Re: [PATCH 7/8] mm: use vm_unmapped_area() on powerpc architecture

[Michel Lespinasse]

On Tue, Jan 8, 2013 at 6:15 PM, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, 2013-01-08 at 17:28 -0800, Michel Lespinasse wrote:
>> Update the powerpc slice_get_unmapped_area function to make use of
>> vm_unmapped_area() instead of implementing a brute force search.
>>
>> Signed-off-by: Michel Lespinasse <walken@xxxxxxxxxx>
>>
>> ---
>>  arch/powerpc/mm/slice.c |  128 +++++++++++++++++++++++++++++-----------------
>>  1 files changed, 81 insertions(+), 47 deletions(-)
>
> That doesn't look good ... the resulting code is longer than the
> original, which makes me wonder how it is an improvement...

Well no fair, the previous patch (for powerpc as well) has 22
insertions and 93 deletions :)

The benefit is that the new code has lower algorithmic complexity, it
replaces a per-vma loop with O(N) complexity with an outer loop that
finds contiguous slice blocks and passes them to vm_unmapped_area()
which is only O(log N) complexity. So the new code will be faster for
workloads which use lots of vmas.

That said, I do agree that the code that looks for contiguous
available slices looks kinda ugly - just not sure how to make it look
nicer though.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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