Re: gettimeofday "slow" in RHEL4 guests

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From: Hollis Blanchard

Subject: Re: gettimeofday "slow" in RHEL4 guests

Date: Tuesday, November 25, 2008 - 10:20 am

On Mon, 2008-11-24 at 21:41 -0700, David S. Ahern wrote:
quoted text> 
> 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

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