IBM I5/OS - Manual (Page 16)

i Series. my Series.
Processor Concepts
Dedicated, Shared, Capped or Uncapped ?
How to decide on using :
Dedicated or shared processors ?
If using shared processors, it is to be capped or uncapped ?
And ... how many virtual processors do I allocate ?
The best performance may be achieved by using dedicated processors ...
However, dedicated processors cannot utilizes excess capacity !
Shared uncapped will allow use of excess capacity of the processor, but ...
Setting virtual processor number too low will limit uncapped usage
Setting virtual processor number too high will degrade performance
What does IBM recommends ?
It depends ...
What do I recommend ?
Keep things simple !
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i Series. my Series.
Processor Concepts
Simple Recommendations
On small configurations, use shared uncapped processor :
Allocate desired processing units according needs
Keep a little portion of unused processing units
Configure uncapped weight according partition importance
Set virtual processor number to a “reasonable” value :
Set a value “close” to the number of allocated processing units
0.4 PU running on 1 VCPU
1.8 PU running on 2 VCPU
Take care of your workload pattern
1.0 PU running on 1 or 2 VCPU ?
Single job, single thread applications will not take benefits when running on multiple VCPU
Multiple job, multiple thread applications will run more efficiently when running on multiple VCPU
Take care of SMT capabilities !
Do not exceed the number of physical processors
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