Hi,

I would really set MULTIPROCESSOR to 1 on a
multiprocessor machine, regardless of the
SINGLE_CPU_VP setting.

The reason is that with MULTIPROCESSOR being set
to 0 (zero), there are certain assumption being made
on the basis that there's only one CPU in the machine.

[ IDS is always running several processes at the same
time, as there are not only CPUVP processes, but
other VPs as well.

If there's only one CPU in a machine, then there is no
true parallelism happening on that machine. Only one
process can run on the one CPU at any one time.
Therefore it is safe to optimize certain interactions
between processes, as it is certain that two processes
cannot run at the same time. The OS only makes
things look like parallel by (clever) timeslicing of
the CPU to the many processes, but as said above,
there is no true parallelism.

On a multiprocessor machine, however, things are
different. Processes can actually and truly run in
parallel. Therefore interaction between processes
needs to account for that fact and do extra things
to make the interactions in a still safe way.

With MULTIPROCESSOR set to 0 (zero) on a real
multiprocessor machine you actually trick IDS into
thinking that two processes cannot truly run in parallel.
Therefore IDS will think it does not have to do those
extra provisions for interactions among processes.
This can (and sooner or later will) end badly. Therefore
I would not do it. ]

Surely there will soon be a question about Hyperthreading
(or whatever it's called on non-Linux platforms), Dualcore
CPUs and the like. For safety, I would set
MULTIPROCESSOR to 1 on such systems.

Regards,
Martin
--
Martin Fuerderer
IBM Informix Development Munich, Germany
Information Management

owner-informix-list@iiug.org wrote on 13.12.2005 22:10:52:
> Is it a must to set MULTIPROCESSOR to 1 on a multiple CPU server even if
> you only using 1 CPUVP and SINGLE_CPU_VP is set to 1?
>
> Some posts on c.d.i say yes, some say no and the IDS docs do not give a
> concrete answer.
>
> Also, it has been said that when configuring ipcshm poll threads you
> should configure 1 per CPUVP to load balance the polling work on all of
> the CPUVPs. On HP/UX this can cause your number of shared memory
> segments to be greater than the optimal value of 3. Anyone have any
> thoughts on which is better, load balancing the polling work or keeping
> the number of segments at 3?
>
> Thanks,
>
> Andrew Ford
> sending to informix-list
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