Re: Review of Thomas's >2GB ext2fs proposal

[Ognyan Kulev]

Thomas Bushnell BSG wrote:
> 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