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

[Avi Kivity <avi@xxxxxxxxxx>]

On 10/06/2009 04:19 PM, Dan Magenheimer wrote:
> > From: Jeremy Fitzhardinge [mailto:jeremy.fitzhardinge@xxxxxxxxxx]
> > 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@xxxxxxxxxx]
> > 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.
--
error compiling committee.c: too many arguments to function


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