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

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Date:	Thu, 31 Mar 2011 08:50:36 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Robin Holt <holt@....com>, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC 2/2] Make x86 calibrate_delay run in parallel.


* Yinghai Lu <yinghai@...nel.org> wrote:

> On Tue, Dec 14, 2010 at 5:58 PM,  <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.
> >
> > 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
> > fans on the machine to ramp up to full speed where the normal sequential
> > calls did not cause them to budge at all.
> 
> please check attached patch, that could calibrate correctly.
> 
> Thanks
> 
> Yinghai

> [PATCH -v2] x86: Make calibrate_delay run in parallel.
> 
> 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.

The risk wit tat suggestion is that it will spectacularly miscalibrate on 
hyperthreading systems.

Thanks,

	Ingo
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