From: Song Liu <songliubraving@fb.com>
To: "Michal Koutný" <mkoutny@suse.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>,
Kernel Team <Kernel-team@fb.com>,
"peterz@infradead.org" <peterz@infradead.org>,
"vincent.guittot@linaro.org" <vincent.guittot@linaro.org>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"mingo@redhat.com" <mingo@redhat.com>,
"cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/7] introduce cpu.headroom knob to cpu controller
Date: Tue, 21 May 2019 16:27:02 +0000 [thread overview]
Message-ID: <D9376488-F290-4917-9124-292AA649948C@fb.com> (raw)
In-Reply-To: <20190521134730.GA12346@blackbody.suse.cz>
Hi Michal,
> On May 21, 2019, at 6:47 AM, Michal Koutný <mkoutny@suse.com> wrote:
>
> Hello Song.
>
> On Wed, Apr 10, 2019 at 07:43:35PM +0000, Song Liu <songliubraving@fb.com> wrote:
>> The load level above is measured as requests-per-second.
>>
>> When there is no side workload, the system has about 45% busy CPU with
>> load level of 1.0; and about 75% busy CPU at load level of 1.5.
>>
>> The saturation starts before the system hitting 100% utilization. This is
>> true for many different resources: ALUs in SMT systems, cache lines,
>> memory bandwidths, etc.
> I have read through the thread continuation and it appears to me there
> is some misunderstanding on the latency metric (scheduler latency <=
> your latency <= request wall time?).
We define "your latency" == "request wall time" > "scheduler latency".
The application is a web server. It works as:
for a few iterations:
(a) request data from cache/db
(b) wait for data
(c) data is ready, wait to be scheduled
(d) render web page based on data
We need to do a few iterations because we need some data to decide what
other data to fetch.
The overall latency (or wall latency) contains:
(1) cpu time, which is (a) and (d) in the loop above;
(2) time waiting for data, which is (b);
(3) schedule latency, time between data is ready and the thread wakes
up, which is (c);
In our experiment, we can measure (1) and "(1)+(2)+(3)". For data in the
following table. "cpu time" is (1), "wall time" is (1)+(2)+(3), and
"wall - cpu" is "(2)+(3)". We assume (2) doesn't change, so changes in
"wall - cpu" is mostly due to changes in (3).
side job | cpu.headroom | cpu-idle | wall time | cpu time | wall - cpu
------------------------------------------------------------------------
none | n/a | 42.4% | 1.00 | 0.31 | 0.69
ffmpeg | 0 | 10.8% | 1.17 | 0.38 | 0.79
ffmpeg | 25% | 22.8% | 1.08 | 0.35 | 0.73
Does this make sense?
Thanks,
Song
next prev parent reply other threads:[~2019-05-21 16:28 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-04-08 21:45 [PATCH 0/7] introduce cpu.headroom knob to cpu controller Song Liu
2019-04-08 21:45 ` [PATCH 1/7] sched: refactor tg_set_cfs_bandwidth() Song Liu
2019-04-08 21:45 ` [PATCH 2/7] cgroup: introduce hook css_has_tasks_changed Song Liu
2019-04-08 21:45 ` [PATCH 3/7] cgroup: introduce cgroup_parse_percentage Song Liu
2019-04-08 21:45 ` [PATCH 4/7] sched, cgroup: add entry cpu.headroom Song Liu
2019-04-08 21:45 ` [PATCH 5/7] sched/fair: global idleness counter for cpu.headroom Song Liu
2019-04-08 21:45 ` [PATCH 6/7] sched/fair: throttle task runtime based on cpu.headroom Song Liu
2019-04-08 21:45 ` [PATCH 7/7] Documentation: cgroup-v2: add information for cpu.headroom Song Liu
2019-04-10 11:59 ` [PATCH 0/7] introduce cpu.headroom knob to cpu controller Morten Rasmussen
2019-04-10 19:43 ` Song Liu
2019-04-17 12:56 ` Vincent Guittot
2019-04-22 23:22 ` Song Liu
2019-04-28 19:47 ` Song Liu
2019-04-29 12:24 ` Vincent Guittot
2019-04-30 6:10 ` Song Liu
2019-04-30 16:20 ` Vincent Guittot
2019-04-30 16:54 ` Song Liu
2019-05-10 18:22 ` Song Liu
2019-05-14 20:58 ` Song Liu
2019-05-15 10:18 ` Vincent Guittot
2019-05-15 15:42 ` Song Liu
2019-05-21 13:47 ` Michal Koutný
2019-05-21 16:27 ` Song Liu [this message]
2019-06-26 8:26 ` Michal Koutný
2019-06-26 15:56 ` Song Liu
2019-04-15 16:48 ` Song Liu
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