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

Glauber Costa
2009-10-28T09:29:48Z

On Wed, Oct 28, 2009 at 07:52:04AM +0200, Avi Kivity wrote:
> On 10/27/2009 08:20 PM, Jeremy Fitzhardinge wrote:
>> On 10/27/09 10:29, Dan Magenheimer wrote:
>>
>>> Is there any way for an application to conclusively determine
>>> programmatically if the "fast vsyscall" pvclock is functional
>>> vs the much much slower gettimeofday/clock_gettime equivalents?
>>>
>>> If not, might it be possible to implement some (sysfs?)
>>> way to determine this, that would also be backwards compatible
>>> to existing OS's that don't have pvclock+vsyscall supported?
>>>
>>>
>> It would probably be simplest and most portable for the app to just
>> measure the performance of gettimeofday and see if it meets its needs.
>>
>
> How can you reliably measure performance in a virtualized environment?
If we loop gettimeofday(), I would expect the vsyscall-based version not to show
up in strace, right?