Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation

[Zhi Yong Wu]

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.
  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 i remember correctly, it's the reason.

> 
> Stefan
> 

-- 
Regards,

Zhi Yong Wu