linux-kernel - Re: [PATCH] ftrace: Provide trace clock monotonic raw

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Message-ID: <20150508100903.39641009@gandalf.local.home>
Date:	Fri, 8 May 2015 10:09:03 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Drew Richardson <drew.richardson@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	John Stultz <john.stultz@...aro.org>,
	Wade Cherry <Wade.Cherry@....com>,
	Pawel Moll <Pawel.Moll@....com>
Subject: Re: [PATCH] ftrace: Provide trace clock monotonic raw

On Tue, 5 May 2015 07:54:46 -0700
Drew Richardson <drew.richardson@....com> wrote:
 
> CLOCK_MONOTONIC_RAW will advance more constantly than CLOCK_MONOTONIC.
> 
> Imagine someone is trying to optimize a particular program to reduce
> instructions executed for a given workload while minimizing the effect
> on runtime. Also suppose that ntp is running and potentially making
> larger adjustments to CLOCK_MONOTONIC. If ntp is adjusting
> CLOCK_MONOTONIC to advance more rapidly, the program will appear to
> use fewer instructions per second but run longer than it would if
> CLOCK_MONOTONIC_RAW had been used. The total number of instructions
> observed would be the same regardless of the clock source used, but
> how it's attributed to time would be affected.
> 
> Conversely if ntp is adjusting CLOCK_MONOTONIC to advance more slowly,
> the program will appear to use more instructions per second but run
> more quickly. Of course there are many sources that can cause jitter
> in performance measurements on modern processors, but I'd like to
> remove ntp from the list.

This is a nice description, and was expecting to see it in the change
log of the latest patch. Can you resend with a more detailed
description (like the above).

Thanks!

-- Steve
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