Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems
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- To: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
- Subject: Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems
- From: Robin Holt <holt@xxxxxxx>
- Date: Wed, 7 Jan 2009 03:43:28 -0600
- Cc: Nick Piggin <npiggin@xxxxxxx>, Robin Holt <holt@xxxxxxx>, "Luck, Tony" <tony.luck@xxxxxxxxx>, Dimitri Sivanich <sivanich@xxxxxxx>, "linux-ia64@xxxxxxxxxxxxxxx" <linux-ia64@xxxxxxxxxxxxxxx>, Greg KH <greg@xxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Gregory Haskins <ghaskins@xxxxxxxxxx>, Tony Luck <tony.luck@xxxxxxxxx>
- In-reply-to: <1231313289.11687.172.camel@twins>
- List-id: <linux-ia64.vger.kernel.org>
- References: <20090106162741.GA7991@xxxxxxx> <57C9024A16AD2D4C97DC78E552063EA35CB95575@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> <20090106201950.GA3850@xxxxxxx> <57C9024A16AD2D4C97DC78E552063EA35CB955B4@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> <1231275441.11687.110.camel@twins> <20090106225054.GB3850@xxxxxxx> <1231283763.11687.135.camel@twins> <20090107030030.GH3390@xxxxxxxxxxxxx> <1231313289.11687.172.camel@twins>
- User-agent: Mutt/1.5.18 (2008-05-17)
On Wed, Jan 07, 2009 at 08:28:09AM +0100, Peter Zijlstra wrote:
> That's basically what the HAVE_UNSTABLE_SCHED_CLOCK code does. It takes
> a tick timestamp and tries to improve on that by using strict per cpu
> sched_clock() deltas.
>
> What we do to obtain remote time, is basically calculate local time and
> pull remote time fwd if that was behind.
But what happens when the remote clock then takes a single tick and
pulls forward a different remote clock by both your tick and its tick.
Multiply that by 4096 and I am starting to feel that time is likely to
go racing forward in a really random fashion.
> While doing that, it filters out any backward motion and large fwd leaps
> so as to stay no worse than a jiffie clock.
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