Re: shmmax sizing recommendations

   First, ORA-4030 is at PGA level. So, I am not sure why SHMMAX is playing a role in a 64 bit environment. Also, How much swap space is allocated? You can encounter ORA-4030 errors if the swap space is insufficient too. Further, You might want to understand which type of PGA heap is growing and ORA-4030 trace files usually can give you a good understanding of the growth Another option is to trigger PGA heapdump at a process level if ORA-4030 encountered.

   I just reviewed that note and it is for Linux x86 platform only. I would go with OSS recommendation. There is nothing wrong in having few more shared memory segments for an instance as long as SHMMAX is meaningful (like 4GB-1). Only downside is that each connect and disconnect will execute more attach and detach calls. This can be a problem in few applications with excessive concurrent connects and disconnects. But, that is an exception.

   I had encountered a client issue related to this setting, but this problem is rare though.
http://orainternals.wordpress.com/2008/10/31/performance-issue-high-kernel-mode-cpu-usage/

Cheers

Riyaj Shamsudeen
Principal DBA,
Ora!nternals - http://www.orainternals.com - Specialists in Performance, Recovery and EBS11i
Blog: http://orainternals.wordpress.com
OakTable member http://www.oaktable.com
Co-author: "Expert Oracle practices: Oracle Database Administration from the Oak Table" http://www.apress.com/book/view/9781430226680

On Wed, Nov 18, 2009 at 8:56 AM, Don Seiler <don_at_seiler.us> wrote:

> While troubleshooting an ORA-04030 issue, Oracle Support also took
> notice that we had shmmax a bit out of whack. Whether or not this
> would contribute to the 04030, I'll not get into right now. This is
> Oracle 11.1.0.7 64-bit on OEL 5.3. The server has 64gb of physical
> RAM. It hosts 3 RDBMS instances and 1 ASM instance, with a sum total
> SGA sizes of ~42 gb and HugePages configured for 44gb. The largest
> instance has an SGA of just under 30gb.
>
> OSS first suggested setting shmmax to "RAM times 0.5 but not greater
> then 4GB-1" and then pointed me to Doc Id 567506.1:
>
> === BEGIN QUOTE ===
> Oracle Global Customer Support officially recommends a " maximum" for
> SHMMAX of just less than 4Gb, or 4294967295.
>
> The maximum size of a shared memory segment is limited by the size of
> the available user address space. On 64-bit systems, this is a
> theoretical 2^64bytes. So the "theoretical limit" for SHMMAX is the
> amount of physical RAM that you have. However, to actually attempt to
> use such a value could potentially lead to a situation where no system
> memory is available for anything else. Therefore a more realistic
> "physical limit" for SHMMAX would probably be "physical RAM - 2Gb".
>
> In an Oracle RDBMS application, this "physical limit" still leaves
> inadequate system memory for other necessary functions. Therefore, the
> common "Oracle maximum" for SHMMAX that you will often see is "1/2 of
> physical RAM". Many Oracle customers chose a higher fraction, at their
> discretion.
>
> One last data point for 64-bit Linux systems: A SHMMAX value of 4Gb or
> greater will lead to coredump limitations. If a customer is willing to
> forfeit complete and successful core file generation under any and all
> circumstances, then a SHMMAX value of 4Gb or greater is fine. It is
> for this reason that Oracle Global Customer Support officially
> recommends a " maximum" for SHMMAX of just less than 4Gb, or
> 4294967295.
> === END QUOTE ===
>
> I'm wondering what the masters out in the field think. Would you
> advise setting SHMMAX to fit the largest SGA? Is there any known
> performance hit by having more, smaller segments for an instance
> rather than 1 large one?
>
> --
> Don Seiler
> http://seilerwerks.wordpress.com
> ultimate: http://www.mufc.us
> --
> http://www.freelists.org/webpage/oracle-l
>
>
>

--
http://www.freelists.org/webpage/oracle-l