Re: [PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

Robin Holt
2009-01-07T09:43:28Z

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.