Re: [PATCH 0/6] In kernel PIT patch

[Dor Laor <dor.laor@gmail.com>]

On Wed, 2008-03-05 at 19:30 +0200, Avi Kivity wrote:
> Anthony Liguori wrote:
> > Playing a movie is a bit subjective.  I presume you're talking about the 
> > standard HAL as presumably the ACPI HAL is using the pm timer?
> >   
> 
> ACPI HAL uses the apic timer, IIRC; perhaps the pm timer as well.
> 
> > So the two cases I'm hearing where timer accuracy should improve is 
> > standard HAL on Windows and clock=pit on Linux?  I'd still like to see 
> > what the actual difference in timer accuracy is.
> 
> It depends on the load.  As the load increases, the host process starts 
> to miss timer signals.  With both pic and pit in userspace, you can 
> detect those missed interrupts and inject them later once you get your 
> timeslice.  With the pic in kernel, there is no way to do this.
> 
> The same thing happens with the apic timer, only there, it is easy to 
> compensate because both parts are in the kernel.
> 
> >   I have no doubt that 
> > moving the pit into the kernel is more efficient.  Moving everything 
> > into the kernel is more efficient because light weight exits are cheaper 
> > than heavy weight exits.
> >   
> 
> Efficiency is only a secondary goal here.  The userspace PIT does not 
> consume large amounts of CPU.
> 
> > The thing I'm trying to get at is a quantitative statement about why 
> > moving the pit into the kernel is the right thing.  I'll try to give the 
> > patches a try myself in the next couple of days.  I don't think it's 
> > obvious that it's the right thing to do without some sort of benchmark 
> > supporting it.
> >   
> 
> Playing a movie is better than any benchmark; it reflects actual user 
> experience in a real and important use case.  Benchmarks are substitutes 
> for real use cases, not the goal of the optimization.
> 

I forgot to mention that the benchmark is measuring time drift in the
guest. Playing a movie in winxp changes the guest clock frequency from
100HZ to 1000HZ, thus causing a 250HZ host to coalesce pit irqs.

So good pic & pit combination can handle guest multimedia without drifts
while insufficient implementation just can't.


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