UNIX for Advanced & Expert Users

Have a p570 LPAR capped/smt set to 4 physical processors. And currently have virtual processors set to 6 which gives 12 logical processors at AIX. This is for an Oracle9i database server running 4 instances.

What should virtual processors be set to? at least initially?
And how to determine when to increase/decrease?

I did read something about setting virtual processors to high can cause ovehead....

Thanks.

According to your text the partition you are using has 4 physical processor and is capped. That means that under no circumstances can it claim more than 4 physical processors or 8 cores in that matter.
So here it really makes no sense to provide it with more virtual processors than 4, which will give 8 logical processors in AIX. If you have more processors than that for this lpar the only thing that will happen is that one of the physical processors will have to act as it is two processors - that is switch context more often. This will give you performance decline, probably only small, but on some workloads it can be quite some acctually.

So here I would have the virtual processors set to the same amount as your physical processors. If your partition was uncapped and part of a shared processor pool I would reccomend the virtual processors set to the same number as physical processors in the pool.

That makes sense.
Thank you for the reply.

Sprellarinn was almost correct. It all depends on you version of AIX! Version 5.2 then 4 physical and 4 virtual is correct! Plus I would consider running it in a dedicated setup anyway, which means the virtual element does not count anymore! With version 5.3 things changed drastically! The word virtualisation arrived and with it came the ability to have many virtual CPUs on a few physical ones. There is a limit though and it comes down to the 0.1 slice rule and micro-partitioning! Think in terms of 10 to 1 this would mean an LPAR configured with 2 processing units could effectively have the execution time of 20 physical processors.

The best way to work out the virtual configuration is to take into account the amount of physical storage allocated to the machine. If this is high (high tens of Giga Bytes) then the number of virtual CPUs should be kept to a sensible figure otherwise the paging out of shared memory can kill the performance. I would suggest that with 4 CPUs then a virtual setting of 8 to 10 virtual will not be a bad thing. However if your TL is 4 (oslevel -s) then turn off the vmm_mpsize_support with the command:

# vmo -r -o vmm_mpsize_support=0
# bosboot -a
# shutdown -Fr

Hope this helps!