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

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Message-ID: <715.1271709945@neuling.org>
Date:	Tue, 20 Apr 2010 06:45:45 +1000
From:	Michael Neuling <mikey@...ling.org>
To:	Peter Zijlstra <peterz@...radead.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

In message <1271688543.1488.253.camel@...top> you wrote:
> 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.

Ok.

> > 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).

Oops, yes of course :-)

<from before>
> Hrmm, my brain seems muddled but I might have another solution, let me
> ponder this for a bit..

Let me know if/when you come up this solution or if I can help.  

Mikey
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