RE: Cost-Based Optimizer and AIX Virtual Processors
- From: "Crisler, Jon" <Jon.Crisler@xxxxxxx>
- To: <david.barbour1@xxxxxxxxx>, <rjoralist@xxxxxxxxxxxxxxxxxxxxx>
- Date: Thu, 30 Apr 2009 23:30:47 -0400
IIRC the cpu_count parameter is dynamic- you can change it within
certain limits after startup. We have done this on certain Sun T1 and
T2 based machines to alter behavior of parallel query and RMAN
processes, and I had a short class where it was mentioned you can do
this on AIX LPAR as well. However, it was more in the context of
software licensing for Oracle, as AIX LPAR licensing can get
tricky....as if multi-core and multi-socket licensing was not confusing
enough.
________________________________
From: oracle-l-bounce@xxxxxxxxxxxxx
[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of David Barbour
Sent: Thursday, April 30, 2009 12:34 PM
To: rjoralist@xxxxxxxxxxxxxxxxxxxxx
Cc: Oracle-L Freelists
Subject: Re: Cost-Based Optimizer and AIX Virtual Processors
Don't have a sample yet. This incident (which actually may be
incidents) occurred on new hardware where we've installed a sandbox as
part of a looming SAP upgrade. I'm taking snapshots and doing some
other monitoring now. When our development team takes a breath, I'm
going to ask them to run the job when the box is ramped up and when it's
relatively quiet.
Rich's recollection sort of matches my gut feeling (we had TONS of
issues in Production with application servers and load balancing until
we stopped the virtual CPU/Memory) but until I can collect some
empirical evidence or someone can point me to a discussion or article on
the subject, I really don't have much to go on.
Keep you informed.
On Thu, Apr 30, 2009 at 10:13 AM, Rich Jesse
<rjoralist@xxxxxxxxxxxxxxxxxxxxx> wrote:
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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