On 29/03/2016 19:16, David Matlack wrote: > On Tue, Mar 29, 2016 at 8:57 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: >> >> Windows lets applications choose the frequency of the timer tick, >> and in Windows 10 the maximum rate was changed from 1024 Hz to >> 2048 Hz. Unfortunately, because of the way the Windows API >> works, most applications who need a higher rate than the default >> 64 Hz will just do >> >> timeGetDevCaps(&tc, sizeof(tc)); >> timeBeginPeriod(tc.wPeriodMin); >> >> and pick the maximum rate. This causes very high CPU usage when >> playing media or games on Windows 10, even if the guest does not >> actually use the CPU very much, because the frequent timer tick >> causes halt_poll_ns to kick in. >> >> There is no really good solution, especially because Microsoft >> could sooner or later bump the limit to 4096 Hz, but for now >> the best we can do is lower a bit the upper limit for >> halt_poll_ns. :-( > > This is a good solution for now. I don't think we lose noticeable > performance by lowering the max to 400 us. Good, then I'll submit it for 4.6 and applicable stable kernels. > Do you think it's ever useful to poll for a timer interrupt? It seems > like it wouldn't be. We don't need polling to deliver accurate timer > interrupts, KVM already delivers the TSC deadline timer slightly early > to account for injection delay. Maybe we can shrink polling anytime a > timer interrupt wakes up a VCPU. That's a clever idea. Because this really applies only to Windows we can just extend the special casing of GSI 8 (the RTC GSI), which exists exactly because Windows uses it as a periodic interrupt timer. On the other hand, that wouldn't work for split irqchip. For that you would probably add a new "no polling required" flag to struct kvm_irq_routing_entry, but finding a suitable place to call shrink_halt_poll_ns might be a bit hackish. Paolo >> >> Reported-by: Jon Panozzo <jonp@xxxxxxxxxxxxxxxxxxx> >> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx> >> --- >> arch/x86/include/asm/kvm_host.h | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h >> index f62a9f37f79f..b7e394485a5f 100644 >> --- a/arch/x86/include/asm/kvm_host.h >> +++ b/arch/x86/include/asm/kvm_host.h >> @@ -43,7 +43,7 @@ >> >> #define KVM_PIO_PAGE_OFFSET 1 >> #define KVM_COALESCED_MMIO_PAGE_OFFSET 2 >> -#define KVM_HALT_POLL_NS_DEFAULT 500000 >> +#define KVM_HALT_POLL_NS_DEFAULT 400000 >> >> #define KVM_IRQCHIP_NUM_PINS KVM_IOAPIC_NUM_PINS >> >> -- >> 1.8.3.1 >> > -- > To unsubscribe from this list: send the line "unsubscribe kvm" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
