Re: SGA_MAX_SIZE on Solaris

Ah ha... Excellent info in the note... This bit explains the behavior I saw...

Solaris limitation

Keep in mind that on most Unix platforms, Oracle9i will allocate sufficient shared memory segment(s) to have a total sized at, or just above the size specified by SGA_MAX_SIZE when the instance is started. The default behavior on Solaris platforms is to lock this/these shared memory segment(s) into physical memory by forcing the use of ISM (Intimate Shared Memory). This is the default behavior of Oracle and is due to the operating system limitation in the shared memory management area. It is not recommended to disable this behavior as a serious performance penalty will be incured.

In Solaris 8, thanks to DISM support (Dynamic Intimate Shared Memory), it is possible to avoid the above limitation and allocate the shared memory segmentsuch that only the active granules in the SGA are located in physical memory. All unused granules will be stored in swap until needed.

Thanks Sten....

Rognes, Sten wrote:

>Be careful with using sga_max_size on Solaris (I am assuming that's your
>platform). There is a Solaris bug which is claimed to can cause up to a 90%
>performance degradation, bug#4675878. This bug affects both Solaris 8 and
>earlier releases of Solaris 9. Although there are a Solaris 8 patch
>available or in the works for this, I would not recommend using this feature
>unless you are on Solaris 9 update 2(4/03) where large page support is
>introduced for DISM segments and DISM and ISM performance is similar.
>
>There are some notes on Metalink that will answer your questions below,
>check out # 151222.1.
>
>
>Sten
>
>-----Original Message-----
>From: DENNIS WILLIAMS [mailto:DWILLIAMS_at_LIFETOUCH.COM]
>Sent: Thursday, March 11, 2004 2:08 PM
>To: 'oracle-l_at_freelists.org'
>Subject: SGA_MAX_SIZE on Solaris
>
>
>Has anyone experimented with SGA_MAX_SIZE? Does Oracle request that amount
>of memory on startup? If memory of the components like SHARED_POOL_SIZE and
>DB_CACHE_SIZE are smaller than SGA_MAX_SIZE, does Oracle only request enough
>memory for the components? If the size of one of the components is reduced
>dynamically, does Solaris eventually reclaim that space?
> I have a test system with many Oracle9i instances. Most are little used,
>and I allocate only a small amount of memory. However, now and then a
>development group will start pounding an instance. Often they run out of
>memory. So I have to audit the memory settings of all instances, then notify
>developers on other instances that I need to bounce their instance to
>retrieve some memory. A hassle for everyone concerned.
> I'm wondering if I can set the SGA_MAX_SIZE high on all instances, but
>then just allocate a small amount of memory. Then if an instance needs more
>memory, I could allocate it dynamically, and also dynamically reduce the
>memory allocated in other instances.
> Any thoughts appreciated.
>
>Dennis Williams
>DBA
>Lifetouch, Inc.
>dwilliams_at_lifetouch.com
>
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-- 
Regards,
Tim Johnston
Tel: 978-322-4226
Fax: 978-322-4100


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