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

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Date:	Wed, 28 Oct 2009 10:47:37 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Glauber Costa <glommer@...hat.com>,
	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 10/28/09 02:34, Avi Kivity wrote:
> On 10/28/2009 11:29 AM, Glauber Costa wrote:
>>> 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?
>>    
>
> Much better to have an API for this.  Life is hacky enough already.

My point is that if an app cares about property X then it should just
measure property X.  The fact that gettimeofday is a vsyscall is just an
implementation detail that apps don't really care about.  What they care
about is whether gettimeofday is fast or not.

If the environment has such unstable timing that the effect can't be
measured, then it is moot whether its a vsyscall or not (but in that
case its almost certainly better to use the standard API rather than
trying to roll your own timesource with rdtsc).

    J
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