linux-kernel - Re: [PATCH 1/1] add StartTimeMonotomic, StartTimeBootTime to per pid in /proc

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Date:	Fri, 24 Jan 2014 13:49:39 +0100
From:	Lennart Poettering <mzxreary@...inter.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dan Ballard <dan@...dstab.net>, kay.sievers@...y.org,
	Rob Landley <rob@...dley.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Minto Joseph <mvaliyav@...hat.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	Rik van Riel <riel@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mel Gorman <mgorman@...e.de>, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] add StartTimeMonotomic, StartTimeBootTime to per
 pid   in   /proc

On Fri, 24.01.14 12:32, Peter Zijlstra (peterz@...radead.org) wrote:

> > The process starttime is useful for a variety of things, like figuring
> > out creation ordering of processes. Or it is useful to detect PID
> > reuses in a somewhat reliable way. 
> 
> OK, maybe. Changelog should have said so.
> 
> > It is useful information to show the admin in "ps".
> 
> Does the one jiffy rounding really matter there? I doubt it, ps
> typically shows in second granularity.

Well, it's just annoying. Much of userspace uses CLOCK_MONOTONIC
throughout all the local timestamping needs these days, however the jiffy rounding
and the fact that "starttime" is based on CLOCK_BOOTTIME makes it hard
to compare process timestamps currently with other timestamps...

> > Profilers like "bootchart" can use this information to
> > plot when precisely specific process got started. From the outside it is
> > often useful to see for how long a specific process has already been
> > running, for accounting needs, and so on.
> 
> Profilers have far better interfaces than /proc to get information
> from.

That is true, but note that at least on Fedora taskstats and thing are
actually disabled these days in the kernel, since they slow things down
too much. The /proc interface is certainly much nicer there, since it
relies on a the timestamping the kernel does anyway...

> > Note that Dan's patch doesn't add any new timestamp logic to the kernel,
> > it just exposes the existing timestamps in a way to userspace that is
> > more in line with the rest of timestamps exposed. 
> 
> Yeah, Dan was also too lazy to explain the need, and had like 3 typoes
> in the inadequate changelog he had.
> 
> He also fails to explain why he needs the timestamp twice, as do you for
> that matter.

Well, I am mostly interesting int the monotonic timestamp. But given
that the kernel keeps the boottime clock value as well, and already
exposes it in a skewed way to userspace it looked like a natural choice
to also expose that time in a clean way, while we are it...

Lennart

-- 
Lennart Poettering, Red Hat
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