Question regarding sibling field in cpuinfo and hyperthreading enabled/disabled in bios

[Richard Wohlstadter <rwohlsta@watson.wustl.edu>]

Hello,

I've recently been told that the siblings field in /proc/cpuinfo should 
have a count of 2 when hyperthreading is enabled in the bios of a 
machine and should drop to 1 when hyperthreading is disabled in the bios 
of a machine.  I have a vendor who is using this algorithm to decide 
whether they should divide by 2 on the number of processors showing in 
/proc/cpuinfo to obtain the real count of physical processors in the 
machine.  I've noticed on my Dell 2-cpu Xeon, that siblings field does 
not change when you enable/disable the hyperthreading feature in the 
bios (only the number of processors seen in /proc/cpuinfo).  My question 
  I guess is this a bug in the bios of my Dell box(should siblings count 
drop to 1 when hyperthreading is disabled?).  I would think a better 
approach for the vendor would be to look at the physical ID field in 
/proc/cpuinfo since I've noticed that each real physical cpu has a 
different ID.

Please CC me on answers since I'm not on the list.  Thanks for any 
insight you provide.

Rich Wohlstadter
Genome Sequencing Center
Washington University of Saint Louis
rwohlsta@watson.wustl.edu