On Sun, May 29, 2011 at 01:01:04PM -0400, Mathieu Desnoyers wrote: > * Mathieu Desnoyers (mathieu.desnoyers@xxxxxxxxxxxx) wrote: > > * Sasha Levin (levinsasha928@xxxxxxxxx) wrote: > [...] > > > Hi Mathieu! > > > > > > In tools/kvm/ we use a rb-tree (same one used by the kernel) with the > > > augmentation feature to support an interval rb-tree - which means that > > > every update to the tree not only updates the nodes directly related to > > > the updated node but also all the nodes on the path to the root of the > > > tree. > > > > Cool !! > > > > I'm adding in copy Phil Howard who has been working on RCU RB tree for > > much longer than myself. > > > > > I see that in liburcu there is an implementation of a rcu linked list > > > but no implementation of a rb-tree. > > > > > > Are you currently working on one? or maybe I should try writing one and > > > sending it to you? > > > > Actually, I started working on one last year, but had to interrupt my > > effort before I got it even working right. > [...] > > We'd have to see how we can go from this implementation of a standard RB > > tree to an interval RB tree too. I guess it will depend whether you need > > the updates from the target node up to the root to be done "all at once" > > from a reader perspective (then you would probably need to replace a > > copy of a part of the tree all at once), or if you can allow the update > > to be done piece-wise on a node-by-node basis as readers go through the > > tree (from root to leafs). > > I've revisited the RCU rbtree implementation this weekend, and it works > much better now. I reimplemented the whole thing from 0 starting from > the CLRS chapter 12 algorithms (to get the non-rcu > (insertion/removal)-only stress-tests working) and incrementally > RCU-ized the updates and then added read-side tests. All along, I used > the test_urcu_rbtree test case that does some basic coherency tests by > searching for some random elements that *should* be there in parellel > with insertion and removals. The implementation I currently have > survives the "search for known elements in parallel with updates" stress > test (so far). (e.g. test_urcu_rbtree 6 2 10 -g 30 : 6 readers, 2 > writers, 30 known random elements, writers are adding/removing 6 random > elements, on a 8-core machine) > > See: git://git.lttng.org/userspace-rcu.git > branch : rbtree2 > > The key idea I used in this implementation is to "decay" the old nodes > (AFAIK, I just made this up) : "decaying" a node could be best described > as creating an exact copy of a node, and putting a pointer to this new > node into the old node to form a "decay chain". This allowed me to keep > the algorithm very much similar to CLRS by just walking the decay chains > whenever needed. The old node "decays" by using call_rcu to free it > after a grace period passes. This imply that the updates must hold the > RCU read-side lock in addition to a mutex to make sure the decaying > nodes stay valid for the duration of their use. > > This implementation never requires the read-side to loop, thus > guaranteeing a wait-free read-side behavior (so search operations will > always be strictly log(n) without any busy-loop delay). > > I have not created stress-tests for next/prev walk of the tree yet. It > is therefore entirely possible that this does not work as expected. > > Comments are welcome, Very cool! The trick Phil Howard used allowed him to avoid duplicating the nodes in some cases in the rotations. I might be missing something, but it looks like you are duplicating in all cases. Would using Phil's trick result in significant performance gain? Thanx, Paul > Thanks, > > Mathieu > > > > > > Thanks, > > > > Mathieu > > > > > > -- > > Mathieu Desnoyers > > Operating System Efficiency R&D Consultant > > EfficiOS Inc. > > http://www.efficios.com > > -- > Mathieu Desnoyers > Operating System Efficiency R&D Consultant > EfficiOS Inc. > http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
