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[PATCH] mips sparsemem howto

Subject: [PATCH] mips sparsemem howto
From: Sundis <sundism@CUPLXSUNDISM01.corp.sa.net>
Date: Mon, 6 Oct 2008 10:31:08 -0700
---
 Documentation/sparsemem.txt |   92 +++++++++++++++++++++++++++++++++++++++++++
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 create mode 100644 Documentation/sparsemem.txt

diff --git a/Documentation/sparsemem.txt b/Documentation/sparsemem.txt
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+++ 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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