[Xen-devel] RE: About pit irq

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

>From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx] 
>Sent: Wednesday, November 19, 2008 4:19 PM
>
>Since the PIT advertises itself as a 32-bit timer, the 
>platform overflow
>logic will not trigger every 27ms. More like once an hour.
>
>The issue with PIT is the 16-bit counter overflows within 10s of
>milliseconds, and we have non-preemptible periods in Xen of 
>longer than that
>(e.g., during domain destruction). If we miss an overflow, 
>time handling
>gets totally screwed so we have to deal with PIT in hardirq 
>context, not
>softirq. We do it at 10ms because that's what we happen to 
>always program
>PIT for. Certainly in this specific case we could program it 
>to tick slower.
>
> -- Keir
>

I see. Yes, a slower tick is better since maximum overflow for
pit ch2 is about 54ms, and xen has already be a tickless
model.

Thanks,
Kevin
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