On Thu, Mar 21, 2013 at 09:18:27AM +0800, Zhi Yong Wu wrote: > On Wed, 2013-03-20 at 16:12 +0100, Stefan Hajnoczi wrote: > > On Wed, Mar 20, 2013 at 03:56:33PM +0100, Benoît Canet wrote: > > > > But I don't understand why bs->slice_time is modified instead of keeping > > > > it constant at 100 ms: > > > > > > > > bs->slice_time = wait_time * BLOCK_IO_SLICE_TIME * 10; > > > > bs->slice_end += bs->slice_time - 3 * BLOCK_IO_SLICE_TIME; > > > > if (wait) { > > > > *wait = wait_time * BLOCK_IO_SLICE_TIME * 10; > > > > } > > > > > > In bdrv_exceed_bps_limits there is an equivalent to this with a comment. > > > > > > --------- > > > /* When the I/O rate at runtime exceeds the limits, > > > * bs->slice_end need to be extended in order that the current > > > statistic > > > * info can be kept until the timer fire, so it is increased and tuned > > > * based on the result of experiment. > > > */ > > > bs->slice_time = wait_time * BLOCK_IO_SLICE_TIME * 10; > > > bs->slice_end += bs->slice_time - 3 * BLOCK_IO_SLICE_TIME; > > > if (wait) { > > > *wait = wait_time * BLOCK_IO_SLICE_TIME * 10; > > > } > > > ---------- > > > > The comment explains why slice_end needs to be extended, but not why > > bs->slice_time should be changed (except that it was tuned as the result > > of an experiment). > > > > Zhi Yong: Do you remember a reason for modifying bs->slice_time? > Stefan, > In some case that the bare I/O speed is very fast on physical machine, > when I/O speed is limited to be one lower value, I/O need to wait for > one relative longer time(i.e. wait_time). You know, wait_time should be > smaller than slice_time, if slice_time is constant, wait_time may not be > its expected value, so the throttling function will not work well. > For example, bare I/O speed is 100MB/s, I/O throttling speed is 1MB/s, > slice_time is constant, and set to 50ms(a assumed value) or smaller, If > current I/O can be throttled to 1MB/s, its wait_time is expected to > 100ms(a assumed value), and is more bigger than current slice_time, I/O > throttling function will not throttle actual I/O speed well. In the > case, slice_time need to be adjusted to one more suitable value which > depends on wait_time.
When an I/O request spans a slice: 1. It must wait until enough resources are available. 2. We extend the slice so that existing accounting is not lost. But I don't understand what you say about a fast host. The bare metal throughput does not affect the throttling calculation. The only values that matter are bps limit and slice time: In your example the slice time is 50ms and the current request needs 100ms. We need to extend slice_end to at least 100ms so that we can account for this request. Why should slice_time be changed? > In some other case that the bare I/O speed is very slow and I/O > throttling speed is fast, slice_time also need to be adjusted > dynamically based on wait_time. If the host is slower than the I/O limit there are two cases: 1. Requests are below I/O limit. We do not throttle, the host is slow but that's okay. 2. Requests are above I/O limit. We throttle them but actually the host will slow them down further to the bare metal speed. This is also fine. Again, I don't see a nice to change slice_time. BTW I discovered one thing that Linux blk-throttle does differently from QEMU I/O throttling: we do not trim completed slices. I think trimming avoids accumulating values which may lead to overflows if the slice keeps getting extended due to continuous I/O. blk-throttle does not modify throtl_slice (their equivalent of slice_time). Stefan