Re: testing io.low limit for blk-throttle

From: Joseph Qi
Date: Thu Apr 26 2018 - 23:28:04 EST

Hi Paolo,

On 18/4/27 01:27, Paolo Valente wrote:
>> Il giorno 25 apr 2018, alle ore 14:13, Joseph Qi <jiangqi903@xxxxxxxxx> ha scritto:
>> Hi Paolo,
> Hi Joseph
>> ...
>> Could you run blktrace as well when testing your case? There are several
>> throtl traces to help analyze whether it is caused by frequently
>> upgrade/downgrade.
> Certainly. You can find a trace attached. Unfortunately, I'm not
> familiar with the internals of blk-throttle and low limit, so, if you
> want me to analyze the trace, give me some hints on what I have to
> look for. Otherwise, I'll be happy to learn from your analysis.

I've taken a glance at your blktrace attached. It is only upgrade at first and
then downgrade (just adjust limit, not to LIMIT_LOW) frequently.
But I don't know why it always thinks throttle group is not idle.

For example:
fio-2336 [004] d... 428.458249: 8,16 m N throtl avg_idle=90, idle_threshold=1000, bad_bio=10, total_bio=84, is_idle=0, scale=9
fio-2336 [004] d... 428.458251: 8,16 m N throtl downgrade, scale 4

In throtl_tg_is_idle():
is_idle = ... ||
(tg->latency_target && tg->bio_cnt &&
tg->bad_bio_cnt * 5 < tg->bio_cnt);

It should be idle and allow run more bandwidth. But here the result shows not
idle (is_idle=0). I have to do more investigation to figure it out why.

You can also filter these logs using:
grep throtl trace | grep -E 'upgrade|downgrade|is_idle'