From: john stultz <johnstul@us.ibm.com>
To: Kasper Pedersen <kernel@kasperkp.dk>
Cc: linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
x86@kernel.org, Peter Zijlstra <a.p.zijlstra@chello.nl>,
Suresh Siddha <suresh.b.siddha@intel.com>
Subject: Re: x86: tsc: make TSC calibration more immune to interrupts
Date: Wed, 20 Apr 2011 12:15:59 -0700 [thread overview]
Message-ID: <1303326959.2796.136.camel@work-vm> (raw)
In-Reply-To: <4DAF2B57.6010100@kasperkp.dk>
On Wed, 2011-04-20 at 20:52 +0200, Kasper Pedersen wrote:
> When a SMI or plain interrupt occurs during the delayed part
> of TSC calibration, and the SMI/irq handler is good and fast
> so that is does not exceed SMI_TRESHOLD, tsc_khz can be a bit
> off (10-30ppm).
>
> We should not depend on interrupts being longer than 50000
> clocks, so always do the 5 tries, and use the best sample we
> get.
> This should work always for any four periodic or rate-limited
> interrupt sources. If we get 5 interrupts with 500ns gaps in
> a row, behaviour should be as without this patch.
>
> This costs us 20-100 microseconds in startup time, as
> tsc_read_refs is called 8 times.
>
> measurements:
> On a 700MHz P3 I see t2-t1=~22000, and 31ppm error.
> A Core2 is similar: http://n1.taur.dk/tscdeviat.png
> (while mostly t2-t1=~1000, in about 1 of 3000 tests
> I see t2-t1=~20000 for both machines.)
> vmware ESX4 has t2-t1=~8000 and up.
I guess I'm curious how useful this is with the refined TSC calibration
that was added not too long ago:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=08ec0c58fb8a05d3191d5cb6f5d6f81adb419798
Are you saying that you see the same 10-30ppm variance in the dmesg
line: "Refined TSC clocksource calibration: XXXX.XXX MHz" ?
thanks
-john
next prev parent reply other threads:[~2011-04-20 19:16 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-04-20 18:52 x86: tsc: make TSC calibration more immune to interrupts Kasper Pedersen
2011-04-20 19:15 ` john stultz [this message]
2011-04-20 19:44 ` Kasper Pedersen
2011-04-20 20:28 ` john stultz
2011-04-20 21:22 ` x86: tsc: v2 " Kasper Pedersen
2011-04-20 22:39 ` Josh Triplett
2011-04-21 2:19 ` john stultz
2011-04-21 4:32 ` Josh Triplett
2011-04-21 19:46 ` Kasper Pedersen
2011-04-23 1:38 ` john stultz
2011-04-21 19:52 ` x86: tsc: v3 " Kasper Pedersen
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