RE: [Xen-devel] Xenoprof in an HVM domain

["Santos, Jose Renato G" <joserenato.santos@xxxxxx>]

>> -----Original Message-----
>> From: Ray Bryant [mailto:raybry@xxxxxxxxxxxxxxxxx] 
>> Sent: Thursday, May 25, 2006 3:02 PM
>> To: xen-devel@xxxxxxxxxxxxxxxxxxx
>> Cc: Santos, Jose Renato G; Eranian, Stephane; Steve Dobbelstein
>> Subject: Re: [Xen-devel] Xenoprof in an HVM domain
>> 
>> On Thursday 25 May 2006 16:44, Santos, Jose Renato G wrote:
>> > If you partition the perf counters and do not share them you still 
>> > need to enable/disable them on context switch. Otherwise 
>> you would be 
>> > counting events that happen when other domains are 
>> running. Probably 
>> > saving and restoring counters should not be much more 
>> expensive than 
>> > disabling/enabling them.
>> 
>> Good point.   I don't know what the trade off is there. 
>> 
>> One other issue that worries me here is counter interference 
>> -- e. g. suppose 
>> we are measuring cache misses.   Then if another domain has 
>> been scheduled in 
>> the meantime it can wash the measured domain's data out of 
>> the cache and 
>> cause a lot of unexpected misses in the measured domain.   
>> Offhand, I don't 
>> know of a good way to fix this other than to pin the 
>> measured domain to its 
>> cpu's for the duration of the measurement experiment.   Of 
>> course, there's 
>> still the pesky issue of interrupt service routines running 
>> in the hypervisor and causing cache damage there as well, or 
>> of dom0 getting invovled to do some I/O for the measured domain.

These are all fair concerns. However I wouldn't consider this a perf
counter issue, but a virtualization issue. Counters are just counting
what is happening. This is really an issue of sharing hardware resources
(cache, TLB, buses, etc) across virtual machines. Whenever resources are
shared there is potential for interference. The interference will be
there regardeless if you measure it or not. 
Perf counters can be used to measure it and help you define resource
policies, such as for example dedicating a CPU to a domain, as you
mention. If your application is suffering from interference from other
domains you would like to know that, when you are profiling.

Renato

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