Originally posted by: SystemAdmin

Hi All,

I have a 9117-MMB (P770), with lpars running in Power 6+ compatibility mode.
The CPU's are 3.1 Ghz/4Cores, however since the compatibity mode is activated the lpar lists it as dual core.

For ex. An lpar running power7/default mode shows 4 cores for each CPU whereas a Power 6/6+ mode lpar shows it with 2 cores.

How does this translate in terms of performance. Are there any performance differences when a Quad core CPU works in dual core mode ?
Are the sizing & performance variance in such a situation reliable inputs for a server consolidation analysis.

Thanks for any insights and support on this!
Somesh
#AIX-Forum

Originally posted by: nagger

Hi,
Your terminology is all over the place - have you been drinking the Intel/Sparc "coolaid"
In Power Systems Land we have:
POWER7 or POWER7+ processor chip that has normally 8 CPU cores but there is also 4 CPU core and 6 CPU core versions too.

Each CPU core can run 1, 2 or 4 SMT threads (these are also called logical processors which is plain confusing) these threads are sharing the lower level processing units of the CPU core like integer maths unit, floating point unit, compare and branch, etc. So instead of doing just one thing at a time it does many.

When in POWER6/6+ mode you can have SMT1 or SMT2 - there is a rule of thumb then switching to SMT4 gives you a 17% boost but that is an average and your mileage may vary. Particularly, to get this boost you need to have lots of running processes or process threads. At least 1 or two for every SMT thread available. If you don;t the SMT threads 3 and 4 have nothing to run and so make no difference. Also not the 17% could be 5% or could be 25% depending on the code being run.

So the answer is "it depends".

cheers, Nigel Griffiths
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