linux-kernel - Re: [PATCH 1/5] sched: fix capacity calculations for SMT4

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Date:	Mon, 19 Apr 2010 16:49:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michael Neuling <mikey@...ling.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [PATCH 1/5] sched: fix capacity calculations for SMT4

On Mon, 2010-04-19 at 07:34 +1000, Michael Neuling wrote:
> > Are there any numbers available on how much they gain? It might be worth
> > to stick in real numbers instead of this alleged 15%.
> 
> I get some gain numbers but obviously the workloads makes a huge
> difference.  From a scheduler perspective, I assume an
> average/representative gain is best rather than an optimistic or
> pessimistic one?

Yeah, average would be best.

> We'll have different gains for SMT2 and SMT4, so we could change the
> gain dynamically based on which SMT mode we are in.  Does that seem like
> something we should add as an arch hook?  

That's the sort of thing you can use arch_scale_smt_power() for. But be
weary to not fall into the same trap I did with x86, where I confused
actual gain with capacity (When idle the actual gain is 0, but the
capacity is not).




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