Re: BFS cpu scheduler and skip list implementation

Willy Tarreau
2011-09-29T20:45:20Z

Hi Con,

On Sat, Sep 24, 2011 at 06:38:06PM +1000, Con Kolivas wrote:
> That's great then. I'm sure we'd explode in other weird and wonderful ways
> before the CPU load ever got to 64k. Plus all that would happen is that it
> would start degenerating from O(log n) insertion to O(n) as the number way
> surpassed 64k. The number 16 for levels was simply chosen as the one
> originally used by William Pugh in his sample code, but seems to be ample for
> this application.

If you're interested, during the early CFS benchmarks a few years ago, I
reworked my old binary tree code to make it kernel-compatible. By this, I
mean that it never needs to allocate memory, it's used just like rbtrees
or kernel lists, by having a node in your structure and inserting it from
the root of a tree. It offers the following features :
- O(log(N)) insertion/lookup
- O(1) removal
- O(1) next/prev walk
- 20 bytes per node on 32-bit pointers, 36-bytes on 64-bit pointers, plus
the key
- supports unique or multiple occurrences of the same key (walked in
insertion order and classed in trees)
- supports 32/64 bit signed/unsigned integers, strings and memory blocks
- supports prefixes (eg. to insert IP addresses with masks)
- supports lookup of greater than or equal to, less than or equal to.

I replaced the rbtree that was used in haproxy's scheduler with this new
code and measured a noticeable performance improvement, since haproxy
does insert/next/remove a lot of times a second, and not having to balance
a tree saves a huge number of cycles.

I remember having conducted some tests on CFS a log time ago with it, but
the only cases where I observed a gain was when running insane amounts of
tasks at insane context switching rates, which was biased and irrealistic.
So I stopped trying to put it into the kernel at that time.

Maybe for your usage it might bring some value. Take a look at the code
here if you want, it's not too much documented but still enough to start
with it. There are a few examples that can help get it right. I know that
a few people use it for their own projects and they did not ask for help :-)

http://git.1wt.eu/web?p=ebtree.git;a=summary

Cheers,
Willy