linux-kernel - RE: [RFC patch 15/15] LTTng timestamp x86

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Date:	Fri, 17 Oct 2008 11:58:31 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: RE: [RFC patch 15/15] LTTng timestamp x86

> Hrm, on such systems
> - *large* amount of cpus
> - no synchronized TSCs
>
> What would be the best approach to order events ?

There isn't a perfect solution for this.  My feeling is
that your best hope is with per-cpu buffers logged with
the local TSC ... together with some fancy heuristics to
post-process the logs to come up with the best approximation
to the actual ordering.

If you have a tight upper bound estimate for the
errors in converting from "per-cpu" TSC values to "global
system time" then the post processing tool will be able
to identify events for which the order is uncertain.

> Do you think we should consider using HPET, event though it's
> painfully slow ? Would it be faster than cache-line bouncing
> on such large boxes ? With a frequency around 10MHz, that
> would give a 100ns precision, which should be enough
> to order events.

This sounds like a poor choice.  Makes all traces very
slow.  100ns precision isn't all that good ... we can
probably do almost as well estimating the delta between
TSC on different cpus.

-Tony
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