Re: testing io.low limit for blk-throttle

From: Joseph Qi <jiangqi903@gmail.com>
To: Paolo Valente <paolo.valente@linaro.org>,
	linux-block <linux-block@vger.kernel.org>,
	Jens Axboe <axboe@kernel.dk>, Shaohua Li <shli@fb.com>,
	Mark Brown <broonie@kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Ulf Hansson <ulf.hansson@linaro.org>
Subject: Re: testing io.low limit for blk-throttle
Date: Mon, 23 Apr 2018 14:05:51 +0800	[thread overview]
Message-ID: <4c6b86d9-1668-43c3-c159-e6e23ffb04b4@gmail.com> (raw)
In-Reply-To: <A749046B-BEB9-4278-ABEF-3007817D59DD@linaro.org>

Hi Paolo,
What's your idle and latency config?
IMO, io.low will allow others run more bandwidth if cgroup's average
idle time is high or latency is low. In such cases, low limit won't get
guaranteed.

Thanks,
Joseph

On 18/4/22 17:23, Paolo Valente wrote:
> Hi Shaohua, all,
> at last, I started testing your io.low limit for blk-throttle.  One of
> the things I'm interested in is how good throttling is in achieving a
> high throughput in the presence of realistic, variable workloads.
> 
> However, I seem to have bumped into a totally different problem.  The
> io.low parameter doesn't seem to guarantee what I understand it is meant
> to guarantee: minimum per-group bandwidths.  For example, with
> - one group, the interfered, containing one process that does sequential
>   reads with fio
> - io.low set to 100MB/s for the interfered
> - six other groups, the interferers, with each interferer containing one
>   process doing sequential read with fio
> - io.low set to 10MB/s for each interferer
> - the workload executed on an SSD, with a 500MB/s of overall throughput
> the interfered gets only 75MB/s.
> 
> In particular, the throughput of the interfered becomes lower and
> lower as the number of interferers is increased.  So you can make it
> become even much lower than the 75MB/s in the example above.  There
> seems to be no control on bandwidth.
> 
> Am I doing something wrong?  Or did I simply misunderstand the goal of
> io.low, and the only parameter for guaranteeing the desired bandwidth to
> a group is io.max (to be used indirectly, by limiting the bandwidth of
> the interferers)?
> 
> If useful for you, you can reproduce the above test very quickly, by
> using the S suite [1] and typing:
> 
> cd thr-lat-with-interference
> sudo ./thr-lat-with-interference.sh -b t -w 100000000 -W "10000000 10000000 10000000 10000000 10000000 10000000" -n 6 -T "read read read read read read" -R "0 0 0 0 0 0"
> 
> Looking forward to your feedback,
> Paolo
> 
> [1] 
> 