Re: [PATCH v2] blk-throttle: simplify logic by token bucket algorithm

[Hong zhi guo]

Hi, Vivek,

Thanks for your elaboration. I got your points. I'll update the patch
to have such logic.

Do you think adding below logic in tg_with_in_bps_limit enough?
if (!sq->nr_queued[rw]) {
    trim the token to bucket depth;
}

Thanks

On Wed, Oct 16, 2013 at 10:14 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Wed, Oct 16, 2013 at 02:09:40PM +0800, Hong zhi guo wrote:
>> Hi, Vivek,
>>
>> Thanks for your careful review. I'll rename t_c to last_dispatch, it's
>> much better.
>>
>> For the big burst issue, I've different opinion. Let's discuss it.
>>
>> Any time a big IO means a big burst. Even if it's throttled at first
>> time, queued in
>> the service_queue, and then waited for a while, when it's delivered
>> out, it IS still
>> a big burst. So throttling the bio for another while makes no sense.
>
> If a malicious application is creating a big BIO and sending it out, then
> effective IO rate as seen by application will be much higher than
> throttling limit.
>
> So yes, a burst is anyway going to happen when big IO is dispatched
> to disk, but the question is when should that burst be allowed. What's
> the effective IO rate application should see.
>
>>
>> If a group has been idle for 5 minutes, then it owns the credit to
>> deliver a big IO
>> (within 300 * bps bytes). And the extra credit will be cut off after
>> the delivery.
>
> I think there are couple of issues here.
>
> - First of all, if you think that a group is entitiled for tokens even
>   when it is not doing IO, then why are you truncating the tokens after
>   dispatch of a BIO.
>
> - Second in general it does not seem right that a group is entitiled to
>   tokens even when no IO is happening or group is not backlogged. That
>   would mean a group will not do IO for 10 hours and then be entitiled
>   to those tokens suddenly after 10 hours with a huge burst.
>
> So I think you also agree that a group should not be entitiled to
> tokens when group is not backlogged and that's why you seem to be
> truncating extra tokens after dispatch of a BIO. If that's the case,
> then even for first BIO, ideally a group should not be given tokens
> for idle time.
>
> Just think that an application has a huge BIO, say size 2MB. And group
> has limit of say 8KB per second. Now if group has been idling long enough,
> this BIO will be dispatched immediately. And effective rate a group
> will be is much higher than 8KB/s. Which is not right, IMO.
>
> If you argue that token entitilement for idle groups is not right and
> doing it for first BIO in a batch is exception for simplicity reasons,
> that still might be fine. But right now that does not seem to be the
> case.
>
> Thanks
> Vivek



-- 
best regards
Hong Zhiguo
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