[Xen-devel] RE: rdtsc strangeness on upstream kernel

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> With vcpus=2, timer0 goes up to about 2000 irq/sec on CPU0
> and timer1 on CPU1 is the same while rdtsc/sec is about 20000.

No, wait, correct that...timer0 and timer1 are 1000 irq/sec.
(I was using "watch -d" which defaults to updating every
two seconds, not every second.)

> Is it possible that the timer interrupt is using rdtsc
> five times each interrupt?

TEN times each interrupt.

> -----Original Message-----
> From: Dan Magenheimer
> Sent: Monday, July 19, 2010 5:51 PM
> To: Jeremy Fitzhardinge
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: rdtsc strangeness on upstream kernel
> 
> > > The number of rdtsc/second goes up dramatically when
> > > there is CPU-intensive load on an upstream kernel!
> >
> > Are you looking at rdtsc emulation traps when running a PV domain?
> 
> Yes, with "for i in {0..99}; do xm debug-key s; sleep 1; done".
> 
> > > I know we both observed some cases where rdtsc/sec
> > > was very high, but I don't think we ever were able to
> > > reproduce this consistently.
> >
> > It would be interesting to compare that to the context switch rate
> (cs
> > column in vmstat output) to see if they correlate.
> 
> Doesn't appear to be related.  Vmstat continues to show
> cs at about 40/sec while the rdtsc/sec is very high.
> 
> > Also, how does it relate to timer interrupts?
> 
> With vcpus=2, timer0 goes up to about 2000 irq/sec on CPU0
> and timer1 on CPU1 is the same while rdtsc/sec is about 20000.
> Is it possible that the timer interrupt is using rdtsc
> five times each interrupt?
> 
> > > First, this is a single vcpu, 64-bit 2.6.32 (RHEL6b2)
> > > kernel.
> > >
> > > I am observing ~300 rdtsc/sec on an idle VM.  When
> > > I run a load of:
> > >
> > > main() {while(1);}
> > >
> > > I am observing about 10000 rdtsc/sec!!
> > >
> > > This is a CONFIG_HZ_1000=y kernel, so I would
> > > expect 1000 rdtsc/sec, or maybe 2000 rdtsc/sec,
> > > but 10000 makes me wonder if there is some hidden
> > > bug.
> >
> > Do you have preemption running?  And why HZ=1000?
> 
> CONFIG_PREEMPT is off, though there are some other
> PREEMPT-related configs that are on.  Are any others
> relevant?
> 
> HZ=1000 is because this is a straight RHEL6b2 kernel,
> but running as a PV guest.
> 
> Dan

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel