linux-kernel - Re: [PATCH 0/6] x86-64: Micro-optimize vclock

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Message-ID: <20110406201442.GV21838@one.firstfloor.org>
Date:	Wed, 6 Apr 2011 22:14:42 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Andrew Lutomirski <luto@....edu>
Cc:	Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/6] x86-64: Micro-optimize vclock_gettime

On Wed, Apr 06, 2011 at 04:10:22PM -0400, Andrew Lutomirski wrote:
> I ran Ingo's time-warp-test w/ 6, 7, and 8 threads on Sandy Bridge and
> on a Xeon 5600 series chip.  My C2D laptop thinks that its TSC halts
> in idle and my only AMD system has unsynchronized TSCs.

I think you should have coverage on more systems. The original
problems that motivated the barriers were on older K8 AMD systems.

You can ask people on l-k to run such tests for you if you don't
have the hardware.

> > I did a similar attempt recently for the in kernel timers.
> > You won't see any difference in a micro benchmark loop, but you may
> > in a workload that dirties lots of cache between timer calls.
> 
> For CLOCK_REALTIME they're already in one cache line.  I tried the
> prefetch and couldn't measure a speedup even after playing with

Did you run a cache pig between the calls? With a tight loop it's obviously
useless.

> Agreed.  In fact, I could do both in one fell swoop: have a flag for
> the mode and have one option be "just issue the syscall."  Static
> branch stuff scares me because this stuff runs in userspace and, in
> theory, userspace might have COWed the page with this code in it.

The vdso is never cowed.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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