WARNING: Do not use AMM with Linux Hugepages

28 December 2008

A very common error on Linux platforms is attempting to implement automatic memory management with the Linux hugepages support.

Oracle does NOT recommend using Automatic Memory Management (AMM, e.g. setting memory_target) with Linux hugepages. See MOSC note 749851.1 "HugePages and Oracle Database 11g Automatic Memory Management (AMM) on Linux" for important details. The message ORA-00845 addresses this error:

ORA-00845: MEMORY_TARGET not supported on this system

Cause: The MEMORY_TARGET parameter was not supported on this operating system or /dev/shm was not sized correctly on Linux.

Action: Refer to documentation for a list of supported operating systems. Or, size /dev/shm to be at least the SGA_MAX_SIZE on each Oracle instance running on the system.

AMM is a memory-morphing tool that adjusts the sizes of the SGA pool reactively, as workloads change. It works well on tiny systems, but for large databases, the DBA can perform the SGA pool size adjustment proactively, before a workload change cripples performance.

In sum, AMM is not compatible with Linux Hugepages.

Also, beware that AMM re-size operations can cripple Oracle performance in some cases.

For mission-critical databases, I recommend building intelligent SGA monitors that predict workload changes before they occur and change the SGA just-in-time. For complete details, see my book Oracle Tuning: The Definitive Reference.

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