linux-kernel - Re: LinuxPPS low-level IRQs timestamps & ldisc

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Message-ID: <20080602175946.366c5d64@core>
Date:	Mon, 2 Jun 2008 17:59:46 +0100
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
Cc:	Rodolfo Giometti <giometti@...eenne.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: LinuxPPS low-level IRQs timestamps & ldisc

> Of course if you have to check of the DCD changed, that may not be
> constant.  A check for whether this device currently has anyone that
> wants the timestamp though should be a constant time.

Not having to take the cost of that for other users is far more important
than a time variance of a jump prediction. CPU cache effects alone will
add way more variance already. It's not a meaningful number to chase
compared to the randomness introduced by the IRQ handling itself.

If you want perfect accuracy do what I²IT did years ago and use a card
which timestamps the physical IRQ bus transition and exposes it to the OS.

Alan
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