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

To: Avi Kivity <avi@xxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation
From: Glauber Costa <glommer@xxxxxxxxxx>
Date: Wed, 28 Oct 2009 10:29:01 +0100
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, Jeremy Fitzhardinge <jeremy.fitzhardinge@xxxxxxxxxx>, kurt.hackel@xxxxxxxxxx, the arch/x86 maintainers <x86@xxxxxxxxxx>, Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, zach.brown@xxxxxxxxxx, Glauber de Oliveira Costa <gcosta@xxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, chris.mason@xxxxxxxxxx
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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?

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