Thank you Michael! I will try it out, and will post results later, but not next couple of days, since I have some stuff on hands approaching the deadline.
John
--- On Mon, 10/6/08, C Michael Sundius <Michael.sundius@xxxxxxxxxx> wrote:
> From: C Michael Sundius <Michael.sundius@xxxxxxxxxx>
> Subject: Re: Have ever checked in your mips sparsemem code into mips-linux tree?
> To: "Andy Whitcroft" <apw@xxxxxxxxxxxx>, "Dave Hansen" <dave@xxxxxxxxxxxxxxxxxx>, linux-mm@xxxxxxxxx, linux-mips@xxxxxxxxxxxxxx, "VomLehn, David" <dvomlehn@xxxxxxxxx>, me94043@xxxxxxxxx
> Date: Monday, October 6, 2008, 1:15 PM
> adding patch 2 containing Documentation:
>
>
>
>
> - - - - - Cisco
> - - - - -
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> computer.From e01ad377b29c0e5c39289bece382e1f78f6e7e2c Mon
> Sep 17 00:00:00 2001
> From: Sundis <sundism@xxxxxxxxxxxxxxxxxxxxxxxxxx>
> Date: Mon, 6 Oct 2008 10:31:08 -0700
> Subject: [PATCH] mips sparsemem howto
>
> ---
> Documentation/sparsemem.txt | 92
> +++++++++++++++++++++++++++++++++++++++++++
> 1 files changed, 92 insertions(+), 0 deletions(-)
> create mode 100644 Documentation/sparsemem.txt
>
> diff --git a/Documentation/sparsemem.txt
> b/Documentation/sparsemem.txt
> new file mode 100644
> index 0000000..0b36412
> --- /dev/null
> +++ b/Documentation/sparsemem.txt
> @@ -0,0 +1,92 @@
> +Sparsemem divides up physical memory in your system into N
> sections of M
> +bytes. Page tables are created for only those sections
> that
> +actually exist (as far as the sparsemem code is
> concerned). This allows
> +for holes in the physical memory without having to waste
> space by
> +creating page descriptors for those pages that do not
> exist.
> +When page_to_pfn() or pfn_to_page() are called there is a
> bit of overhead to
> +look up the proper memory section to get to the
> page_table, but this
> +is small compared to the memory you are likely to save.
> So, it's not the
> +default, but should be used if you have big holes in
> physical memory.
> +
> +Note that discontiguous memory is more closely related to
> NUMA machines
> +and if you are a single CPU system use sparsemem and not
> discontig.
> +It's much simpler.
> +
> +1) CALL MEMORY_PRESENT()
> +Existing sections are recorded once the bootmem allocator
> is up and running by
> +calling the sparsemem function "memory_present(node,
> pfn_start, pfn_end)" for each
> +block of memory that exists in your physical address
> space. The
> +memory_present() function records valid sections in a data
> structure called
> +mem_section[].
> +
> +2) DETERMINE AND SET THE SIZE OF SECTIONS AND PHYSMEM
> +The size of N and M above depend upon your architecture
> +and your platform and are specified in the file:
> +
> + include/asm-<your_arch>/sparsemem.h
> +
> +and you should create the following lines similar to
> below:
> +
> + #ifdef CONFIG_YOUR_PLATFORM
> + #define SECTION_SIZE_BITS 27 /* 128 MiB */
> + #endif
> + #define MAX_PHYSMEM_BITS 31 /* 2 GiB */
> +
> +if they don't already exist, where:
> +
> + * SECTION_SIZE_BITS 2^M: how big each section
> will be
> + * MAX_PHYSMEM_BITS 2^N: how much memory we
> can have in that
> + space
> +
> +3) INITIALIZE SPARSE MEMORY
> +You should make sure that you initialize the sparse memory
> code by calling
> +
> + bootmem_init();
> + + sparse_init();
> + paging_init();
> +
> +just before you call paging_init() and after the
> bootmem_allocator is
> +turned on in your setup_arch() code.
> +
> +4) ENABLE SPARSEMEM IN KCONFIG
> +Add a line like this:
> +
> + select ARCH_SPARSEMEM_ENABLE
> +
> +into the config for your platform in
> arch/<your_arch>/Kconfig. This will
> +ensure that turning on sparsemem is enabled for your
> platform.
> +
> +5) CONFIG
> +Run make *config, as you like, and turn on the sparsemem
> +memory model under the "Kernel Type" -->
> "Memory Model" and then build your
> +kernel.
> +
> +
> +6) Gotchas
> +
> +One trick that I encountered when I was turning this on
> for MIPS was that there
> +was some code in mem_init() that set the
> "reserved" flag for pages that were not
> +valid RAM. This caused my kernel to crash when I enabled
> sparsemem since those
> +pages (and page descriptors) didn't actually exist. I
> changed my code by adding
> +lines like below:
> +
> +
> + for (tmp = highstart_pfn; tmp < highend_pfn; tmp++) {
> + struct page *page = pfn_to_page(tmp);
> +
> + + if (!pfn_valid(tmp))
> + + continue;
> + +
> + if (!page_is_ram(tmp)) {
> + SetPageReserved(page);
> + continue;
> + }
> + ClearPageReserved(page);
> + init_page_count(page);
> + __free_page(page);
> + physmem_record(PFN_PHYS(tmp), PAGE_SIZE,
> physmem_highmem);
> + totalhigh_pages++;
> + }
> +
> +
> +Once I got that straight, it worked!!!! I saved 10MiB of
> memory.
> --
> 1.5.4.1
