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From: "deebeeay@gmail.com" <deebeeay@gmail.com>
Newsgroups: comp.databases.oracle.server
Subject: Re: sga_max_size - pre-allocated at instance startup?
Date: 3 Mar 2006 01:16:53 -0800
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Thanks for your reply Laurenz. Our platform is HP-UX. I managed to find
a discussion on the HP forums which seems to suggest that Oracle grabs
all the memory specified in SGA_MAX_SIZE at startup.

And indeed running an ipcs -mb shows that Oracle has grabbed that
amount of memory

Cheers
Dee

Laurenz Albe wrote:
> deebeeay@gmail.com <deebeeay@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