Re: sparcemem or discontig?

[C Michael Sundius <Michael.sundius@sciatl.com>]

Andy Whitcroft wrote:
> On Tue, Jul 29, 2008 at 02:51:21PM -0700, Dave Hansen wrote:
>   
>> On Tue, 2008-07-29 at 11:11 -0700, C Michael Sundius wrote:
>>     
>>> My understanding is that SPARCEMEM is the way of the future, and since
>>> I don't really have a NUMA machine, maybe sparcemem is more appropriate,
>>> yes? On the other hand I can't find much info about how it works or how
>>> to add support for it on an architecture that has here-to-fore not
>>> supported that option.
>>>
>>> Is there anywhere that there is a paper or rfp that describes how the
>>> spacemem (or discontig) features work (and/or the differences between
>>> then)?
>>>       
>> I think you're talking about sparsemem. :)
>>
>> My opinion is that NUMA and DISCONTIG are too intertwined to be useful
>> apart from the other.  I use sparsemem on my non-NUMA 2 CPU laptop since
>> it has a 1GB hole.  It is *possible* to use DISCONTIG without NUMA, and
>> I'm sure people use it this way, but I just personally think it is a bit
>> of a pain.  
>>
>> Basically, to add sparsemem support for an architecture, you need a
>> header like these:
>>
>> dave@nimitz:~/lse/linux/2.5/linux-2.6.git$ find | grep sparse | xargs
>> grep -c '^.*$'
>> ./include/asm-powerpc/sparsemem.h:32
>> ./include/asm-x86/sparsemem.h:34
>> ./include/asm-sh/sparsemem.h:16
>> ./include/asm-mips/sparsemem.h:14
>> ./include/asm-ia64/sparsemem.h:20
>> ./include/asm-s390/sparsemem.h:18
>> ./include/asm-arm/sparsemem.h:10
>>
>> These are generally pretty darn small (the largest is 34 lines).  You
>> also need to tweak some things in your per-arch Kconfig.  ARM looks like
>> a pretty simple use of sparsemem.  You might want to start with what
>> they've done.  We tried really, really hard to make it easy to add to
>> new architectures.
>>     
Pardon my ignorance, but is sparcemem independent of the bootmem allocator?

We also use highmem. I noticed that all of our kmap and kmap_atomic code 
is located
in the arch/mips directory. Is the sparcemem also independent of that? 
should I expect
that I will have to make some changes in that...
>> Feel free to cc me and Andy (cc'd) on the patches that you come up with.
>> I'd be happy to sanity check them for you.  If *you* want to document
>> the process for the next guy, I'm sure we'd be able to find some spot in
>> Documentation/ so the next guy has an easier time. :)
>>     
I'm happy to write a how to for sparcemem.



     - - - - -                              Cisco                            - - - - -         
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