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[Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>, "Xen-Devel (E-mail)" <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Re: TSC scaling and softtsc reprise, and PROPOSAL
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 27 Jul 2009 20:55:10 +0100
Cc: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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On 27/07/2009 18:25, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

>> I believe this affects the guest OS executing RDTSC, not
>> guest apps, and is
>> only to delay the TSC to not 'run past' pending timer ticks
>> (typically where
>> they have been delayed due to the guest being preempted).
>>  -- Keir
> Could be.  The text would lead me to believe otherwise
> though.  Read the section on "Virtual TSC" in the
> above PDF. Specifically the Virtual TSC "advances even
> when the the virtual CPU is not running" and "In the
> past, this feature had sometimes been recommended to
> improve performance of APPLICATIONS that read the
> TSC frequently..." (my emphasis)

Yes, then it sounds like they virtualise it for apps too. Also there is an
option to virtualise the TSC at a specified frequency -- that would be
pretty weird if it applied only to guest-OS RDTSCs but not guest-app RDTSCs.


 -- Keir

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