From:Steve Adams
Date:09-Feb-2001 14:47
Subject:   SGA size on 32-bit Solaris

Yes, that information is correct. The default sgabeg is 0x80000000 and the default address at which shared libraries are mapped into memory is 0xEF000000. That allows 0x6F000000 of address space for the SGA, which is 1776M, or a bit more than 1.73G.

The 3.7G number is not firm. It depends how little space you need for PGA memory. If you set sgabeg to 0x02333333, that gives you 3.7G for the SGA and leaves about 35M for the oracle executable and the PGA. That should be fine in most OLTP or web environments, but may not be enough for data warehouse environments.

If you do adjust sgabeg to allow large SGAs, it is advisable to limit shmmax so that the SGA will be created in two or more shared memory segments, rather than one large one, to avoid an ISM bug.

The size of the 32-bit address space places no restriction on the total amount of memory that may be used for the SGAs of multiple instances. There is a restriction on that, but it has to do with how much shared virtual memory can be locked into physical memory. That limit is a little less than three quarters of physical memory.

Can you please validate these comments? Can we have databases exceeding 1.7GB in Solaris without adjusting the sgabeg address?

In theory, a 32-bit word size can address 4Gb of memory. I think the total 1.7GB restriction is for HP-UX (32-bit).

Solaris is different. If you don't change sgabeg, the maximum SGA size is 1.73Gb. If you change sgabeg, it's up to 3.7Gb. New additional memory is taken from where PGAs reside. You can exceed 3.7Gb in total.

We have 6GB of physical memory in one of the Sun boxes (32 bit OS). There are around 24 databases and using up around 3.7GB of total memory. If I understand correctly, cumulative size of thes SGAs can not exceed 1.7GB without adjusting the sgabeg address. We have 4 different versions of the software and I checked the sgabeg address in each one of them and they are set at default (0x80000000). Since the cumulative size can not be more than 1.7GB, how are we able to bring up all these database? What am I missing here?