Re: [Qemu-block] [Qemu-devel] [PATCH v2 2/7] throttle: Update the throttle_fix_bucket() documentation

[Eric Blake]

On 08/24/2017 08:24 AM, Alberto Garcia wrote:
> The way the throttling algorithm works is that requests start being
> throttled once the bucket level exceeds the burst limit. When we get
> there the bucket leaks at the level set by the user (bkt->avg), and
> that leak rate is what prevents guest I/O from exceeding the desired
> limit.
> 
> If we don't allow bursts (i.e. bkt->max == 0) then we can start
> throttling requests immediately. The problem with keeping the
> threshold at 0 is that it only allows one request at a time, and as
> soon as there's a bit of I/O from the guest every other request will
> be throttled and performance will suffer considerably. That can even
> make the guest unable to reach the throttle limit if that limit is
> high enough, and that happens regardless of the block scheduler used
> by the guest.
> 
> Increasing that threshold gives flexibility to the guest, allowing it
> to perform short bursts of I/O before being throttled. Increasing the
> threshold too much does not make a difference in the long run (because
> it's the leak rate what defines the actual throughput) but it does
> allow the guest to perform longer initial bursts and exceed the
> throttle limit for a short while.
> 
> A burst value of bkt->avg / 10 allows the guest to perform 100ms'
> worth of I/O at the target rate without being throttled.
> 
> Signed-off-by: Alberto Garcia <address@hidden>
> ---
>  util/throttle.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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