SHMVIRTSIZE configuration parameter

Use the SHMVIRTSIZE configuration parameter to specify the initial size of a virtual shared-memory segment.

onconfig.std value
Platform dependent
if not present
If SHMADD is present: the value of the SHMADD configuration parameter.

If SHMADD is not present: 8192.

values
32-bit platforms: Positive integer with a maximum value of 2 GB

64-bit platforms: Positive integer with a maximum value of 4 TB

The maximum value might be less on some platforms due to operating-system limitations. For the actual maximum value for your UNIX platform, see the machine notes.

units
KB
takes effect
After you edit your onconfig file and restart the database server.

Usage

To determine the appropriate value for the SHMVIRTSIZE configuration parameter, use the following algorithm to determine the size of the virtual portion of shared memory:
shmvirtsize = fixed overhead + ((stack size + heap) * number of users)
Variable Value to use
fixed overhead This includes the size of the AIO vectors, sort memory, dbspace backup buffers, dictionary size, size of stored-procedure cache, histogram pool, other pools, and other overhead.
To obtain an estimate of the fixed overhead, start the database server and see how many additional memory segments are allocated, if any. The number of users that you have on the system when you start the server, impacts the allocation of memory segments. When you start the server:
  • If the number of users is typical for your environment, then add the size of the memory segments to the current value for the SHMVIRTSIZE configuration parameter and restart the server.
  • If the number of users is far less than what is typical for your environment, you must calculate the appropriate overhead value to use for the memory segments. You can determine how many memory segments each user consumes by dividing the number of additional memory segments that are allocated when you started the server by the number of users that you had on the server then. Multiply the value for the memory segments for each user by the number of users that you typically have on the system. Add this calculated value for the memory segments to the current value for SHMVIRTSIZE configuration parameter and restart the server.
stack size On 32-bit systems, use 32 KB for the stack size. Typically on 64-bit systems, you use 64 KB for the stack size. However, some 64-bit systems use a different value.
heap Use 30 KB per user.
number of users Use the maximum number of concurrent user sessions that you anticipate on the server.

If possible, create a virtual portion of shared memory of a size that is more than you require for your daily processing.

Use the onstat -g seg command to determine peak usage and lower the value of the SHMVIRTSIZE configuration parameter accordingly.


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