linux-kernel - Re: [PATCH] cpufreq: governor: Be friendly towards latency-sensitive bursty workloads

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Date:	Sat, 7 Jun 2014 11:55:59 +0200
From:	Pavel Machek <pavel@....cz>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	rjw@...ysocki.net, viresh.kumar@...aro.org,
	svaidy@...ux.vnet.ibm.com, ego@...ux.vnet.ibm.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: governor: Be friendly towards
 latency-sensitive bursty workloads

Hi!

> We just want to avoid the stupidity of dropping down the frequency to a minimum
> and then enduring a needless (and long) delay before ramping it up back again.
> So, let us simply carry forward the previous load - that is, let us just pretend
> that the 'load' for the current time-window is the same as the load for the
> previous window. That way, the frequency and voltage will continue to be set
> to whatever values they were set at previously. This means that bursty workloads
> will get a chance to influence the CPU frequency at which they wake up from
> cpu-idle, based on their past execution history. Thus, they might be able to
> avoid suffering from slow wakeups and long response-times.
> 
> [ The right way to solve this problem is to teach the CPU frequency governors
> to track load on a per-task basis, not a per-CPU basis, and set the appropriate
> frequency on whichever CPU the task executes. But that involves redesigning
> the cpufreq subsystem, so this patch should make the situation bearable until
> then. ]

Are you sure? For example "./configure" load consists of a lot of
short-lived tasks. Per-task basis may not be option for that.

> A rudimentary and somewhat approximately latency-sensitive workload such as
> sleeping-ebizzy itself showed a consistent, noticeable performance improvement
> with this patch. Hence, workloads that are truly latency-sensitive will benefit
> quite a bit from this change. Moreover, this is an overall win-win since this
> patch does not hurt power-savings at all (because, this patch does not reduce
> the idle time or idle residency; and the high frequency of the CPU when it goes
> to cpu-idle does not affect/hurt the power-savings of deep idle
> states).

Are you sure about win-win?

AFAICT, your patch helps

##########.........#########.........###########...........##########............

case (not surprising, that's why you wrote the patch :-), but what happens in

##########.........#.................#.....................#.....................

case? (That is idle system, with some tasks taking very small ammounts
of CPU).

AFAICT, it will remember the (high) prev_load over the idle period,
and use too high frequency for mostly idle system, no?

									Pavel

-- 
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