From mboxrd@z Thu Jan  1 00:00:00 1970
From: Hollis Blanchard <hollisb@us.ibm.com>
Subject: Re: gettimeofday "slow" in RHEL4 guests
Date: Tue, 25 Nov 2008 11:20:06 -0600
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References: <492AE8AC.2090502@cisco.com>  <492B8204.5@cisco.com>
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Cc: kvm-devel <kvm@vger.kernel.org>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Glauber de Oliveira Costa <gcosta@redhat.com>,
	Avi Kivity <avi@redhat.com>
To: "David S. Ahern" <daahern@cisco.com>
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On Mon, 2008-11-24 at 21:41 -0700, David S. Ahern wrote:
> 
> RHEL3 (which is based on the 2.4.21 kernel) gets microsecond
> resolutions
> by reading the TSC. Reading the TSC from within a guest is very fast
> on kvm.
> 
> RHEL4 (which is basd on the 2.6.9 kernel) allows multiple time
> sources:
> pmtmr (ACPI power management timer which is the default), pit, hpet
> and TSC.
> 
> The pmtmr and pit both do ioport reads to get microsecond resolutions
> (see read_pmtmr and get_offset_pit, respectively). For the tsc as the
> timer source gettimeofday is *very* lightweight, but time drifts very
> badly and ntpd cannot acquire a sync.

Why aren't you seeing severe time drift when using RHEL3 guests with the
TSC time source?

-- 
Hollis Blanchard
IBM Linux Technology Center