Re: [RFC 1/3] Framework for accurate node based statistics

[Andi Kleen <ak@suse.de>]

On Tue, Dec 06, 2005 at 11:36:43AM -0800, Christoph Lameter wrote:
> On Tue, 6 Dec 2005, Andi Kleen wrote:
> 
> > > Yuck. That code uses atomic operations and is not aware of atomic64_t.
> > Hmm? What code are you looking at? 
> include/asm-generic/local.h. this is the default right? And 
> include/asm-ia64/local.h.
>  
> > At least i386/x86-64/generic don't use any atomic operations, just
> > normal non atomic on bus but atomic for interrupts local rmw.
> 
> inc/dec are atomic by default on x86_64?

They are atomic against interrupts on the same CPU. And on Linux
also atomic against preempt moving you to another CPU. And all that
without the cost of a bus lock. And that is what local_t is about.

>  
> > Do you actually need 64bit? 
> 
> 32 bit limits us in the worst case to 8 Terabytes of RAM (assuming a very 
> small page size of 4k and 31 bit available for an atomic variable 
> [sparc]). SGI already has installations with 15 Terabytes of RAM.

Ok we'll need a local64_t then. No big deal - can be easily added.
Or perhaps better a long_local_t so that 32bit doesn't need to
pay the cost.

-Andi

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