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Date: Fri, 23 Sep 2011 18:21:06 -0700 From: Andi Kleen <andi@...stfloor.org> To: Con Kolivas <kernel@...ivas.org> Cc: linux-kernel@...r.kernel.org Subject: Re: BFS cpu scheduler and skip list implementation Con Kolivas <kernel@...ivas.org> writes: > Many of you may know about Skip lists as an alternative to balanced binary > search trees. They feature O(log n) insertion, lookup and removal of table > entries. Anyway I've been looking for some time at the O(n) lookup of BFS > (which is O(1) insertion and removal) to try and find a solution that didn't > cost us at the desktop level since O(n) of small numbers of n is very fast. > The problem is of course at higher numbers of n (or server type loads), where > it gets linearly slower, and the cache trashing aspect of scanning linked > lists becomes expensive. The big problem with skiplists is that it is hard to resize the pointer arrays: so you either waste a lot of memory/cache or you have a highest limit after which they start performing poorly. I investigated them some time ago to replace the non scalable rbtrees we have currently, but got discouraged by these problems. > +struct nodeStructure { > + int level; /* Levels in this structure */ > + keyType key; > + valueType value; > + skiplist_node *next[16]; > + skiplist_node *prev[16]; > +}; That's 128 byte / 2 cache lines, not too bad, but it limits the maximum number of tasks that can be efficiently handled (my guess to around 64k with maxlevel == 16, but someone may correct me on that) -Andi -- ak@...ux.intel.com -- Speaking for myself only -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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