Messages in this thread | | | Date | Thu, 29 Sep 2011 22:45:05 +0200 | From | Willy Tarreau <> | Subject | Re: BFS cpu scheduler and skip list implementation |
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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
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