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Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computa
From: |
Zhi Yong Wu |
Subject: |
Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation |
Date: |
Thu, 21 Mar 2013 09:18:27 +0800 |
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
- [Qemu-devel] [PATCH] Fix I/O throttling pathologic oscillating behavior, Benoît Canet, 2013/03/20
- [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Benoît Canet, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Zhi Yong Wu, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Stefan Hajnoczi, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Stefan Hajnoczi, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Benoît Canet, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Stefan Hajnoczi, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation,
Zhi Yong Wu <=
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Stefan Hajnoczi, 2013/03/21
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Zhi Yong Wu, 2013/03/21
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Stefan Hajnoczi, 2013/03/21
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Benoît Canet, 2013/03/20
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Stefan Hajnoczi, 2013/03/21
- Re: [Qemu-devel] [PATCH] block: fix bdrv_exceed_iops_limits wait computation, Benoît Canet, 2013/03/21