Re: [Xen-devel] [PATCH] Add a timer mode that disables pending missed ticks

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 9/11/07 19:22, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:

> Since I had a high error (~.03%) for the ASYNC method a couple of days ago,
> I ran another ASYNC test. I think there may have been something
> wrong with the code I used a couple of days ago for ASYNC. It may have been
> missing the immediate delivery of interrupt after context switch in.
> 
> My results indicate that either SYNC or ASYNC give acceptable accuracy,
> each running consistently around or under .01%. MIXED has a fairly high
> error of
> greater than .03%. Probably too close to .05% ntp threshold for comfort.
> I don't have an overnight run with SYNC. I plan to leave SYNC running
> over the weekend. If you'd rather I can leave MIXED running instead.
> 
> It may be too early to pick the protocol and I can run more overnight tests
> next week.

I'm a bit worried about any unwanted side effects of the SYNC+run_timer
approach -- e.g., whether timer wakeups will cause higher system-wide CPU
contention. I find it easier to think through the implications of ASYNC. I'm
surprised that MIXED loses time, and is less accurate than ASYNC. Perhaps it
delivers more timer interrupts than the other approaches, and each interrupt
event causes a small accumulated error?

Overall I would consider MIXED and ASYNC as favourites and if the latter is
actually more accurate then I can simply revert the changeset that
implemented MIXED.

Perhaps rather than running more of the same workloads you could try idle
VCPUs and I/O bound VCPUs (e.g., repeated large disc reads to /dev/null)? We
don't have any data on workloads that aren't CPU bound, so that's really an
obvious place to put any further effort imo.

 -- Keir



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