SHMVIRTSIZE

SHMVIRTSIZE

onconfig.std value

8000 on UNIX and 8192 on Windows

if not present

If SHMADD is present: SHMADD If SHMADD is not present: 8

units

Kilobytes

range of values

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

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

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.

takes effect

When the database server is shut down and restarted

utilities

onstat -g seg (see page ***)

refer to

The following material:

• Virtual portion of shared memory, in the shared-memory chapter of the IBM Informix Administrator's Guide
• Chapter on configuration effects on memory utilization, in your IBM Informix Performance Guide

SHMVIRTSIZE specifies the initial size of a virtual shared-memory segment. Use the following algorithm to determine the size of the virtual portion of shared memory:

shmvirtsize = fixed overhead + shared structures +
             mncs * private structures) + other buffers

This algorithm includes the following values.

Value

Description

fixed_overhead

Global pool + thread pool after booting (partially dependent on the number of virtual processors)

shared_structures

AIO vectors + sort memory + dbspace backup buffers + dictionary size + size of stored-procedure cache + histogram pool + other pools (See the onstatdisplay.)

mncs

Maximum number of concurrent sessions

private_structures

Stack (generally 32 kilobytes but dependent on recursion in SPL routines and triggers) + heap (about 30 kilobytes) + session-control-block structures

If messages in the message file indicate that the database server is adding segments to the virtual portion of shared memory for you, add the amount that these messages indicate to the value of SHMVIRTSIZE. It is recommended that you initially create a virtual portion of shared memory of a size that is more than sufficient for your daily processing, if possible.

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