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SubjectRe: [PATCH] make radix tree gang lookup faster by using a bitmap search
James Bottomley <James.Bottomley@SteelEye.com> wrote:
>
> On Sun, 2005-08-28 at 17:52 -0700, Andrew Morton wrote:
> > > +#if BITS_PER_LONG == 32
> > > +#define RADIX_TREE_MAP_SHIFT 5
> > > +#elif BITS_PER_LONG == 64
> > > ...
> > > struct radix_tree_node {
> > > - unsigned int count;
> > > void *slots[RADIX_TREE_MAP_SIZE];
> > > - unsigned long tags[RADIX_TREE_TAGS][RADIX_TREE_TAG_LONGS];
> > > + unsigned long tags[RADIX_TREE_TAGS];
> > > };
> >
> > I don't see why the above change was necessary? Why not stick with the
> > current more flexible sizing option?
> >
> > Note that the RADIX_TREE_MAP_FULL trick can still be used. It just has to
> > be put inside a `for (idx = 0; idx < RADIX_TREE_TAG_LONGS; idx++)' loop,
> > which will vanish if RADIX_TREE_TAG_LONGS==1.
>
> Well ... It's my opinion (and purely unsubstantiated, I suppose) that
> it's more efficient on 32 bit platforms to do bit operations on 32 bit
> quantities, which is why I changed the radix tree map shift to 5 for
> that case.
>
> It also makes much cleaner code than having to open code checks on
> variable sized bitmaps.
>

It does make the tree higher and hence will incur some more cache missing
when descending the tree.

We changed the node size a few years back. umm....
http://www.ussg.iu.edu/hypermail/linux/kernel/0206.2/0141.html

It would be a little bit sad to be unable to make such tuning adjustments
in the future. Not a huge loss, but a loss.
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