RE: [PATCHv6 0/8] zswap: compressed swap caching

From: Dan Magenheimer
Date: Thu Feb 21 2013 - 10:54:16 EST

> From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx]
> Subject: [PATCHv6 0/8] zswap: compressed swap caching
> Changelog:
> v6:
> * fix improper freeing of rbtree (Cody)

Cody's bug fix reminded me of a rather fundamental question:

Why does zswap use a rbtree instead of a radix tree?

Intuitively, I'd expect that pgoff_t values would
have a relatively high level of locality AND at any one time
the set of stored pgoff_t values would be relatively non-sparse.
This would argue that a radix tree would result in fewer nodes
touched on average for lookup/insert/remove.

Do you have evidence that rbtree is better here?
(Preferably over a set of workloads larger than
kernbench and SPECjbb ;-) Or are there other
important design issues that disqualify a radix tree?

In the end, I guess either one (rbtree or radix tree)
will work, but it would be nice to get this kind of
fundamental design issue properly solved before merging
is to be considered.

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