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

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

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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