Modern data structures

Question

I just realized all the data structures I regularly use are really old and really simple. Linked lists, hash tables, trees, and even the more complex variants such as VLists or RBTrees are all pretty old inventions.

Most of them were conceived for a serial, single CPU world and require adapting to work in parallel environments.

What kind of newer, better data structures do we have? Why are they not widely used?

I understand using a plain old linked list if you have to implement it and prefer the simplicity, but having huge STLs and piles of third party libraries like Guava or Boost, why am I still placing locks around hashes?

Don't we have potentially standard, hard-proven modern data structures that can actually replace the trusty old-timers?

Answer 1

There is nothing wrong with the old ones. A good way to keep flexibility is to separate concerns. Normal (old style) datastructures are concerned with the way how data is stored. Locking is a completely different concern, which should not be part of the datastructure.

Locking is a potentially expensive operation, so if you can, you should lock multiple structures at once to optimize your code. I.e. lock critical sections not datastructures. If you directly add locking to your datastructures, you do not have the possibility to optimize this way. Also this will introduce implicit synchronisation points, that you may not want and canot control.

This does not answer a different aspect of your question. The part of "Why do we need locking at all". The answer is, that sometimes there is just no way around it. You either need to have lock somewhere, completely rely on atomic operations or disallow mutation altogether.

Method one is not feasible, as I have pointed out above, because you loose potential for optimization and you have implicit synchronisation points.

Only using atomic operations in your data structure (i.e. non-locking structures) is still an open research question, and might not always be possible. I know of some non-locking structures, i.e. queues, lists etc, but I have never heard of a non locking tree. Also non-locking structures tend to become much more complicated and slower, so we still need some better structure for thread local data, and can only add these to our datastructure zoo.

Not having mutable datastructures at all is in my opinion the best way of all of them. Mutability is often more of a hassle than it is worth. However this is a concept from functional programming and only makes sense in such an environment. Functional programming however is regarded as an esoteric concept by most programmers. Most languages which are actually used in production work mainly use non-functional concepts (this does not mean it actually is more complicated or is more error prone, it is just reflecting the current state of training among developers). In my opinion functional programming will become more wide spread, once people start to note it solves their threading problems automatically in a blink. Several other languages are now borrowing already from functional languages, so this is probably where we will find the next evolution of data structures.

Answer 2

If you want lock-free data structures, study persistent data structures. These are mostly popular in the functional programming world, but are applicable in other domains as well. Most persistent DSs are variants of plain lists, trees etc. but newer ones such as hash tries have surfaced in recent years.