From: Hollis Blanchard <hollisb@us.ibm.com>
To: "David S. Ahern" <daahern@cisco.com>
Cc: kvm-devel <kvm@vger.kernel.org>,
Marcelo Tosatti <mtosatti@redhat.com>,
Glauber de Oliveira Costa <gcosta@redhat.com>,
Avi Kivity <avi@redhat.com>
Subject: Re: gettimeofday "slow" in RHEL4 guests
Date: Tue, 25 Nov 2008 11:20:06 -0600 [thread overview]
Message-ID: <1227633606.7434.14.camel@localhost.localdomain> (raw)
In-Reply-To: <492B8204.5@cisco.com>
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
next prev parent reply other threads:[~2008-11-25 17:20 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-11-24 17:47 gettimeofday "slow" in RHEL4 guests David S. Ahern
2008-11-25 4:41 ` David S. Ahern
2008-11-25 10:14 ` Andi Kleen
2008-11-25 11:17 ` Alexander Graf
2008-11-25 11:48 ` Andi Kleen
2008-11-25 12:13 ` Alexander Graf
2008-11-25 12:52 ` Andi Kleen
2008-12-28 18:38 ` Marcelo Tosatti
2008-12-29 12:37 ` Yang, Sheng
2008-12-29 13:11 ` Avi Kivity
2008-12-29 16:12 ` Dor Laor
2008-12-29 16:27 ` Avi Kivity
2008-12-29 16:29 ` Avi Kivity
2008-11-25 17:20 ` Hollis Blanchard [this message]
2008-11-25 19:09 ` David S. Ahern
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