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. ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ kvm-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/kvm-devel
