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Re: [Xen-devel] [PATCH 0/2] Improve hpet accuracy

On 10/6/08 00:34, "Dave Winchell" <dwinchell@xxxxxxxxxxxxxxx> wrote:

I don't recall what prompted me to try end-of-interrupt,
but I saw a significant improvement. I may have been running
a monotonicity test at the same time to explain the lock
contention mentioned in the write-up.

Doesn’t this policy guarantee that you actually deliver interrupts at consistently too low a rate? Since the delivery period is now timer period + latency of interrupt handling? I suppose it works for this guest type because it doesn’t actually care about getting interrupts at the correct rate, so long as the ticks are always a bit late?

For those that do need missed ticks to be delivered, do you track missed ticks at the absolute correct rate?

This is perhaps a fine tradeoff for all platform timers — those guests that can handle missed ticks obviously do not care about getting their timer interrupts  at absolutely the correct rate, and delivering a little late is what they are geared to handle (getting delivered consistently early is just weird!). Whereas guests that need all ticks also want them (at least over the long run) at exactly the correct rate.

I think there’s good empirical analysis in the work you’ve done. We just need the patches cleaned up and generalised for vpt.c now.

 -- Keir
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