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Re: [Xen-devel] vgettimeofday disabled?

To: Chris Lalancette <clalance@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] vgettimeofday disabled?
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Wed, 03 Sep 2008 17:01:01 +0100
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On 3/9/08 16:48, "Chris Lalancette" <clalance@xxxxxxxxxx> wrote:

> Hello,
>      Based on a comment from a customer, I looked briefly at the vgettimeofday
> implementation in Xen.  I noticed that vgettimeofday is disabled in the 64-bit
> linux-2.6.18-xen.hg tree, basically by setting sysctl_vsyscall = 0 in
> vsyscall_init().  Is there some known problem with enabling this?  In a quick
> test, I turned it on, and went from 2m14s in my micro benchmark to 30s for the
> benchmark, but I'm not sure if there will be any ill effects from the same.

We probably don't update vxtime structure, so you likely get dodgy results
from do_vgettimedofday(). However, forcing vxtime to track vcpu0's time info
obtained from Xen would probably be quite easy to do and may be accurate

But, without that extra engineering, I recommend you check what answers
you're getting from gettimeofday(). Could be quite a significant
performance/correctness tradeoff ;-)

 -- Keir

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