I only cache mappings, so it doesn't really matter. Creating mappings is very very cheap (especially if you allow Mach to pick the addresses by using the anywhere flag). Creating mappings is just vm_map, which is speedy and quick--it's the same as vm_allocate. But I would use LRU for lack of something else.
Mappings carry no overhead, and are quick and easy to set up and tear
down.
This is new to me. I always thought that vm_map/vm_unmap is expensive.
Currently, the ext2fs patch uses fixed-size cache of pages, which are used to map and unmap different disk blocks. But it will be much more flexible if we have set of vm_mapped pages. Requesting unmapped disk block vm_maps page (where Mach decides) that contains that block. Evicting this page automatically (by ext2fs) vm_unmaps it too. All these pages are part of the same pager that grows as necessary.
Regards, ogi
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