Re: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL
From: Christoph Lameter
Date: Mon Apr 02 2007 - 11:37:50 EST
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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