Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

Avi Kivity
2009-10-06T15:12:55Z

On 10/06/2009 04:19 PM, Dan Magenheimer wrote:
>> From: Jeremy Fitzhardinge [mailto:jeremy.fitzhardinge@citrix.com]
>> With this in place, I can do a gettimeofday in about 100ns on a 2.4GHz
>> Q6600. I'm sure this could be tuned a bit more, but it is
>> already much better than a syscall.
>>
> To evaluate the goodness of this, we really need a full
> set of measurements for:
>
> a) cost of rdtsc (and rdtscp if different)
> b) cost of vsyscall+pvclock
> c) cost of rdtsc emulated
> d) cost of a hypercall that returns "hypervisor system time"
>
> On a E6850 (3Ghz but let's use cycles), I measured;
>
> a == 72 cycles
> c == 1080 cycles
> d == 780 cycles
>
> It may be partly apples and oranges, but it looks
> like a good guess for b on my machine is
>
> b == 240 cycles
>

Two rdtscps should suffice (and I think they are much faster on modern
machines).

> Not bad, but is there any additional context switch
> cost to support it?
>

rdtscp requires an additional msr read/write on heavyweight host context
switches. Should be negligible compared to the savings.

>> From: Avi Kivity [mailto:avi@redhat.com]
>> Instead of using vgetcpu() and rdtsc() independently, you can
>> use rdtscp
>> to read both atomically. This removes the need for the
>> preempt notifier.
>>
> Xen does not currently expose rdtscp and so does not emulate
> (or context switch) TSC_AUX. Context switching TSC_AUX
> is certainly possible, but will likely be expensive.
> If the primary reason for vsyscall+pvclock is to maximize
> performance for gettimeofday/clock_gettime, this cost
> would need to be added to the mix.
>

It will cost ~100 cycles on heavyweight host context switch
(guest-to-guest).

>> preempt notifiers are per-thread, not global, and will upset
>> the cycle
>> counters. I'd drop them and use rdtscp instead (and give up if the
>> processor doesn't support it).
>>
> Even if rdtscp is used, in the Intel processor lineup
> only the very latest (Nehalem) supports rdtscp, so
> "give up" doesn't seem like a very good option, at least
> in the near future.
>

Why not? we still fall back to the guest kernel. By the time guest
kernels with rdtscp support are in the field, these machines will be
quiet old.

--
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