From: Steve Muckle <steve.muckle@linaro.org>
To: Juri Lelli <juri.lelli@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, devicetree@vger.kernel.org,
peterz@infradead.org, vincent.guittot@linaro.org,
robh+dt@kernel.org, mark.rutland@arm.com, linux@arm.linux.org.uk,
sudeep.holla@arm.com, lorenzo.pieralisi@arm.com,
catalin.marinas@arm.com, will.deacon@arm.com,
morten.rasmussen@arm.com, dietmar.eggemann@arm.com,
broonie@kernel.org
Subject: Re: [PATCH v3 0/6] CPUs capacity information for heterogeneous systems
Date: Tue, 9 Feb 2016 09:30:05 -0800 [thread overview]
Message-ID: <56BA221D.1030907@linaro.org> (raw)
In-Reply-To: <20160209103748.GP11415@e106622-lin>
On 02/09/2016 02:37 AM, Juri Lelli wrote:
>> I'm still concerned that there's no way to obtain optimal boot time on a
>> > heterogeneous system. Either the dynamic benchmarking is enabled, adding
>> > 1 sec, or the benchmarking is skipped, and task distribution on the
>> > heterogeneous CPUs is determined by the platform's CPU numbering and
>> > chance, potentially impacting performance nondeterministically until
>> > userspace sets the correct capacity values via sysfs.
>> >
>> > I believe you tested the impact on boot time of using equal capacity
>> > values and saw little difference. I'm wondering though, what was the CPU
>> > numbering on that target?
>> >
>
> My targets (Juno and TC2) had big cluster on 1,2 and little on the
> remaining cpus. Why do you think this might matter?
There's a natural bias in the scheduler AFAIK towards lower-numbered
CPUs since they are typically scanned in numerically ascending order. So
when all capacities are initially defaulted to be the same I think
you'll be more likely to use the lower numbered CPUs.
I'd be curious what the performance penalty is on a b.L system where the
lowest numbered CPUs are small. I don't have such a target but maybe
it's possible to compare booting just with bigs vs just with littles, at
least until userspace intializes and a script can bring up the others,
which is the same point at which capacities could be properly set. That
would give something of an upper bound.
> Anyway, IMHO boot time performance is not what we are targeting here, so
> I wouldn't be too worried about this particular point.
It may not be the most important thing but it is a factor worth
considering - as mentioned earlier there are applications where boot
time is critical such as automotive. It seems unfortunate that actual
performance may be left on the table due to (IMO anyway) a tenuous
concern over DT semantics. But it looks like that may just be my
position :/ .
thanks,
Steve
next prev parent reply other threads:[~2016-02-09 17:30 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-03 11:59 [PATCH v3 0/6] CPUs capacity information for heterogeneous systems Juri Lelli
2016-02-03 11:59 ` [PATCH v3 1/6] ARM: initialize cpu_scale to its default Juri Lelli
2016-02-03 11:59 ` [PATCH v3 2/6] drivers/cpufreq: implement init_cpu_capacity_default() Juri Lelli
2016-02-03 21:04 ` Vincent Guittot
2016-02-04 9:36 ` Morten Rasmussen
2016-02-04 12:03 ` Vincent Guittot
2016-02-04 12:16 ` Juri Lelli
2016-02-04 12:35 ` Vincent Guittot
2016-02-04 14:13 ` Juri Lelli
2016-02-04 15:44 ` Vincent Guittot
2016-02-04 15:46 ` Vincent Guittot
2016-02-05 9:30 ` Juri Lelli
2016-02-09 15:54 ` Dietmar Eggemann
2016-02-10 14:25 ` Juri Lelli
2016-02-03 11:59 ` [PATCH v3 3/6] arm: Enable dynamic CPU capacity initialization Juri Lelli
2016-02-03 11:59 ` [PATCH v3 4/6] arm64: " Juri Lelli
2016-02-08 12:28 ` Dietmar Eggemann
2016-02-08 13:13 ` Mark Brown
2016-02-08 13:41 ` Dietmar Eggemann
2016-02-03 11:59 ` [PATCH v3 5/6] arm: add sysfs cpu_capacity attribute Juri Lelli
2016-02-03 11:59 ` [PATCH v3 6/6] arm64: " Juri Lelli
2016-02-05 17:19 ` Dietmar Eggemann
2016-02-05 17:49 ` Juri Lelli
2016-02-08 23:59 ` [PATCH v3 0/6] CPUs capacity information for heterogeneous systems Steve Muckle
2016-02-09 10:37 ` Juri Lelli
2016-02-09 17:30 ` Steve Muckle [this message]
2016-02-09 17:40 ` Juri Lelli
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