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

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Date:	Wed, 24 Sep 2008 10:54:37 -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

> Why "tsc_shifted"?
>
> I think 27 bits is probably fine, but not by removing precision. Instead
> of shifting it so it will fit (and dropping low bits as uninteresting), do
> it by encoding it as a delta against the previous thing. 27 bits would
> still be sufficient for any high-performance thing that has tons and tons
> of packets, and if you only have a few trace events you can afford to have
> the "TSC overflow" event type (and if you want it that dense, you could
> just make 'data' be the high bits, for a total of 59 bits rather than 64
> bits of TSC.
>
> 59 bits of cycle counters is perfectly fine unless you are talking trace
> events over a year or so (I didn't do the math, but let's assume a 4GHz
> TSC as a reasonable thing even going forward - even _if_ CPU's get faster
> than that, the TSC is unlikely to tick faster since it's just not worth it
> from a power standpoint).
>
> Ok, I did the math. 1<<27 seconds (assuming the low 32 bits are just
> fractions) is something like 4+ years. I _really_ don't think we need more
> than that (or even close to that) in TSC timestamps for tracing within one
> single buffer.

Mmm. Either I'm confused, or we're talking about different things.

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 ...
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