Re: [Xen-devel] Align periodic vpts to reduce timer interrupts and save power

[Gary Grebus <ggrebus@xxxxxxxxxxxxxxx>]

On Mon, 2009-02-09 at 14:26 +0800, Wei, Gang wrote:
...
> Is there any objections, comments or concerns for such a enhancement?
> 
> BTW, current vpt code try to handle vLAPIC specially to give vLAPIC ticks a 
> offset from other timer ticks like below:
>     pt->scheduled = NOW() + delta;
>     /*
>      * Offset LAPIC ticks from other timer ticks. Otherwise guests which use
>      * LAPIC ticks for process accounting can see long sequences of process
>      * ticks incorrectly accounted to interrupt processing.
>      */
>     if ( pt->source == PTSRC_lapic )
>         pt->scheduled += delta >> 1;
> 
> But I don't think this way can really reach the offset purpose, because the 
> additional (delta >> 1) can't prevent other timers created in other time to 
> expires right before lapic ticks.
> 
> Will it really bring issues if vlapic ticks and vhpet ticks were aligned?

In that case, it was the vlapic timer ticking too close to the pit.  We
saw a userspace application getting the wrong answer because long CPU
bound sequences appeared to run with zero CPU time.  Skewing the vlapic
was sufficient to fix the problem.  This only showed up with old Linux
kernels (IIRC, it was with Red Hat 3 U8). 

If the effect of your proposed change is to always deliver all timer
interrupts to the guest in immediate succession, then I think you will
cause a regression on this problem.

-- 
Gary Grebus
Virtual Iron Software, Inc.



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