Oracle FAQ Your Portal to the Oracle Knowledge Grid
HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US
 

Home -> Community -> Usenet -> c.d.o.server -> Re: sga_max_size - pre-allocated at instance startup?

Re: sga_max_size - pre-allocated at instance startup?

From: Laurenz Albe <invite_at_spam.to.invalid>
Date: 03 Mar 2006 08:15:24 GMT
Message-ID: <1141373722.984104@proxy.dienste.wien.at>


deebeeay_at_gmail.com <deebeeay_at_gmail.com> wrote:
> Is sga_max_size pre-allocated in memory at instance startup?
>
> I can't seem to find a definitive answer from docs or the web. Some
> suggest yes but only on certain OSs.

I can shed light on this as I was also dismayed by the woefully vague documentation and opened a service request to get the word from the source.

We asked specifically for AIX and the answer was: yes, all the shared memory in SGA_MAX_SIZE is allocated at instance startup, but only SGA_TARGET of it is actually used.

The documentation states that on Windows and Solaris the amount of allocated shared memory will change dynamically with SGA_TARGET.

From that and the handwaving in the documentation ('On some UNIX platforms that do not support dynamic shared memory') I would guess that most UNIX platforms will have it like AIX.

This is a little aggrevating as AIX is a UNIX platform that does support dynamic shared memory.

You can try it out by setting SGA_MAX_SIZE different from SGA_TARGET and use the ipcs command to see how much shared memory is actually allocated.

However, even if all the shared memory is allocated at instance startup, that is not a problem because as long as it is not actually accessed, it will get paged out and stay there.

Yours,
Laurenz Albe Received on Fri Mar 03 2006 - 02:15:24 CST

Original text of this message

HOME | ASK QUESTION | ADD INFO | SEARCH | E-MAIL US