Re: [patch 0/4] V2 ia64 SPARSEMEM

Bob Picco
2005-05-25T23:20:35Z

Jack Steiner wrote: [Wed May 25 2005, 06:44:06PM EDT]
> On Wed, May 25, 2005 at 11:13:47AM -0400, Bob Picco wrote:
> ...
> > and HPSIM. An early version of ia64 with SPARSEMEM was tested by Jesse. It
> > would be optimal to have another test pass on SGI NUMA hardware.
>
>
> I'm away from the office this week, but next week, I'll give the patch
> another spin on a big SGI system.
Great!
>
> >
> >
> > VIRTUAL_MEM_MAP was introduced on ia64 because of memory holes on ia64
> > platforms. The mem_map of the pglist_data is a pointer to a virtual
> > contiguous array of page structures in memory for a node. To
> > eliminate memory waste contiguous memory holes don't have page structures.
> > The alternative would be for holes in memory to be represented by reserve
> > page structures. The VIRTUAL_MEM_MAP solutions requires ia64_pfn_valid
> > and a hook in the buddy allocator to check for holes in memory.
> >
> > SPARSEMEM has eliminated mem_map, ia64_pfn_valid and the buddy allocator
> > hook. There is a small cost for SPARSEMEM. Any section which has both
> > memory and holes requires reserved pages for the holes. I can see
>
> Our systems are notorious for having large holes in memory on nodes. For example,
> a node may have memory at node offsets 0, 16GB, 32GB & 48GB. At each offset, you
> can have a chunk of 1 to 16GB. For example, a typical system might have memory
> at the following node offsets:
> Memory at
> 0 - 4GB
> 16 - 20GB
> 32 - 36GB
> 48 - 52GB
>
> You are guaranteed to have memory at node offset 0. Memory may or
> may not exist at the other offsets.
>
> I haven't had a chance to look at the patch, but will page struct entries
> will be allocated for the pages in the holes (4GB-16GB, etc).
No there won't be page structures allocated for regions which are holes.
The default SECTION_BITS value of 28 is for a 256Mb aligned section. So memory
not reported (via efi memory descriptor walk) in a section won't have page
structs. The function register_sparse_mem in discontig.c calls SPARSEMEM
to validate a memory section. Only valid sections are subject to page
structure allocation later in initialization.
>
>
>
> --
> Thanks
>
> Jack Steiner (steiner@sgi.com) 651-683-5302
> Principal Engineer SGI - Silicon Graphics, Inc.
>
your welcome and thanks,

bob
>