linux-kernel - Re: [PATCH 1/2] perf tools: Add reference timestamp to perf header

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Message-ID: <1292268146.6803.492.camel@twins>
Date:	Mon, 13 Dec 2010 20:22:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"David S. Ahern" <daahern@...co.com>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] perf tools: Add reference timestamp to perf header

On Mon, 2010-12-13 at 12:15 -0700, David S. Ahern wrote:
> >   monotonic-clock
> 
> Name seems off. The 'event' is to grab time-of-day and have the 'time'
> attribute set which gives the correlation between wall-clock and perf
> timestamps. 

Well, you really want CLOCK_MONOTONIC, not CLOCK_REALTIME (nothing what
so ever to do with Real-Time computing, just another brilliant POSIX
misnomer), or possibly even CLOCK_MONOTONIC_RAW.

gettimeofday() usually returns CLOCK_REALTIME  which is subject to both
NTP and timezone adjustments.

CLOCK_MONOTONIC is only subject to NTP

CLOCK_MONOTONIC_RAW is neither.

At some point we'll try and have CLOCK_TRACING and provide a similar
time to user-space that we currently use for perf (using the VDSO to
avoid any actual system-calls).

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