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RE: [Xen-devel] reading time value in dom0 and domU kernels

To: "sanjay kumar" <sanjay.kushwaha@xxxxxxxxx>, "Keir Fraser" <Keir.Fraser@xxxxxxxxxxxx>
Subject: RE: [Xen-devel] reading time value in dom0 and domU kernels
From: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Date: Sat, 15 Apr 2006 13:10:34 +0100
Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
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Thread-index: AcZgRDeYZiFOILblROeoo9yrxcGGQgAQRTCQ
Thread-topic: [Xen-devel] reading time value in dom0 and domU kernels
> Hi Keir/All,
> If I want to measure time at the application level, is there 
> any API available? can gettimeofday in domU give me real time value?

On xen linux, gettimeofday will return an accurate 'wall clock' time


> On 4/9/06, Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> wrote:
>       On 9 Apr 2006, at 16:19, sanjay kumar wrote:
>       > Hi Folks,
>       >  I want to calculate latency in transferring a buffer 
> from domU kernel
>       > to dom0 kernel and vice versa. for that I need a time 
> 'flavour' (cycle 
>       > counter time?) which reads the same in dom0 and domU. 
> Could someone
>       > please let me know if cycle counter time is the right 
> time to use? if
>       > not then which one (system time or wall clock time)? 
> Also could 
>       > someone please tell me how to read to read this timer value.
>       >
>       >  if I use rdtsc() in both dom0 and domU, will it give 
> me the same time
>       > value?
>       It should be close enough as Xen attempts to 
> synchronise the TSCs of 
>       all CPUs during boot and on most x86 platforms the TSCs will not
>       diverge.
>       If you're measuring in the kernel then something like 
> 'sched_clock()'
>       will get you system time in nanoseconds. That would be slightly 
>       preferable as system time is actively synchronised 
> across all domains.
>       With TSC you are winging it a tiny bit (although it's 
> usually an okay
>       strategy on x86).
>         -- Keir
> --
> ----------------------
> PhD Student, Georgia Tech
> http://www.cc.gatech.edu/~ksanjay/

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