On Fri, 15 Aug 2008 11:17:21 -0700 C Michael Sundius wrote:
> Ah, compromise :] that's why you get paid the big bux dave. thanks.
Here are some documentation comments/corrections.
And please try to use inline patches instead of attachments.
See Documenation/email-clients.txt for some help on this.
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diff --git a/Documentation/sparsemem.txt b/Documentation/sparsemem.txt
new file mode 100644
index 0000000..89656e3
--- /dev/null
+++ b/Documentation/sparsemem.txt
@@ -0,0 +1,96 @@
+Sparsemem divides up physical memory in your system into N section of M
sections
+bytes. Page descriptors 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 discriptors for those pages that do not exist.
descriptors
+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 descriptors, 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()
+Once the bootmem allocator is up and running, you should call the
+sparsemem function "memory_present(node, pfn_start, pfn_end)" for each
+block of memory that exists on your system.
+
+2) DETERMINE AND SET THE SIZE OF SECTIONS AND PHYSMEM
...
+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 menuconfig or make gconfig, as you like, and turn on the sparsemem
+memory model under the "Kernel Type" --> "Memory Model" and then build your
+kernel.
Wow! A gconfig user? I see more people using menuconfig or xconfig IIRC.
Anyway, we usually just say something like "run make *config"...
+
+
+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.
Please don't end patch lines with whitespace. (like above)
---
~Randy
Linux Plumbers Conference, 17-19 September 2008, Portland, Oregon USA
http://linuxplumbersconf.org/
