Question & Answer
Question
How to get accurate information about memory consumption on AIX Server by using svmon command?
Answer
In order to get accurate information on how much memory is consumed and how much still available to OS and applications issue command
# svmon -G -O unit=auto
Observe 'memory available' which shows physical memory still available before the system starts paging to disks. (memory available ->2nd column from right).
Memory consumption percentage is virtual memory (memory-virtual) divided by the amount of physical memory (memory size).
In our example 'memory virtual' ~ 66 GB and 'memory size' ~ 128 GB so memory consumption is 66 ÷ 128 = 0.515 or 51.5%.
In our example 'memory virtual' ~ 66 GB and 'memory size' ~ 128 GB so memory consumption is 66 ÷ 128 = 0.515 or 51.5%.
Note:
Do not focus on (1) memory inuse or (2) memory free.
- memory inuse includes filesystem cache, which is in fact available memory
- memory free because it does not show memory used for filesystem caching
Note:
All modern operating systems use any physical memory not used by the OS - Applications for filesystem caching. RAM access takes between => 500-600 CPU cycles <= , disk access takes up to => 20 million <=. So, if there is free memory available, OS will try to cache all files it reads for future use. It means the memory inuse grow to approach 100% in most systems eventually. The current size of the file cache can be calculated as (memory size) - (memory virtual). The minimum and maximum size is defined by minperm and maxperm respectively.
AIX also maintains a collection of free page frames (memory free) that it used to quickly satisfy any requests for new virtual or file pages. AIX always starts to replenish this list when it gets below maxfree. When the system is booted, this free page collection will be quite large, but will eventually fall to something approaching maxfree as pages are used for applications and file cache. It is the expected behavior.
AIX favors OS-Application virtual memory, so if new virtual memory pages are required and there are no free frames, AIX discards file cache and use those frames for virtual memory. Most systems operate with nearly 100% memory inuse and with a small memory free list sitting around maxfree. Memory is considered overcommitted only when memory virtual is more than 100% of physical memory minus minperm (97% with default settings).
AIX also maintains a collection of free page frames (memory free) that it used to quickly satisfy any requests for new virtual or file pages. AIX always starts to replenish this list when it gets below maxfree. When the system is booted, this free page collection will be quite large, but will eventually fall to something approaching maxfree as pages are used for applications and file cache. It is the expected behavior.
AIX favors OS-Application virtual memory, so if new virtual memory pages are required and there are no free frames, AIX discards file cache and use those frames for virtual memory. Most systems operate with nearly 100% memory inuse and with a small memory free list sitting around maxfree. Memory is considered overcommitted only when memory virtual is more than 100% of physical memory minus minperm (97% with default settings).
Related Information
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Document Information
Modified date:
30 September 2021
UID
ibm16477326
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