Re: [Slightly off topic] A question about R/B trees.

From: Pekka Enberg
Date: Sat Oct 18 2008 - 04:46:04 EST

Hi Andi,

On Sat, Oct 18, 2008 at 10:53 AM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> The problem with hash tables is that if they're big enough
> or if the rest of the workload is memory intensive
> each hit will be a cache miss. And you can do a lot of branch
> mispredicts in the time of a single cache miss.
> In general trees can be much better for cache usage, although
> it's generally better to use some tree that has nodes near
> the cache line size. Binary trees like RB are too small for that.

Right, but even for binary trees, you can get some of the benefits by
packing all the nodes in a slab cache of their own. That way many of
the neighboring nodes will share the same cache line if you're
allocating memory for the nodes in a top-down order. Of course, you
lose the benefits if the tree is updated a lot because you're back to
worst case allocation again.

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