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Re: Have ever checked in your mips sparsemem code into mips-linux tree?

To: Andy Whitcroft <apw@shadowen.org>, Dave Hansen <dave@linux.vnet.ibm.com>, linux-mm@kvack.org, linux-mips@linux-mips.org, "VomLehn, David" <dvomlehn@cisco.com>, C Michael Sundius <Michael.sundius@sciatl.com>
Subject: Re: Have ever checked in your mips sparsemem code into mips-linux tree?
From: John <me94043@yahoo.com>
Date: Mon, 6 Oct 2008 17:33:19 -0700 (PDT)
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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@sciatl.com> wrote:

> From: C Michael Sundius <Michael.sundius@sciatl.com>
> Subject: Re: Have ever checked in your mips sparsemem code into mips-linux 
> tree?
> To: "Andy Whitcroft" <apw@shadowen.org>, "Dave Hansen" 
> <dave@linux.vnet.ibm.com>, linux-mm@kvack.org, linux-mips@linux-mips.org, 
> "VomLehn, David" <dvomlehn@cisco.com>, me94043@yahoo.com
> 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@CUPLXSUNDISM01.corp.sa.net>
> 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


      

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