > the byte_disp[w] = 4k. The slice may be extended to > [jiffies1, jiffies1+1100]. In tg_dispatch_one_bio, the slice will be trimed > to [jiffies1+1000, jiffies1+1100], the byte_disp[w] will set 0. > > After the 4k SYNC bio be dispatched, the WAITING ASYNC bio still cann't > be dispatched because of overlimit within this slice. > > The same thing may happen DISPACH_SYNC_FACTOR times if alway there is a SYNC > bio be queued in the SYNC queue when the ASYNC bio is waiting. > > This means that in nearly 5s, we have dispathed 4 4k SYNC bios and a 1m > ASYNC bio. > > In our test, with 100M/s bps limit setted, the bps only reach to ~80m/s > when a fio generate buffer write ios and fsync continuous generated by > dbench in same cgroup. That makes sense to me. Thanks for the explanation. It'd probably a good idea to expand the explanation in the comment tho. Thanks. -- tejun
