linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Message-ID: <33307c790809241104k2af4b900w781aba1a1393004b@mail.gmail.com>
Date:	Wed, 24 Sep 2008 11:04:04 -0700
From:	"Martin Bligh" <mbligh@...gle.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Peter Zijlstra" <peterz@...radead.org>,
	"Steven Rostedt" <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...e.hu>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	"Mathieu Desnoyers" <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	"David Wilder" <dwilder@...ibm.com>, hch@....de,
	"Tom Zanussi" <zanussi@...cast.net>,
	"Steven Rostedt" <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer

> If we just record the TSC unshifted, in 27 bits, at 4GHz, that gives us
> about 1/30 of a second? So we either shift, use > 27 bits, or record
> at least 30 events a second, none of which I like much ...

If we use 32 bits instead of 27, then the timestamp events are only
about once per second, which is probably fine for overhead ... ?

I think we're OK losing 5 bits of precision, that's only 32 cycles,
given all the CPU reordering stuff we've talked about here,
not-quite-synced TSCs, etc. I suspect you thought we were
shifting by much more than this, in reality it was 5-10 bits,
with timestamp events inbetween, though we put wall time in
them, which I think was a mistake.

(note: I'm not suggesting we have to use this compact a format,
at least by default)
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