linux-kernel - Re: [RFT] Please test rdtsc on various x86-64 hardware (app included)

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Date:	Tue, 19 Apr 2011 09:10:38 +0100
From:	Frank Kingswood <frank@...gswood-consulting.co.uk>
To:	linux-kernel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>, x86 <x86@...nel.org>
Subject: Re: [RFT] Please test rdtsc on various x86-64 hardware (app included)

On 18/04/11 20:32, Andrew Lutomirski wrote:
> Hi all-
>
> I'd appreciate some help testing rdtsc's ordering wrt memory on
> various hardware.  You can download evil-clock-test code at:
>
> https://gitorious.org/linux-test-utils/linux-clock-tests/blobs/raw/master/evil-clock-test.cc

A low power core2 server:

CPU vendor   : GenuineIntel
CPU model    : Intel(R) Core(TM)2 Duo CPU     T8100  @ 2.10GHz
CPU stepping : 6
TSC flags    : tsc constant_tsc
Using lfence_rdtsc because you have an Intel CPU
Will test the "lfence;rdtsc" clock.
Now test passed  : margin 84 with 39213496 samples
Load3 test passed: margin 20947164 with 15 samples
Load test passed : margin 52 with 12466437 samples
Store test failed as expected: worst error 1397 with 11151528 samples

That margin number seemed a bit high, rerunning gives:

CPU vendor   : GenuineIntel
CPU model    : Intel(R) Core(TM)2 Duo CPU     T8100  @ 2.10GHz
CPU stepping : 6
TSC flags    : tsc constant_tsc
Using lfence_rdtsc because you have an Intel CPU
Will test the "lfence;rdtsc" clock.
Now test passed  : margin 84 with 39201440 samples
Load3 test passed: margin 7991 with 15 samples
Load test passed : margin 52 with 12482714 samples
Store test failed as expected: worst error 2457 with 11129294 samples

Frank

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