Re: [PATCH v3 2/5] bcache: implement PI controller for writeback rate

[Coly Li <i@coly.li>]

On 2017/10/8 下午12:57, Michael Lyle wrote:
> Coly--
> 
> 
> On 10/07/2017 09:22 PM, Coly Li wrote:
> [snip]
>> rate:        488.2M/sec
>> dirty:        91.7G
>> target:        152.3G
>> proportional:    -1.5G
>> integral:    10.9G
>> change:        0.0k/sec
>> next io:    0ms
> [snip]
> 
>> The backing cached device size is 7.2TB, cache device is 1.4TB, block
>> size is 8kB only. I write 700G (50% of cache device size) dirty data
>> onto the cache device, and start writeback by echo 1 to
>> writeback_running file.
>>
>> In my test, writeback spent 89 minutes to decrease dirty number from
>> 700G to 147G (dirty target number is 152G). At this moment writeback
>> rate was still displayed as 488.2M/sec. And after 22 minutes writeback
>> rate jumped to 4.0k/sec. During the 22 minutes, (147-15.8=) 131.2G dirty
>> data written out.
> 
> I see it-- if we can write faster than 488MB/sec, we inappropriately
> clamp the write rate to 488MB/sec-- this is from the old code.  In turn,
> if we're keeping up at that speed, the integral term can wind up.  I
> will fix this and clean up a couple related things.
> 
>> Is it as expected ?
> 
> It is supposed to overshoot the target, but not by this much.
> 
> I think the implementation must have changed at some point in the past
> for bch_ratelimit, because the clamping doesn't match. bch_ratelimit
> really needs a rewrite for other reasons, but I'll send the minimal fix
> now.

Hi Mike,

Copied, I will continue my test. And when the new version comes, I will
run same workload again to confirm.

Thanks.


-- 
Coly Li