Date:   Mon, 20 Nov 2017 20:36:57 -0800
From:   Khazhismel Kumykov <khazhy@...gle.com>
To:     Shaohua Li <shli@...nel.org>
Cc:     shli@...com, vgoyal@...hat.com, tj@...nel.org,
        linux-kernel@...r.kernel.org, linux-block@...r.kernel.org
Subject: Re: [RFC PATCH] blk-throttle: add burst allowance.

On Fri, Nov 17, 2017 at 11:26 AM, Shaohua Li <shli@...nel.org> wrote:
> On Thu, Nov 16, 2017 at 08:25:58PM -0800, Khazhismel Kumykov wrote:
>> On Thu, Nov 16, 2017 at 8:50 AM, Shaohua Li <shli@...nel.org> wrote:
>> > On Tue, Nov 14, 2017 at 03:10:22PM -0800, Khazhismel Kumykov wrote:
>> >> Allows configuration additional bytes or ios before a throttle is
>> >> triggered.
>> >>
>> >> This allows implementation of a bucket style rate-limit/throttle on a
>> >> block device. Previously, bursting to a device was limited to allowance
>> >> granted in a single throtl_slice (similar to a bucket with limit N and
>> >> refill rate N/slice).
>> >>
>> >> Additional parameters bytes/io_burst_conf defined for tg, which define a
>> >> number of bytes/ios that must be depleted before throttling happens. A
>> >> tg that does not deplete this allowance functions as though it has no
>> >> configured limits. tgs earn additional allowance at rate defined by
>> >> bps/iops for the tg. Once a tg has *_disp > *_burst_conf, throttling
>> >> kicks in. If a tg is idle for a while, it will again have some burst
>> >> allowance before it gets throttled again.
>> >>
>> >> slice_end for a tg is extended until io_disp/byte_disp would fall to 0,
>> >> when all "used" burst allowance would be earned back. trim_slice still
>> >> does progress slice_start as before and decrements *_disp as before, and
>> >> tgs continue to get bytes/ios in throtl_slice intervals.
>> >
>> > Can you describe why we need this? It would be great if you can describe the
>> > usage model and an example. Does this work for io.low/io.max or both?
>> >
>> > Thanks,
>> > Shaohua
>> >
>>
>> Use case that brought this up was configuring limits for a remote
>> shared device. Bursting beyond io.max is desired but only for so much
>> before the limit kicks in, afterwards with sustained usage throughput
>> is capped. (This proactively avoids remote-side limits). In that case
>> one would configure in a root container io.max + io.burst, and
>> configure low/other limits on descendants sharing the resource on the
>> same node.
>>
>> With this patch, so long as tg has not dispatched more than the burst,
>> no limit is applied at all by that tg, including limit imposed by
>> io.low in tg_iops_limit, etc.
>
> I'd appreciate if you can give more details about the 'why'. 'configuring
> limits for a remote shared device' doesn't justify the change.

This is to configure a bursty workload (and associated device) with
known/allowed expected burst size, but to not allow full utilization
of the device for extended periods of time for QoS. During idle or low
use periods the burst allowance accrues, and then tasks can burst well
beyond the configured throttle up to the limit, afterwards is
throttled. A constant throttle speed isn't sufficient for this as you
can only burst 1 slice worth, but a limit of sorts is desirable for
preventing over utilization of the shared device. This type of limit
is also slightly different than what i understand io.low does in local
cases in that tg is only high priority/unthrottled if it is bursty,
and is limited with constant usage

Khazhy

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