Re: [Lse-tech] Re: 10.31 second kernel compile

On Sat, Mar 16, 2002 at 09:05:04PM +0100, Andi Kleen wrote:
> On Sat, Mar 16, 2002 at 12:57:11PM -0700, [e-mail] wrote:
> > On Sat, Mar 16, 2002 at 08:32:26PM +0100, Andi Kleen wrote:
> > > x86-64 aka AMD Hammer does hardware (or more likely microcode) search of 
> > > page tables.
> > > It has a 4 level page table with 4K pages. Generic Linux MM code only sees
> > > the first slot in 4th level page limit user space to 512GB with 3 levels. 
> > 
> > What about 2M pages?
> 
> They are not supported for user space, but used in private mappings
> for kernel text and direct memory mappings. Generic code never sees them.
> 
> That will hopefully change eventually because 2M pages are a bit help for
> a lot of applications that are limited by TLB thrashing, but needs some 
> thinking on how to avoid the fragmentation trap (e.g. I'm considering
> to add a highmem zone again just for that and use rmap with targetted
> physical freeing to allocating them) 

To me, once you have a G of memory, wasting a few meg on unused process 
memory seems no big deal.

> > There was something in some AMD doc about preventing tlbflush on process
> > switch - through a context like thing perhaps? Any idea?
> 
> There are global pages which are normally not flushed over context switch.
> That is used for all kernel mappings. 
> 
> There is also some optimization in the CPU that tries to do a selective
> flush only when you reload CR3, but as far as I can see doesn't help
> for the Linux context switch. It only works around broken TLB flushing
> algorithms in some Windows version.

They say:
	Hammer microarchitecture features a flush filter allowing multiple
	processes to share TLB without SW intervention.

Not a lot of technical detail in that.

> 
> -Andi

-- 
---------------------------------------------------------
Victor Yodaiken 
Finite State Machine Labs: The RTLinux Company.
 www.fsmlabs.com  www.rtlinux.com