Jan,

it appears the monitor is trying to tell you that you have a low "capture 
ratio". Searching for that term should give you a good understanding of what 
the message really means, and in a lot of good advice, e.g. in the "Effective 
zSeries Performance Monitoring Using Resource Measurement Facility" 
publication. (IMHO the term "overhead" is misleading, 'management time' would 
be more accurate).
If you believe there might be a problem you may want to start with verifying 
LPAR configuration and IEAOPTxx settings (any obscure timing parameters? 
Anything violating recommendations?). You could also verify that the capture 
ratio is indeed low using SMF70/72 data.

Phantom weight is irrelevant in this context. It is only a vehicle to tell 
PR/SM how to cap the partition. The fact that the LPAR *is* capped at that time 
(to what extent?) may very well be relevant, though: More work, longer work 
queues...
CPUMF counters won't help you diagnosing anything. CPUMF sampling could help 
-theoretically- but the data may be very hard to evaluate.

Horst Sinram - IBM z/OS Workload Management

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