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

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Date:	Wed, 16 Oct 2013 14:09:40 +0800
From:	Hong zhi guo <honkiko@...il.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Jens Axboe <axboe@...nel.dk>, cgroups@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, linux-kernel@...r.kernel.org,
	Hong Zhiguo <zhiguohong@...cent.com>
Subject: Re: [PATCH v2] blk-throttle: simplify logic by token bucket algorithm

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 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.

Waiting for your comments.

Zhiguo


On Wed, Oct 16, 2013 at 1:32 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Mon, Oct 14, 2013 at 05:09:17PM +0800, Hong Zhiguo wrote:
>
> [..]
>
> Hi Hong,
>
> Thanks for the token based throttling implementation. In general it looks
> good and it simplies the logic. Couple of comments/concerns below.
>
>> @@ -133,14 +135,13 @@ struct throtl_grp {
>>       /* IOPS limits */
>>       unsigned int iops[2];
>>
>> -     /* Number of bytes disptached in current slice */
>> -     uint64_t bytes_disp[2];
>> -     /* Number of bio's dispatched in current slice */
>> -     unsigned int io_disp[2];
>> +     /* Token for throttling by bps */
>> +     uint64_t bytes_token[2];
>> +     /* Token for throttling by iops */
>> +     unsigned int io_token[2];
>>
>> -     /* When did we start a new slice */
>> -     unsigned long slice_start[2];
>> -     unsigned long slice_end[2];
>> +     /* Time check-point */
>> +     unsigned long t_c[2];
>
> Can we rename this variable to say last_dispatch[2].
>
> [..]
>>
>> @@ -852,41 +708,26 @@ static bool tg_with_in_bps_limit(struct throtl_grp *tg, struct bio *bio,
>>                                unsigned long *wait)
>>  {
>>       bool rw = bio_data_dir(bio);
>> -     u64 bytes_allowed, extra_bytes, tmp;
>> -     unsigned long jiffy_elapsed, jiffy_wait, jiffy_elapsed_rnd;
>> -
>> -     jiffy_elapsed = jiffy_elapsed_rnd = jiffies - tg->slice_start[rw];
>> -
>> -     /* Slice has just started. Consider one slice interval */
>> -     if (!jiffy_elapsed)
>> -             jiffy_elapsed_rnd = throtl_slice;
>> -
>> -     jiffy_elapsed_rnd = roundup(jiffy_elapsed_rnd, throtl_slice);
>> +     u64 extra_bytes, token;
>> +     unsigned long jiffy_wait;
>>
>> -     tmp = tg->bps[rw] * jiffy_elapsed_rnd;
>> -     do_div(tmp, HZ);
>> -     bytes_allowed = tmp;
>> +     token = (u64)tg->bps[rw] * (jiffies - tg->t_c[rw]);
>
> So here we seem to be calculating how many tokens a group is eligible
> for. This is done based on since last time check. And last time check
> was updated in last dispatch from the group.
>
> What happens if no IO happens in a group for some time, say for 5 minutes,
> and then one big IO comes in. IIUC, we will allow a big burst of IO for
> the first IO (tokens worth of full 5 minutes will be given to this group).
> I think we should have a mechanism where we don't allow too big a burst
> for the first IO. (Possibly limit the burst to THROTL_BURST_IO and
> THROTL_BURST_BYTES until and unless a bigger IO is already queued and
> we are waiting for it).
>
> In previous time slice logic, I took care of it by extend time slice
> by 100ms and if no IO happens in the group for 100ms, time slice will
> expire and next IO will get at max of 100ms of worth of credit (equivalent
> of burst limits).
>
> Thanks
> Vivek



-- 
best regards
Hong Zhiguo
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