linux-kernel - Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86

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Message-ID: <20080527071900.25329ae1@infradead.org>
Date:	Tue, 27 May 2008 07:19:00 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	svaidy@...ux.vnet.ibm.com
Cc:	balbir@...ux.vnet.ibm.com,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	venkatesh.pallipadi@...el.com, suresh.b.siddha@...el.com,
	Michael Neuling <mikey@...ling.org>,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86

\> > 
> > that's a case where it really makes sense; it's the case where the
> > thing that controls the cpu P-state actually learns about how much
> > work was done to reevaluate what the cpu frequency should be going
> > forward. Eg it's a case of comparing actual frequency (APERF/MPERF)
> > to see what's useful to set next.
> > IDA makes this all needed due to the dynamic nature of the concept
> > of "frequency".
> 
> Scaled statistics relative to maximum CPU capacity is just a method of
> exposing the actual CPU utilisation of applications independent of CPU
> frequency changes.
> 
> Reason behind the metric is same as the above fact that you have
> mentioned.  The CPU frequency governors cannot make decisions only
> based on idle time ratio.  It needs to know current utilisation (used
> cycles) relative to maximum capacity so that the frequency can be
> changed to next higher level.
> 
> Higher level management software that wants to control CPU capacity
> externally will need similar information.
>
I entirely understand that desire.
But you're not giving it that information!
The patch is giving it a really poor approximation, an approximation
that will get worse and worse in upcoming cpu generations.
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