Re: Cost-Based Optimizer and AIX Virtual Processors

Hey David,

> Has anyone seen execution plans change using virtual processors on AIX? 
How about VM/Linux or VM/Windows?  For the purposes of the optimizer, I'd
think they'd all react in a similar manner.
>
> We've got several partitions on an AIX system.  If a specific instance has
a certain allotment of processors (generally >=2) when a query kicks off,
it runs differently than when the instance is basically idle (<= 1
processor allocated) and the query starts.

2-3 years ago, I briefly tested micro-partioning using 10.2 on a 4-core
blade with 3 LPARs.  Each LPAR had .1 CPUs with a maximum of 2.  IIRC, it
was the CPU_COUNT parameter on DB *startup* as well as the gathered system
statistics that had the most impact on execution plan (at least as far as
micro-partitioning was concerned).

I have some dim memory that system statistics widely varied, which made it
difficult to collect, since other LPARs could easily grab more CPU away from
the one that was collecting stats.

All said, I didn't care for it.  Made planning very difficult, if not
impossible.

Rich




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