Re: VDSO pvclock may increase host cpu consumption, is this a problem?

[Andy Lutomirski]

On Tue, Apr 1, 2014 at 5:12 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> On Tue, Apr 01, 2014 at 12:17:16PM -0700, Andy Lutomirski wrote:
>> On Tue, Apr 1, 2014 at 11:01 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
>> > On Mon, Mar 31, 2014 at 10:33:41PM -0700, Andy Lutomirski wrote:
>> >> On Mar 31, 2014 8:45 PM, "Marcelo Tosatti" <mtosatti@xxxxxxxxxx> wrote:
>> >> >
>> >> > On Mon, Mar 31, 2014 at 10:52:25AM -0700, Andy Lutomirski wrote:
>> >> > > On 03/29/2014 01:47 AM, Zhanghailiang wrote:
>> >> > > > Hi,
>> >> > > > I found when Guest is idle, VDSO pvclock may increase host consumption.
>> >> > > > We can calcutate as follow, Correct me if I am wrong.
>> >> > > >       (Host)250 * update_pvclock_gtod = 1500 * gettimeofday(Guest)
>> >> > > > In Host, VDSO pvclock introduce a notifier chain, pvclock_gtod_chain in timekeeping.c. It consume nearly 900 cycles per call. So in consideration of 250 Hz, it may consume 225,000 cycles per second, even no VM is created.
>> >> > > > In Guest, gettimeofday consumes 220 cycles per call with VDSO pvclock. If the no-kvmclock-vsyscall is configured, gettimeofday consumes 370 cycles per call. The feature decrease 150 cycles consumption per call.
>> >> > > > When call gettimeofday 1500 times,it decrease 225,000 cycles,equal to the host consumption.
>> >> > > > Both Host and Guest is linux-3.13.6.
>> >> > > > So, whether the host cpu consumption is a problem?
>> >> > >
>> >> > > Does pvclock serve any real purpose on systems with fully-functional
>> >> > > TSCs?  The x86 guest implementation is awful, so it's about 2x slower
>> >> > > than TSC.  It could be improved a lot, but I'm not sure I understand why
>> >> > > it exists in the first place.
>> >> >
>> >> > VM migration.
>> >>
>> >> Why does that need percpu stuff?  Wouldn't it be sufficient to
>> >> interrupt all CPUs (or at least all cpus running in userspace) on
>> >> migration and update the normal timing data structures?
>> >
>> > Are you suggesting to allow interruption of the timekeeping code
>> > at any time to update frequency information ?
>>
>> I'm not sure what you mean by "interruption of the timekeeping code".
>> I'm suggesting sending an interrupt to the guest (via a virtio device,
>> presumably) to tell it that it has been paused and resumed.
>
> code:
>
> 1) disable interrupts
> 2) A = RDTSC
> 3) B = SCALE(A, TSC.FREQ)
>
> If migration happens between 2 and 3, you've got an incorrect value.
>

Fair enough.

I guess

1) disable interrupts
2) A = RDTSC
3) B = SCALE(A, TSC.FREQ)

is also bad if (3) blocks due to magic invalidation of the physical page.

--Andy
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