Re: RCU red-black tree (was: Re: [PATCH 4/6] kvm tools: Add rwlock wrapper)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sun, May 29, 2011 at 01:01:04PM -0400, Mathieu Desnoyers wrote:
> * Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> > * Sasha Levin (levinsasha928@gmail.com) 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