package art

Adaptive Radix Tree (ART) in OCaml

This is an implementation in OCaml of ART. An Adaptive Radix Tree is like a simple Hashtbl with order:

# let tree = Art.make () ;;
# Art.insert tree (Art.key "foo") 42 ;;
# Art.insert tree (Art.key "bar") 21 ;;
# Art.find tree (Art.key "foo")
- : int = 42

Operation like minimum or maximum are available (which don't exist for a simple Hashtbl.t):

# let tree = Art.make () ;;
# Art.insert tree (Art.key "0") 0 ;;
# Art.insert tree (Art.key "1") 1 ;;
# Art.insert tree (Art.key "2") 2 ;;
# Art.minimum tree
- : int = 0
# Art.maximum tree
- : int = 2

If you want the order and the speed of Hashtbl.t, Art is your library:

• Benchmark on [find][find-bechamel]

• Benchmark on [insert][insert-bechamel]

Read Optimised Write Exclusion (ROWEX) in OCaml

ROWEX is a second implementation of ART with atomic operations. It's a functor which expects an implementation of atomic operations such as load or store.

Parallelism, atomic operation & OCaml

The current version of OCaml has a global lock for the GC. By this way, it's not possible for us to execute ROWEX operations (find/insert) with true parallelism if we use the same OCaml runtime. Even if you use LWT or ASYNC, you execute jobs concurrentely.

However, ROWEX wants to provide an implementation where find/insert can be executed in parralel without any problems (race condition or ABA problem). So ROWEX provides an implementation, persistent, which implements atomic operations on a memory area. Then, we are able, as [parmap][parmap], to simulate true parallelism as long as each operations are executed into their own [fork()][fork].

The goal of this library is provide:

• the most easy way to switch the implementation to [ocaml-multicore][ocaml-multicore]

• a baby step to be able to manipulate a file by several processes (consumers/find, producers/insert) in parallel

Status: experimental

This part of the distribution is experimental - even if the distribution comes with several tests to ensure that the implementation works, ROWEX is fragile! It still need a synchronization mechanism fsync() which is added pervasively in some parts of the code according to outcomes of errors.