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

Dan Magenheimer
2009-11-05T14:55:15Z

> From: Avi Kivity [mailto:avi@redhat.com]
>
> Within a process, yes. Across processes, not without writable shared
> memory.
>
> That's why I'm trying to understand what the actual
> requirements are.
> Real monotonic, accurate, high resolution, low cost time sources are
> hard to come by.

Hmmm... this has significant implications for the rdtsc
emulation discussion on xen-devel. Since that's not
a Linux question, I'll start another thread on xen-devel
with a shorter cc list.

> > Actually, I think for many/most profiling applications,
> > just knowing a discontinuity occurred between two
> > timestamps is very useful as that one specific measurement
> > can be discarded. If a discontinuity is invisible,
> > one clearly knows that a negative interval is bad,
> > but if an interval is very small or very large,
> > one never knows if it is due to a discontinuity or
> > due to some other reason.
> >
> > This would argue for a syscall/vsyscall that can
> > "return" two values: the "time" and a second
> > "continuity generation" counter.
>
> I doubt it. You should expect discontinuities in user space due to
> being swapped out, scheduled out, migrated to a different
> cpu, or your
> laptop lid being closed. There are no guarantees to a userspace
> application. Even the kernel can expect discontinuities due
> to SMIs.
> So an explicit notification about one type of discontinuity
> adds nothing.

Good point. I'm interested in enterprise apps that have more
control over the machine (and rarely suffer from laptop lid
closures :-) and would intend for all discontinuities visible
to a hypervisor or kernel to increment "AUX", but bare-metal-
kernel-invisible discontinuities such as SMI do throw a wrench
in the works.

Well, all this discussion has convince me that
my original proposals do make sense for enterprise apps to be
virtualization-aware and use rdtsc/p directly for timestamping
needs rather than OS APIs (with the hypervisor deciding
whether or not to emulate rdtsc/p based on the underlying
physical machine and whether or not migration is enabled
or has occurred).