linux-kernel - Re: [2.6.24.x] UML select()/poll() oversleeping reproducibly

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Message-ID: <87k5k112kf.fsf@hades.wkstn.nix>
Date:	Mon, 17 Mar 2008 21:08:16 +0000
From:	Nix <nix@...eri.org.uk>
To:	Jeff Dike <jdike@...toit.com>
Cc:	clowncoder <vincent-perrier@...b-internet.fr>,
	user-mode-linux-devel@...ts.sourceforge.net,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...esys.com>
Subject: Re: [2.6.24.x] UML select()/poll() oversleeping reproducibly

On 17 Mar 2008, Jeff Dike verbalised:

> Below is the same patch with another kluge, which cuts down the
> requested sleep by 10% in hopes of getting the actual sleep closer to
> what's wanted.

Eeuuuuw. :)

> This is unusable in anything resembling mainline, but I'd like to see
> how your various systems react to it.  I'm getting very close to the
> sleeps I asked for (with slight undersleeping, which is a bug).

OK.

Tests on host with clocksource pit:

bash-3.2# bin/select-sleep 5
Slept for 5 seconds.
bash-3.2# bin/select-sleep 10
Slept for 11 seconds.
bash-3.2# bin/select-sleep 30
Slept for 31 seconds.
bash-3.2# bin/select-sleep 60
Slept for 61 seconds.

... so much better than the 4x error without this patch.

Tests on host with clocksource tsc:

bash-3.2# bin/select-sleep 5
Slept for 5 seconds.
bash-3.2# bin/select-sleep 10
Slept for 10 seconds.
bash-3.2# bin/select-sleep 30
Slept for 30 seconds.
bash-3.2# bin/select-sleep 60
Slept for 61 seconds.

Distinctly better than without this patch.

(Am I the only person who finds it strange that (some) clocksource
hackers are arguing about accuracy problems in the ppm range while we're
glad to get an error of `only' single seconds per minute out of it? 
Maybe next year we can invent the `pendulum' clocksource :) )

-- 
`The rest is a tale of post and counter-post.' --- Ian Rawlings
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