linux-kernel - Re: [PATCH 2/2] powerpc: implement arch_scale_smt

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Date:	Wed, 20 Jan 2010 16:44:01 -0600
From:	Joel Schopp <jschopp@...tin.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	ego@...ibm.com, linuxppc-dev@...ts.ozlabs.org,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	benh@...nel.crashing.org,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [PATCH 2/2] powerpc: implement arch_scale_smt_power for Power7

Peter Zijlstra wrote:
> On Wed, 2010-01-20 at 14:04 -0600, Joel Schopp wrote:
>   
>> On Power7 processors running in SMT4 mode with 2, 3, or 4 idle threads 
>> there is performance benefit to idling the higher numbered threads in
>> the core.  
>>     
>
> So this is an actual performance improvement, not only power savings?
>   
Yes.

>
>
> And you just wrecked x86 ;-)
>
> It has an smt_power implementation that tries to measure smt gains using
> aperf/mperf, trouble is that this represents the actual performance not
> the capacity. This has the problem that when idle it represents 0
> capacity and will not attract work.
>
> Coming up with something that actually works there is on the todo list,
> I was thinking perhaps temporal maximums from !idle.
>
> So if you want to go with this, you'll need to stub out
> arch/x86/kernel/cpu/sched.c
>   

OK.  Guess I now will have a 3 patch series, with a patch to stub out 
the x86 broken version.

Care to take Gautham's bugfix patch (patch 1/2) now, since it just fixes 
a bug?  You'll need it if you ever try to make the x86 broken version work.
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