[CentOS] Questions on cpu frequency scaling AMD vs. Intel

Ned Slider ned at unixmail.co.uk
Mon Aug 4 09:34:57 UTC 2008

Kai Schaetzl wrote:
> S.Tindall wrote on Sun, 3 Aug 2008 21:47:06 -0400:
>> The cpuspeed changelog may be relevant:
>> [quote]
>> * Thu Mar 06 2008 Jarod Wilson <jwilson at redhat.com>
>> - Disable freq scaling by default on AMD rev F and earlier cpus
>> when running xen, due to clock instability (#435321)
>> [/quote]
> Thanks, it didn't occur to me that cpuspeed may also be relevant to this. 
> However, I don't think it's relevant for the wrong cpu frequency reading 
> on the 3.2 Xen kernels (which in turn is responsible for the missing 
> scalability). Cpuspeed is not part of the kernel and did not change during 
> all my tests. See below for possible explanation.
>> I didn't look up your cpu, but I think it's a revision F.
> Hm, /proc/cpuinfo doesn't show any revision number. A bit googling tells 
> me that the CPUs, at least the second one, are more likely to be rev. H or 
> above. The older one is a 3800+ EE and the newer one is a 4850e which I 
> bought right after it became available. Unless rev. G and up are only quad 
> core CPUs at least the latter 45nm one should be rev G or up, too. But I 
> can't find a definitive list, shouldn't there be one on the AMD site?

Maybe this is relevant to you:


     # Frequency scaling on AMD rev F CPUs under Xen can result in
     # timekeeping problems for fully virtualized guests, so we disable
     # it by default.
     if [ -d /proc/xen ] && [ "$cpu_vendor" == AuthenticAMD ] \
        && [ "$cpu_family" -le 15 ]; then

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