From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: rdtsc strangeness on upstream kernel Date: Mon, 19 Jul 2010 16:34:59 -0700 Message-ID: <4C44E123.3030308@goop.org> References: <0784c7e7-1e9c-4abd-8e90-837051fe6b8b@default> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <0784c7e7-1e9c-4abd-8e90-837051fe6b8b@default> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dan Magenheimer Cc: xen-devel@lists.xensource.com List-Id: xen-devel@lists.xenproject.org On 07/19/2010 04:19 PM, Dan Magenheimer wrote: > In trying to understand some strange behavior I was > seeing on a RHEL6b2 guest, I ran across an interesting > anomaly, and it seems to be true on different upstream > (pvops) kernels. At first I thought it was a result > of the xen_sched_clock->xen_clocksource_read patch > you recently posted, but after some testing this > appears to be unrelated. > > 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? > 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. Also, how does it relate to timer interrupts? > 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? Thanks, J