2.13 Notes about the amount of free space in physical memory

This section provides notes about the amount of free space in physical memory.

For details about the amount of free space in physical memory, see 4.2.2(1) computerSystem group and 4.3.2(23) computerSystem64 group.

• Solaris physical memory

  Solaris allows physical memory to be used not just for ordinary program operations, but also as a file cache (buffer cache). In this case, the portion of physical memory that can be used as a reusable file cache is not freed as soon as its current use ends. Instead, it is retained as a cache in case the file is referenced again. Therefore, after the system has run continuously for a certain period of time, the values of computerSystemFreeMemory and computerSystem64FreeMemory converge toward a fixed value (which depends on the system). Because of this, we recommend that users who want to check either the free space in system memory or memory usage in Solaris monitor virtual memory (swap space) instead of physical memory. The objects computerSystemSwapConfig, computerSystemFreeSwap, computerSystem64SwapConfig, and computerSystem64FreeSwap are useful for monitoring virtual memory (swap space). Use computerSystemFreeMemory and computerSystem64FreeMemory to monitor the amount of free space in physical memory that includes the file cache.

• The amount of free space in physical memory in AIX

  In AIX, file access speed improves when physical memory is used as a file cache. The file cache is therefore included in the amount of physical memory in use, and the amount of free space in physical memory obtained by SNMP Agent is the actual amount of free memory that is available.

  If you want to determine the amount of free memory capacity while excluding the file cache from the amount of physical memory in use, set the environment variable SNMP_HTC_AIX_EXCEPT_FILECACHE to Y in the environment variable definition file SnmpHpunix. Then, the amount of free space in physical memory obtained by SNMP Agent is the sum of the free memory capacity and the file cache value.

  Note that the environment variable SNMP_HTC_AIX_EXCEPT_FILECACHE that is set in the environment variable definition file SnmpHpunix can be used for both computerSystemFreeMemory and computerSystem64FreeMemory.

  For details about the path of the environment variable definition file, see Appendix A. SNMP Agent Files.

  The following shows a specification example for the SNMP_HTC_AIX_EXCEPT_FILECACHE environment variable.

  Example

  SNMP_HTC_AIX_EXCEPT_FILECACHE=Y
  export SNMP_HTC_AIX_EXCEPT_FILECACHE

• The amount of free space in physical memory in Linux

  Linux actively allocates free memory to buffer memory and cache memory. If a memory allocation request is issued from an application, buffer memory and cache memory are freed as necessary, and memory is allocated to the application. Therefore, by default, the amount of free memory in the physical memory acquired by SNMP Agent is the sum of the amount of free memory, buffer memory, and cache memory.

  If you specify Y for the environment variable SNMP_HTC_LINUX_INACTIVE_MEM in the environment variable definition file SnmpHpunix, SNMP Agent will acquire the sum of the amount of free memory, inactive buffer memory, and inactive cache memory as the amount of free memory in the physical memory.

  Note that the environment variable SNMP_HTC_LINUX_INACTIVE_MEM that is set in the environment variable definition file SnmpHpunix can be used for both computerSystemFreeMemory and computerSystem64FreeMemory.

  For details about the path of the environment variable definition file, see Appendix A. SNMP Agent Files.

  The following shows a specification example for the SNMP_HTC_LINUX_INACTIVE_MEM environment variable.

  Example

  SNMP_HTC_LINUX_INACTIVE_MEM=Y
  export SNMP_HTC_LINUX_INACTIVE_MEM

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