Re: Oracle RAC - Any formula to calculate/preempt the right number of LMS processes?

Are you running them in real time priority? If yes, you can reduce it to 4 or (# of instances-1) and monitor the latencies of the RAC wait events/CPU usage of the LMS processes. AFAIK there is no magic formula for  calculating the # of LMSs.

One of the most common practices is to schedule the number of RT processes < # of CPUs to avoid spin/hang on CPUs. If your current settings works fine for your workload, I would stick to it.

-Gopal

On Tue, Jun 21, 2011 at 6:02 AM, PD Malik <pdthedba_at_gmail.com> wrote:

> Hello,
>
> We are on 10gR2 5 node RAC (a huge and very busy DB) and also very high
> number of CPUs (80ish) hence the default number of LMS processes that Oracle
> generates for us (num_cpu dependant) is way too high and is causing CPU
> consumption issue (due to their high priority) for some time now so we need
> to look to reduce (and fix) them to a sensible number and going by RAC
> expert Riyaz's advice we shud reduce them down to 4 (one for each 'other'
> RAC node) which makes sense to us particularly becs if we were going 11g and
> left this parameter to Oracle's default calculation it'll come down to 3.
> (Current number of LMS processes : (Due to the varied number of CPUs ) - On
> 2 nodes, 10, on 1 12 and on the other 2 : 8).
>
> Moreover, we've done the analysis and there are no major RAC wait events
> (as in we are not much suffering from RAC) and we "think" we should be fine
> by reducing these and obviously we'll take them thru to Performance Test to
> gain some confidence.
>
> However the question is, is there any other mathematical way of getting to
> just the right number of LMS processes for a RAC instance pls? Is there any
> formula to predict them or even to predict if there will be any latency
> impact by reducing these?
>
> Thanks in advance.
>

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