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

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Date:	Tue, 27 May 2008 09:40:35 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	svaidy@...ux.vnet.ibm.com
Cc:	Linux Kernel <linux-kernel@...r.kernel.org>,
	venkatesh.pallipadi@...el.com, suresh.b.siddha@...el.com,
	Michael Neuling <mikey@...ling.org>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Amit K. Arora" <aarora@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 0/3] Scaled statistics using APERF/MPERF in x86

On Tue, 27 May 2008 19:34:40 +0530
Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com> wrote:
> 
> What we are proposing is a scaled time value that is scaled to the
> current CPU capacity.  If the scaled utilisation is 50% when the CPU
> is at 100% capacity, it is expected to remain at 50% even if the CPU's
> capacity is dropped to 50%, while the traditional utilisation value
> will be 100%.

When you use the word "capacity" I cringe ;(

> 
> The problem in the above two cases is that we had assumed that the
> maximum CPU capacity is 100% at normal capacity (without IDA).
> 
> If the CPU is at half the maximum frequency, then scaled stats should
> show 50%.  

see frequency != capacity.
It's about more than frequency. It's about how much cache you have
available too. If  you run single threaded on a dual core cpu, you have
100% of the cache, but the cpu is 50% idle. But that doesn't mean that
when you double the load, you actually get 2x the performance. So
you're not at 50% of capacity!



> The point I am trying to make is whether scaling should be done
> relative to CPUs designed maximum capacity or maximum capacity under
> current constraints is to be discussed.

now you're back at "capacity".. we were at frequency before ;(


> 
> Case A:
> ------
> 
> Scaled stats is stats relative to maximum designed capacity including
> IDA

you don't know what that is though.

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