Locking a file backed mdconfig into memory

[Alan Cox]

Matthew D Fleming wrote:
> On Fri, Jun 04, 2010 at 08:20:49AM -0400, John Baldwin wrote:
>   
>> Hmmm, I would just try increasing NKPT then.  You might have to poke 
>> around in sys/amd64 to see what the default size is and how to tune 
>> it.
>>     
>
> When Isilon did the stable/7 merge and amd64 default NKPT changed from 
> 240 to 32 amd64 started having weird pmap issues during boot.  At panic 
> time the stack wasn't very useful, and I didn't finish debugging the 
> issue since eventually I just had to get something working.  We just 
> reverted NKPT to 240 and it worked for us.  I didn't see an anything in 
> optsions.amd64 so I hard-coded it in amd64/include/pmap.h.
>
> Supposedly amd64 can deal with a small NKPT and grow dynamically, but it 
> didn't seem to work for us. :-( Perhaps when we do the next merge 
> project I'll have a few days to devote to debugging the root cause.
>   

NKPT controls the number of page table pages that are initially 
allocated at the bottom of the top 2GB of the kernel address space.  
However, the vast majority of the kernel address space, 510GB in FreeBSD 
 >=7.3, is below these page table pages.  The page table pages for this 
region are dynamically allocated as needed.

If you're booting a kernel and modules greater than 64GB in size, then I 
can certainly see why you would need to increase NKPT.

John, is there some way to know at boot time how big the kernel and 
modules were?  Then, we could probably eliminate NKPT.

Alan