Re: RFC: two-level page tables for 16KB pages on LP64 kernels

[Matt Thomas <matt%3am-software.com@localhost>]

On Aug 28, 2011, at 2:32 PM, Manuel Bouyer wrote:

> On Fri, Aug 26, 2011 at 03:35:13PM -0700, Matt Thomas wrote:
>> 
>> Normally, MIPS uses a 3 level page table for a 64-bit kernel which gives 1TB 
>> (40 [12+10+9+9] bits) of user address space when using 4KB pages.  A 3 level 
>> page table using 16KB pages can address 256TB (48 [14+12+11+11] bits) while 
>> a 2 level page table using 16KB pages can address 128GB (37 [14+12+11] bits) 
>> of address space.
>> 
>> Thinking about it, 128GB seems to more than enough on the GDIUM or other 
>> Loongson2 base platforms but it is significantly less the 16TB that it could 
>> use.  I have to think the smaller (but adequate) address space using a 
>> simple 2 level page table might result in a small performance boost.
>> 
>> I figure this could be a config file option.  Maybe even defining 
>> VM_MAX_USER_ADDRESS in an option file and letting the kernel figure out how 
>> many levels are needed to accomodate that maximum.
> 
> 
> How many instructions will it save in the trap handler ?
> With 16k pages we should already have less TLB miss; I'm not sure the
> performance gain justifies the maintenance cost of having another TLB
> handler ...
> but if you have patches to test, I'd be happy to try and benchmark on
> my fuloong.

It will also save 1 16KB page per process (pmap).