Messages in this thread | | | Date | Tue, 6 Oct 2009 07:19:51 -0700 (PDT) | From | Dan Magenheimer <> | Subject | RE: [PATCH 3/5] x86/pvclock: add vsyscall implementation |
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> 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
Not bad, but is there any additional context switch cost to support it?
> 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.
> 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.
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