linux-kernel - Re: [RFC] [PATCH] allow low HZ values?

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Message-ID: <4CB393EB.3060405@zytor.com>
Date:	Mon, 11 Oct 2010 15:47:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	x86@...nel.org, Marcio Saito <marcio@...lades.com>,
	Jiri Slaby <jirislaby@...il.com>,
	Paul Mackerras <paulus@...ba.org>,
	linuxppc-dev@...ts.ozlabs.org, John Stultz <johnstul@...ibm.com>,
	Avantika Mathur <mathur@...ux.vnet.ibm.com>
Subject: Re: [RFC] [PATCH] allow low HZ values?

On 10/11/2010 03:33 PM, Thomas Gleixner wrote:
> On Tue, 12 Oct 2010, Benjamin Herrenschmidt wrote:
> 
>> On Mon, 2010-10-11 at 13:11 -0700, Tim Pepper wrote:
>>> I'm not necessarily wanting to open up the age old question of "what is
>>> a good HZ", but we were doing some testing on timer tick overheads for
>>> HPC applications and this came up...
>>
>> Note that this is also very useful when working on CPU prototypes
>> implemented in FPGAs and running at something like 12Mhz :-)
> 
> /me hands benh 0.5$ for a FPGA upgrade

That's often not possible if the CPU cannot be mapped onto a single FPGA
(either because the core is too large, multiple cores are tested, or
because there is debugging logic is included.)  The interconnects slows
things down tremendously.

	-hpa

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