Introduction:
Starting with AIX V4.3.3, real memory is divided into one or more memory pools depending on the amount of real memory and the number of CPUs in an MP system.
The number of memory pools will be the maximum of #of CPUs/8 or RAMinGB/16, but not more than the # of CPUs and not less than 1. Memory pools are divided into one or
more framesets which allows concurrent memory allocation out of a memory pool.
Each memory pool has its own page replacement daemon, lrud (Least Recently Used Daemon).
Memory allocation is done round-robin across the memory pools where two pages per pool are allocated before moving to the next memory pool.
It is not recommended to change the number of memory pools or number of framesets per memory pool without advice from AIX development.
Memory pools:
To determine the number of memory pools on a machine, use:
- AIX 5L V5.2 and later: vmstat -v | grep “memory pools”
Note: The number of memory pools shown by vmo -a might not represent the actual number of active memory pools, thus vmstat must be used.
- AIX 5L V5.1 and earlier: vmtune -a | grep mempools
The number of memory pools can be changed with:
- AIX 5L V5.2 and later: vmo -r -o mempools=number_of_memory_pools
- AIX 5L V5.1 and earlier: vmtune -m number_of_memory_pools
Changes to the number of memory pools requires bosboot and reboot. While bosboot must be run manually on AIX 5L V5.1 and earlier, the vmo command on
AIX 5L V5.2 and later prompts for running bosboot automatically.
The number of memory pools cannot be changed with vmtune command on AIX 5L V5.2 and later, changes can only be done with vmo.
Framesets:
Each memory pool furthermore consists of one or more framesets. The default is two
framesets per memory pool. This allows concurrent memory allocation out of a single
memory pool. The number of concurrent memory allocations out of one memory pool is equal to the number of framesets in this pool.
The number of framesets in the first memory pool on a system might be different to the number of framesets in all other memory pools.
To determine the number of framesets on a machine:
- AIX 5L V5.2 and later: vmo -a | grep framesets
- AIX 5L V5.1 and earlier: vmtune -a | grep total_frame_sets
As for the number of active memory pools, vmo -a might not show the actual number of active framesets per memory pool.
The number of framesets per memory pool can be changed with:
- AIX 5L V5.2 and later: vmo -r -o framesets=number_of_framesets
- AIX 5L V5.1 and earlier: vmtune -v number_of_framesets
The number of framesets per memory pool cannot be changed with vmtune on AIX 5L V5.2, changes can only be done with vmo.
LRU Buckets:
Rather than scanning the entire page frame list of a memory pool to find pages to steal the page frame list is divided into buckets of page frames. The page replacement algorithm will scan the frames in the bucket and then start over on that bucket for a second scan. Then it will move on to the next bucket.
The default number of frames in a bucket is 131072 and can be changed with vmo/vmtune command.
Reducing the number of frames in an LRU bucket can reduce scanning overhead and latency in page replacement on large memory machines.
