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

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Avi Kivity [mailto:avi@redhat.com]
> 
> On 10/28/2009 07:47 PM, Jeremy Fitzhardinge wrote:
> >> Much better to have an API for this.  Life is hacky enough already.
> >>      
> > My point is that if an app cares about property X then it 
> should just
> > measure property X.  The fact that gettimeofday is a 
> vsyscall is just an
> > implementation detail that apps don't really care about.  
> What they care
> > about is whether gettimeofday is fast or not.
> >    
> 
> But we can not make a reliable measurement.
> 
> > If the environment has such unstable timing that the effect can't be
> > measured, then it is moot whether its a vsyscall or not (but in that
> > case its almost certainly better to use the standard API rather than
> > trying to roll your own timesource with rdtsc).
> >    
> 
> If you're interested in gettimeofday() for a global monotonic counter 
> you can fall back to atomic_fetch_and_add() which will be 
> faster than a 
> syscall even on large systems.  Maybe we should provide a 
> vsyscall for 
> global monotonic counters and implement it using a atomics or tsc 
> instead of these hacks (I'm assuming here that the 
> gettimeofday() calls 
> are used to implement an atomic counter - are they?)

No, the apps I'm familiar with (a DB and a JVM) need a timestamp
not a monotonic counter.  The timestamps must be relatively
accurate (e.g. we've been talking about gettimeofday generically,
but these apps would use clock_gettime for nsec resolution),
monotonically increasing, and work properly across a VM
migration.  The timestamps are taken up to a 100K/sec or
more so the apps need to ensure they are using the fastest
mechanism available that meets those requirements.