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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, 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: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 27 Jul 2009 10:25:08 -0700 (PDT)
Cc: "Dong, Eddie" <eddie.dong@xxxxxxxxx>, John Levon <levon@xxxxxxxxxxxxxxxxx>
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> > Some googling and reading provides evidence that VMware
> > does indeed virtualize the TSC.  The timekeeping paper
> > http://www.vmware.com/pdf/vmware_timekeeping.pdf
> > tells how to turn vTSC off, but says that turning it
> > off is no longer recommended.
> 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)

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