On Tue, Feb 08, 2005 at 02:44:39PM -0700, Pat Maddox wrote: Memory normally moves along the following path:
Wired -> Active -> Inactive -> Cached -> Free
and then when it gets allocated and used it moves back to Wired.
This is not exactly accurate: Active -> Inactive -> Free is better, with Wired and Cached being mostly seperate categories.
The difference between the categories is mainly that "Inactive" and
"Cached" memory still contains data that the system might be able to
reuse, while "Free" memory is completely free and unused.
In order to use Cached or Inactive memory it might need to be flushed
first, with Inactive probably being dirty and Cached probably not.
("Active" memory is almost certainly dirty and is therefore somewhat
more expensive to reuse.
Wired memory is typically the kernel text (executable code), any kernel modules which have been loaded, and dynamic kernel memory used for critical structures like the process table, descriptor table, VM page tables, which tend to be staticly allocated. The pager never touches these, they are always resident in RAM.
More dynamic kernel data structures like the I/O buffer used for disk access, network buffers, and the like are also wired down, but the system will adjust the size and flush pages of data from open files and the like to disk depending on the situation. That pool of memory is the Cached category.
The set of pages which have been accessed by processes "very recently" in the Active category. It includes dirty regions as well as program executable code (which are not dirty).
-- -Chuck
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