linux-kernel - Re: [PATCH] pm_qos: Add system bus performance parameter

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Date:	Fri, 27 Aug 2010 12:17:14 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	skannan@...eaurora.org
Cc:	markgross@...gnar.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	James Bottomley <james.bottomley@...e.de>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jonathan Corbet <corbet@....net>, khilman@...prootsystems.com
Subject: Re: [PATCH] pm_qos: Add system bus performance parameter

On Fri, 2010-08-27 at 01:10 -0700, skannan@...eaurora.org wrote:
> Ignoring other details for now, the biggest problem with throughput/KBps
> units is that PM QoS can't handle it well in its current state. For KBps
> the requests should be added together before it's "enforced". Just picking
> the maximum won't work optimally.
> 
> Another problem with using KBps is that the available throughput is going
> to vary depending on the CPU frequency since the CPU running at a higher
> freq is going to use more bandwidth/throughput than the same CPU running
> at a lower freq.
> 
> A KHz unit will side step both problems. It's not the most ideal in theory
> but it's simple and gets the job done since, in our case, there aren't
> very many fine grained levels of system bus frequencies (and corresponding
> throughputs).
> 
> I understand that other architectures might have different practical
> constraints and abilities and I didn't want to impose the KHz limitation
> on them. That's the reason I proposed a parameter whose units is defined
> by the "enforcer".

Like Mark said, unit-less constraints are impossible to use correctly.
What if a driver moves from one platform to another?

Also, the KHz thing you propose simply doesn't make sense, given a fixed
bus width, KBs and KHz have a fixed ratio, and thus you get the exact
same problem you initially had and refused to fix.


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