linux-kernel - Re: [patch] x86: unify/rewrite SMP TSC sync code

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Message-Id: <1164529484.3147.68.camel@laptopd505.fenrus.org>
Date:	Sun, 26 Nov 2006 09:24:44 +0100
From:	Arjan van de Ven <arjan@...radead.org>
To:	Wink Saville <wink@...ille.com>
Cc:	Robert Hancock <hancockr@...w.ca>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [patch] x86: unify/rewrite SMP TSC sync code

On Sun, 2006-11-26 at 00:16 -0800, Wink Saville wrote:
> Robert Hancock wrote:
> >>>> Actually, we need to ask the CPU/System makers to provide a system wide
> > Generally user mode code should just be using gettimeofday. When the TSC 
> > is usable as a sane time source, the kernel will use it. When it's not, 
> > it will use something else like the HPET, ACPI PM Timer or (at last 
> > resort) the PIT, in increasing degrees of slowness.
> > 
> 
> But gettimeofday is much too expensive compared to RDTSC.

it's the cost of a syscall (1000 cycles?) plus what it takes to get a
reasonable time estimate. Assuming your kernel has enough time support
AND your tsc is reasonably ok, it'll be using that. If it's NOT using
that then that's a pretty good sign that you can't also use it in
userspace....

-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
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