Re: block/throttle and burst bucket

[Peter Lieven]

Am 01.03.21 um 11:59 schrieb Kevin Wolf:
> Am 26.02.2021 um 13:33 hat Peter Lieven geschrieben:
>> Am 26.02.21 um 10:27 schrieb Alberto Garcia:
>>> On Thu 25 Feb 2021 06:34:48 PM CET, Peter Lieven <pl@kamp.de> wrote:
>>>> I was wondering if there is a way to check from outside (qmp etc.) if
>>>> a throttled block device has exceeded the iops_max_length seconds of
>>>> time bursting up to iops_max and is now hard limited to the iops limit
>>>> that is supplied?
>>>>
>>>> Would it be also a good idea to exetend the accounting to account for
>>>> requests that must have waited before being sent out to the backend
>>>> device?
>>> No, there's no such interface as far as I'm aware. I think one problem
>>> is that throttling is now done using a filter, that can be inserted
>>> anywhere in the node graph, and accounting is done at the BlockBackend
>>> level.
>>>
>>> We don't even have a query-block-throttle function. I actually started
>>> to write one six years ago but it was never finished.
>>
>> A quick idea that came to my mind was to add an option to emit a QMP
>> event if the burst_bucket is exhausted and hard limits are enforced.
> Do you actually need to do something every time that it's exceeded, so
> QEMU needs to be the active part sending out an event, or is it
> something that you need to check in specific places and could reasonably
> query on demand?
>
> For the latter, my idea would have been adding a new read-only QOM
> property to the throttle group object that exposes how much is still
> left. When it becomes 0, the hard limits are enforced.
>
>> There seems to be something wrong in the throttling code anyway.
>> Throttling causes addtional i/o latency always even if the actual iops
>> rate is far away from the limits and ever more far away from the burst
>> limits. I will dig into this.
>>
>> My wishlist:
>>
>>  - have a possibility to query the throttling state.
>>  - have counters for no of delayed ops and for how long they were delayed.
>>  - have counters for untrottled <= 4k request performance for a backend 
>> storage device.
>>
>> The later two seem not trivial as you mentioned.
> Do you need the information per throttle node or per throttle group? For
> the latter, the same QOM property approach would work.


Hi Kevin,


per throttle-group information would be sufficient. So you would expose the the 
level of the bucket and

additionally a counter for throttled vs. total ops and total delay?


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

and schedule a timer where it is not necessary.


What am I missing here?



Peter