Re: block/throttle and burst bucket

[Alberto Garcia]

On Mon 01 Mar 2021 01:11:55 PM CET, Peter Lieven <pl@kamp.de> wrote:
> Why we talk about throttling I still do not understand the following part in 
> util/throttle.c function throttle_compute_wait
>
>
>     if (!bkt->max) {
>         /* If bkt->max is 0 we still want to allow short bursts of I/O
>          * from the guest, otherwise every other request will be throttled
>          * and performance will suffer considerably. */
>         bucket_size = (double) bkt->avg / 10;
>         burst_bucket_size = 0;
>     } else {
>         /* If we have a burst limit then we have to wait until all I/O
>          * at burst rate has finished before throttling to bkt->avg */
>         bucket_size = bkt->max * bkt->burst_length;
>         burst_bucket_size = (double) bkt->max / 10;
>     }
>
>
> Why burst_bucket_size = bkt->max / 10?
>
> From what I understand it should be bkt->max. Otherwise we compare the
> "extra" against a tenth of the bucket capacity

1) bkt->max is the burst rate in bytes/second [*]
2) burst_bucket_size is used to decide when to start throttling (you can
   see the code at the end of throttle_compute_wait()).

The important thing is that burst_bucket_size does not actually have an
influence on the actual burst rate. Increasing that value is not going
to make the I/O faster, it just means that I/O will be throttled later.

Once the I/O is throttled, the actual burst rate is define by how quick
the burst bucket leaks (see throttle_leak_bucket()).

The higher burst_bucket_size is, the longer we allow the guest to exceed
the maximum rate. So we divide blk->max by 10 in order to allow the
guest to perform 100ms' worth of I/O without being throttled.

See the commit message of 0770a7a6466cc2dbf4ac91841173ad4488e1fbc7 for
more details.

Berto