DB2 Version 10.1 for Linux, UNIX, and Windows
DB2 Version 10.1 for Linux, UNIX, and Windows
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.
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 has been 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.
Acceptable values are whole numbers in the range of 0 (immediately release any unused database shared memory) to 100 (never release any unused database shared memory). The default is 10 (release unused memory only when more than 10% of database shared memory is currently unused), which should be 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 will be ignored (meaning that unused database shared memory pages will 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 will be ignored.