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/3GB switch on Windows-32bit [message #379880] Thu, 08 January 2009 04:21 Go to next message
Messages: 34
Registered: February 2008
Location: Delhi
I have server with 8GB RAM and 4 Quad core processor. I am running Oracle (32bit). I have implemented /3GB /PAE switch in boot.init and increased the SGA to 2300MB.

Now i want to create one more database into this server so my question is:

Is the memory for another instance will be separate from the previous instance or all the instances will collectively use the limit of 3GB (including 300m for defragmentation)?

So whether another instance will get 3gb memory?

Please suggest.

Re: /3GB switch on Windows-32bit [message #380161 is a reply to message #379880] Fri, 09 January 2009 06:21 Go to previous messageGo to next message
Frank Naude
Messages: 4570
Registered: April 1998
Senior Member
You have more than enough RAM, so a second (or third, but not 4th) instance can also have a 2300MB SGA.
Re: /3GB switch on Windows-32bit [message #380165 is a reply to message #380161] Fri, 09 January 2009 06:30 Go to previous message
Messages: 961
Registered: August 2006
Location: Stockholm/Sweden
Senior Member

Some interesting reading

1. One can have any amount of RAM up to 64GB, but only 2GB of it is addressable by application and Oracle instances on the server without / 3GB and /PAE switches. Another 2GB is used by Kernel (so total of 4GB out of 64GB is acessible/addressable). So, yes, every database instance or application can access up to 2GB, but total memory addressable is also 2GB. Therefore, swapping and out of memory errors will happen if every Oracle instance has set large SGA.

2. If one uses /3GB switch, application and Oracle can address up to 3GB (instead of 2GB). Kernel uses only 1GB. Rest is same as in 1.

3. If /PAE switch is on, then memory above 4GB is accessible to
Oracle, i.e., up to 64GB memory is addressable. However, one can only place database buffers in PAE memory (addressable between 4GB and 64GB). Everything else, shared pool, java pool, pga etc is still limited to 2GB (or 3GB if /3GB switch is on). So same restrictions as in 1 and 2 apply for multiple database instances.

4. If both /3GB and /PAE are set, only 16GB (instead of 6GB) can be addressed by Oracle.

5. PAE memory can be used by multiple database instances. For example,if instance 1 has 4GB of buffer cache, instance 2 12GB, total PAE memory used is 16GB. PAE memory is not as fast as non PAE memory because a few reditections (memeory mappings) are required.

6. Each database instance has its own AWE memory window and it comes from lower 3GB memory (i.e. non PAE memory). But memory window size set using a registery variable is same for all database instances. So if registery variable is set to 256M (in bytes), instance 1 will use 256M from 3GB, instance 2 another 256M from 3GB. Shared pool, java pool, pga etc must also fit in 3GB.

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