Re: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL

[Christoph Lameter <clameter@sgi.com>]

On Sun, 1 Apr 2007, Andi Kleen wrote:

> Hmm, this means there is at least 2MB worth of struct page on every node?
> Or do you have overlaps with other memory (I think you have)
> In that case you have to handle the overlap in change_page_attr()

Correct. 2MB worth of struct page is 128 mb of memory. Are there nodes 
with smaller amounts of memory? Note also that the default sparsemem
section size is (include/asm-x86_64/sparsemem.h)

#define SECTION_SIZE_BITS       27 /* matt - 128 is convenient right now */

128MB ....

So you currently cannot have smaller sections of memory anyways.

> Also your "generic" vmemmap code doesn't look very generic, but
> rather x86 specific. I didn't think huge pages could be easily
> set up this way in many other architectures.  

We do this pmd special casing in other parts of the core VM. I have also a 
patch for IA64 that workks with this.

> Do you have any benchmarks numbers to prove it? There seem to be a few
> benchmarks where the discontig virt_to_page is a problem
> (although I know ways to make it more efficient), and sparsemem
> is normally slower. Still some numbers would be good.

You want a benchmark to prove that the removal of memory references and 
code improves performance?

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