Memory Tuning

Apart from re-writing or modifying the application then there are very parameters that control the memory used by the engine

	The major user of the resident shared memory segment is the buffer pages, where each buffer is either 2k or 4k bytes.
	Limit the amount of memory decision support queries can use with DS_TOTAL_MEMORY Parallel Data Query can use a great deal of memory for parallel processing.You can constrain the amount of memory used by all DSS queries using PDQ by adjusting DS_TOTAL_MEMORY.
	Informix engines have always liked memory and will, given the opportunity, use all that is available if the engine decides it is required. To prevent this from happening SHMTOTAL can be used to restrict the total memory that the engine can allocate. However setting it too low will result in 'Out of Memory' errors (typically errno=12).

Big segments not small ones, drop back to Unix for explanation why it is bad to have lots of small segments

The onstat -g seg command displays the shared memory segment information.

Segment Summary:
(resident segments are not locked)
id        key     addr     size  ovhd class blkused blkfree
0  1381451777  a000000  2727936   868   R     329       4
1  1381451778  a29a000  8192000   720   V     333     667
2  1381451779  aa6a000   155648   596   M      16       3
3  1381451790  aae7000   155648   596   M       0      19
Total: -         -     11075584     -   -      678    693

The onmode -F command will free segments that are not in use. It does this, via the MGM which examines every memory pool and frees the unused memory, and then it examines each segment and will release to the OS any segments that are unused. This can be a CPU intensive operation and sime degradation of performance while this command is being run should be expected.

A Memory Leak

Any driver that uses Kernel Memory Allocator (KMA) resources, but does not specifically release them before exiting can create a memory leak. A memory leak causes the amount of memory allocated by KMA to increase over time. Thus, if the alloc field increases steadily over time, they may be a memory leak. Another indication of a memory leak is failed requests. If this occurs, then it is likely that a memory leak has caused KMA to be unable to reserve and allocate memory.

Detecting memory leaks on a live system can be very difficult. [The DBAudit demo shows a memory leak on a production system, caused by faulty datablade] However, if luxury of a test system is available then it is relatively straightforward to monitor the engine's memory usage and determine whether it is slowly increasing over time.