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Re: [Xen-devel] performance counters

To: Jan Beulich <jbeulich@xxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] performance counters
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Thu, 15 Mar 2007 11:58:58 +0000
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On 15/3/07 11:27, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

> In order to be meaningful and usable together with other measuring methods,
> their use in my opinion should impose as little overhead as possible. With
> that,
> I wonder why per-cpu counters use atomic operations.

Well, they shouldn't be. Nearly all (apart from the array/histogram ones)
are per-cpu anyway. And even if they weren't, a few lost increments wouldn't
matter (assuming the read and write parts of the increment are each
themselves atomic -- otherwise you could get worse write-conflict problems
like word tearing).

 -- Keir

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