Large virtual page size support.

[Julian Elischer]

Jeff Roberson wrote:

> I have implemented support in the vm for PAGE_SIZE values which are a 
> multiple of the hardware page size.  This is primarily useful for two 
> things:


Mach (and the VM system we inherrited from it) had this. I beieve it was 
removed with teh comment
"If we need this and someone is willing to support it it can be added 
back" .

It always seemed like in interesting idea and I'm happy to see that it 
is still being looked at.

>
> 1) Shrinking the size of the vm page array so that very large memory 
> x86 PAE machines may boot.
>
> 2) Improving performance of many operations due to decreased page list 
> sizes as well as improved efficiency of many vm operations.  In the 
> particular application that this was developed for the fs block size, 
> page size, and jumbo frame size were all made equal at 8k on a box 
> with 4k pages.  This made page flipping etc. very fast.
>
> This has been done with full userland backwards compatibility.  
> Userland still has the ability to map things in native page size 
> chunks.  The majority of the system software remains unchanged.  The 
> vm gains some complexity and the elf loader gains some complexity 
> since both need to be able to deal with native page size and virtual 
> page size.
>
> The real page size is now CPU_PAGE_SIZE while PAGE_SIZE is the virtual 
> page size which is the smallest unit of memory handed back by the page 
> allocation routines.  KVA is also managed in PAGE_SIZE chunks.  The 
> x86 pmap code has a small allocator that deals with allocating real 
> pages for page table entries.
>
> I wrote this code for a client who would like for it to be in the 
> freebsd tree.  However, it does add some complexity and so I doubt 
> freebsd wants it unless there is a clear demand for it.  What I'd like 
> to know is, does anyone else find this useful?  Do the developers who 
> work on the vm think this is just a horrible hack?  Does anyone care 
> about PAE anymore?
>
> Let me know what you think.  The patch is available at
> http://www.chesapeake.net/~jroberson/8k.diff.  It will not apply to 
> any version of freebsd that you have.  Please consider it read only 
> and not testable until I decide whether it's worth porting.
>
> Cheers,
> Jeff
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