linux-kernel - Re: [RFC 2/2] Make x86 calibrate

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Date:	Thu, 16 Dec 2010 09:34:37 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Robin@....com, "Holt <holt"@sgi.com
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC 2/2] Make x86 calibrate_delay run in parallel.

On Tue, 14 Dec 2010, Robin@....com wrote:
> On a 4096 cpu machine, we noticed that 318 seconds were taken for bringing
> up the cpus.  By specifying lpj=<value>, we reduced that to 75 seconds.
> Andi Kleen suggested we rework the calibrate_delay calls to run in
> parallel.  With that code in place, a test boot of the same machine took
> 61 seconds to bring the cups up.  I am not sure how we beat the lpj=
> case, but it did outperform.

If you know that all cpus are running at the same speed, then you can
set lpj=firstcpu and juts calibrate the first cpu and take the value
for all others.

> One thing to note is the total BogoMIPS value is also consistently higher.
> I am wondering if this is an effect with the cores being in performance
> mode.  I did notice that the parallel calibrate_delay calls did cause the

We really to know that. I mean the change from:

> bogomips	: 4532.81
> bogomips	: 4532.65
> bogomips	: 4532.64
> bogomips	: 4532.64

to

> bogomips	: 4533.49
> bogomips	: 7890.05
> bogomips	: 9699.67
> bogomips	: 10047.13

looks strange. The deviation is more than factor 2 and this is on the
same socket. So before we push that into the tree we better know
what's going on.

Thanks,

	tglx

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