Re: [PATCH] Radix-tree pagecache for 2.5

[Anton Blanchard]

Hi Ingo,

> Yes, it's very nice. Anton Blanchard has benchmarked both patch variants
> (tree vs. scalable-hash page buckets) for SMP scalability against the
> stock hash, on big RAM, many CPUs boxes, via dbench load. He has found
> performance of radix trees vs. scalable hash to be at least equivalent. (i
> think Anton has a few links to show the resulting graphs.)

Here are some results on a 12 way machine. (2.4.16-splay is the radix
patch):

http://samba.org/~anton/linux/pagecache_locking/1/summary.png

As you can see both patches give pretty much equal improvements.

The other problem with the current pagecache hash is that it maxes out
at order 9 (due to the get_free_pages limitation) which starts to hurt
at 4GB RAM and above. On a 32GB machine the average hashchain depth
was very high:

http://samba.org/~anton/linux/pagecache/pagecache_before.png

There were a few solutions (from davem and ingo) to allocate a larger
hash but with the radix patch we no longer have to worry about this.

So the radix patch solves 2 problems quite nicely :)

Anton