On Mon, 2008-10-27 at 12:49 +0200, Dor Laor wrote: > Beth Kon wrote: > > On Fri, 2008-10-17 at 16:49 +0100, Jamie Lokier wrote: > > > > > Beth Kon wrote: > > > > > > > Clock drift on Linux is in the range of .017% - .019%, loaded and > > > > unloaded. I > > > > haven't found a straightforward way to test on Windows and would > > > > appreciate > > > > any pointers to existing approaches. > > > > > > > Is there any reason why there should be any clock drift, when the > > > guest is using a non-PIT clock? > > > > > > I'm probably being naive, but with 32-bit or 64-bit HPET counters > > > available to the guest, and accurate values from the CMOS clock > > > emulation, I don't see why drift would accumulate over the long term > > > relative to the host clock. > > > > > > > I was measuring with ntpdate, so the drift is with respect to the ntp > > server pool, not the host clock. But in any case, since timer interrupts > > and reads of the hpet counter are at the mercy of the host scheduler > > (i.e., the qemu process can be swapped out at any time during hpet read > > or timer expiration), I'd guess there would always be some amount of > > inaccuracy. Also, qemu checks for timer expiration (qemu_run_timers) as > > part of a bigger loop (main_loop_wait), so the varying amounts of work > > to do elsewhere in the loop from iteration to iteration would also > > introduce irregular delays. > > > This is exactly why hpet as the other clock emulation in qemu (pit, > rtc, pm?) need > to check whether their irq was really injected. Gleb sent patches for > the rtc, pit. > The idea is to check with the irq chip if the injected irq was really > successful. > I assume these are the patches you're referring to? http://thread.gmane.org/gmane.comp.emulators.kvm.devel/18974/focus=18977
Looks like they were never merged. Does anyone know the history on that? Also, HPET generates edge-triggered interrupts (as dictated by Linux and Windows) so I'm not sure if this scheme could work for it. > Dor -- Elizabeth Kon (Beth) IBM Linux Technology Center Open Hypervisor Team email: [EMAIL PROTECTED] -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
