db_mem_thresh - Database memory threshold configuration parameter
This parameter represents the maximum percentage of committed, but currently unused, database shared memory that the database manager will allow before starting to release committed pages of memory back to the operating system.
- Configuration type
- Parameter type
- Configurable online
- Configurable by member in a Db2® pureScale® environment
- Propagation class
- Default [range]
- 100 [0-100 ]
- Unit of measure
This database configuration parameter relates to how the database manager handles excess unused database shared memory. Typically, as pages of memory are touched by a process, they are committed, meaning that a page of memory is allocated by the operating system and occupies space either in physical memory or in a page file on disk. Depending on the database workload, there might be peak database shared memory requirements at a certain times of day. Once the operating system has enough committed memory to meet those peak requirements, that memory remains committed, even after peak memory requirements have subsided.
A value of 0 means to immediately release any unused database shared memory, and a value of 100 means to never release any unused database shared memory. The default for new (not upgraded) databases is 100, which is suitable for most workloads.
This configuration parameter can be updated dynamically. Care should be taken when updating this parameter, as setting the value too low could cause excessive memory thrashing on the box (memory pages constantly being committed and then released), and setting the value too high might prevent the database manager from returning any database shared memory back to the operating system for other processes to use.
This configuration parameter is ignored (meaning that unused database shared memory pages remain committed) if the database shared memory region is pinned through the DB2_PINNED_BP registry variable, configured for large pages through the DB2_LARGE_PAGE_MEM registry variable, or if releasing of memory is explicitly disabled through the DB2MEMDISCLAIM registry variable.
Some versions of Linux® do not support releasing subranges of a shared memory segment back to the operating system. On such platforms, this parameter is ignored.