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"Oracle Global Customer Support officially recommends a " maximum" for SHMMAX of "1/2 of physical RAM".
The maximum size of a shared memory segment is limited by the size of the available user address space. On 64-bit systems, this is a theoretical 2^64bytes. So the "theoretical limit" for SHMMAX is the amount of physical RAM that you have. However, to actually attempt to use such a value could potentially lead to a situation where no system memory is available for anything else. Therefore a more realistic "physical limit" for SHMMAX would probably be "physical RAM - 2Gb".
In an Oracle RDBMS application, this "physical limit" still leaves inadequate system memory for other necessary functions. Therefore, the common "Oracle maximum" for SHMMAX that you will often see is "1/2 of physical RAM". Many Oracle customers chose a higher fraction, at their discretion.
Occasionally, Customers may erroneously think that setting the SHMMAX as recommended in this NOTE limits the total SGA. That is not true. Setting the SHMMAX as recommended only causes a few more "shared memory segments" to be used for whatever total SGA that you subsequently configure in Oracle. For additional detail, please see Document 15566.1, "SGA, SHMMAX, Semaphores and Shared Memory Explained"
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