This section gives an overview of memory as it relates to AIX. I discuss how AIX uses virtual memory to address more memory than is physically on your system. I also explain how the VMM actually works and how it services requests.
Any discussion of memory and AIX must start with a description of the VMM. AIX newbies are sometimes surprised to hear that the VMM services all memory requests from the system, not just virtual memory. When RAM is accessed, the VMM needs to allocate space, even when there is plenty of physical memory left on the system. It implements a process of early allocation of paging space. Using this method, the VMM plays a vital role in helping manage real memory, not only virtual memory. Here is how it works. In AIX, all virtual memory segments are partitioned into pages. In AIX, the default size per page is 4KB. Allocated pages can be either RAM or paging space (virtual memory stored on disk). VMM also maintains what is referred to as a free list, which is defined as unallocated page frames. These are used to satisfy page faults. There are usually a very small amount of unallocated pages (which you configure) that the VMM uses to free up space and reassign the page frames to. The virtual memory pages whose page frames are to be reassigned are selected using the VMM's page replacement algorithm. This paging algorithm determines which virtual memory pages currently in RAM ultimately have their page frames brought back to the free list. AIX uses all available memory, except that which is configured to be unallocated and known as the free list.
To reiterate, the purpose of VMM is to manage the allocation of both RAM and virtual pages. From here, you can determine that its objectives are to help minimize both the response time of page faults and to decrease the use of virtual memory where it can. Obviously, given the choice between RAM and paging space, most people would prefer to use physical memory, if the RAM is available. What VMM also does is classify virtual memory segments into two distinct categories. The categories are working segments using computational memory and persistent segments using file memory. It is extremely important to understand the distinction between the two, as this helps you tune the systems to their optimum capabilities.
Computational memory is used while your processes are actually working on computing information. These working segments are temporary (transitory) and only exist up until the time a process terminates or the page is stolen. They have no real permanent disk storage location. When a process terminates, both the physical and paging spaces are released in many cases. When there is a large spike in available pages, you can actually see this happening while monitoring your system. In the VMM world when free physical memory starts getting low, programs that have not been used recently are moved from RAM to paging space to help release physical memory for more real work.
File memory (unlike computational memory) uses persistent segments and has a permanent storage location on the disk. Data files or executable programs are mapped to persistent segments rather than working segments. The data files can relate to filesystems, such as JFS, JFS2, or NFS. They remain in memory until the time that the file is unmounted, a page is stolen, or a file is unlinked. After the data file is copied into RAM, VMM controls when these pages are overwritten or used to store other data. Given the alternative, most people would much rather have file memory paged to disk rather than computational memory.
When a process references a page which is on disk, it must be paged, which could cause other pages to page out again. VMM is constantly lurking and working in the background trying to steal frames that have not been recently referenced, using the page replacement algorithm I discussed earlier. It also helps detect thrashing, which can occur when memory is extremely low and pages are constantly being paged in and out to support processing. VMM actually has a memory load control algorithm, which can detect if the system is thrashing and actually tries to remedy the situation. Unabashed thrashing can literally cause a system to come to a standstill, as the kernel becomes too concerned with making room for pages than to actually do anything productive.
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