linux-kernel - Re: [patch v2 0/5] Nano/Microsecond resolution for select() and poll()

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Message-ID: <20080831165554.GG19337@1wt.eu>
Date:	Sun, 31 Aug 2008 18:55:54 +0200
From:	Willy Tarreau <w@....eu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, tglx@...x.de, mingo@...e.hu,
	torvalds@...ux-foundation.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch v2   0/5] Nano/Microsecond resolution for select() and poll()

Hi Arjan,

On Sun, Aug 31, 2008 at 09:28:28AM -0700, Arjan van de Ven wrote:
> One thing to note is that on my machine, the current select() implementation
> will return after 1997 microseconds when asked for 1999 microseconds; this
> can be seen in a zoom in of the graph above:
> http://www.tglx.de/~arjan/zoom.png
> E.g. select() is returning too early in current Linux kernels; and this is
> also fixed (by nature) by this patch series.

This is a good news. I've been bothered by this problem for a long time
and got used to add 99999 us to any delay fed to select() (that started
when HZ=100). I remember having noticed performance improvements in my
applications using this trick even on other systems, so I don't think
that Linux is the only one with this anomaly. I think it's simply caused
by the rounding methods which make it hard to compute an accurate deadline
when a value is converted before being added to jiffies.

I do not remember noticing this on epoll though. Maybe it's just because
I did not look close enough :-/

Cheers,
Willy

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