* Problem correlating TSC read from domU with Xentrace's TSC
@ 2011-11-16 21:07 Thawan Kooburat
2011-11-16 23:51 ` Dan Magenheimer
0 siblings, 1 reply; 2+ messages in thread
From: Thawan Kooburat @ 2011-11-16 21:07 UTC (permalink / raw)
To: xen-devel
Hi,
I am trying to correlating performance issue in guest VM with the
scheduling trace from Xentrace. User-mode application in guest VM
periodically dump APIC ID and RDTSC into trace. I also start Xentrace
in Dom0 during the same period.
However, I notice that range of TSC values report both trace is
completely disjointed. TSC values from Xentrace is always greater than
what guest VM see, even if I start capturing Xen's trace 30s before
capturing trace in guest VM.
I understand that Xen don't emulate rdtsc() for PV guest and Xentrace
also read rdtsc() during __insert_record(). However, I don't
understand what cause my issue. Any suggestion? I am using Xen
4.1.1 on Fedora 15
--
Thawan Kooburat
Graduate Student
Department of Computer Science
UW-Madison
^ permalink raw reply [flat|nested] 2+ messages in thread
* RE: Problem correlating TSC read from domU with Xentrace's TSC
2011-11-16 21:07 Problem correlating TSC read from domU with Xentrace's TSC Thawan Kooburat
@ 2011-11-16 23:51 ` Dan Magenheimer
0 siblings, 0 replies; 2+ messages in thread
From: Dan Magenheimer @ 2011-11-16 23:51 UTC (permalink / raw)
To: Thawan Kooburat, xen-devel
> I am trying to correlating performance issue in guest VM with the
> scheduling trace from Xentrace. User-mode application in guest VM
> periodically dump APIC ID and RDTSC into trace. I also start Xentrace
> in Dom0 during the same period.
>
> However, I notice that range of TSC values report both trace is
> completely disjointed. TSC values from Xentrace is always greater than
> what guest VM see, even if I start capturing Xen's trace 30s before
> capturing trace in guest VM.
>
> I understand that Xen don't emulate rdtsc() for PV guest and Xentrace
> also read rdtsc() during __insert_record(). However, I don't
> understand what cause my issue. Any suggestion? I am using Xen
> 4.1.1 on Fedora 15
Hi Thawan --
Depending on the underlying hardware, Xen may emulate rdtsc
for PV guests. To force this off, set tsc_mode=2 in your
guest vm.cfg file (and reboot). For more info on tsc_mode
see tscmode.txt in the Xen source tree (or google for it).
Dan
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2011-11-16 21:07 Problem correlating TSC read from domU with Xentrace's TSC Thawan Kooburat
2011-11-16 23:51 ` Dan Magenheimer
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