package carray

ocaml-carray: contiguous C arrays in OCaml

Motivation

OCaml arrays are not always contiguous piece of memory, requiring accessing different chunks of memory when accessing individual elements. When requiring a value in memory, the CPU will fetch the RAM and load not only the particular value but a memory page (a contiguous piece of memory) and add it to its cache. The CPU will use its cache to load the values in its registers. It is not efficient with large OCaml arrays as the CPU will constantly fetch the RAM to load different memory pages in its cache.

Also, when using the C FFI, the user must know the memory representation of an array and use the non user-friendly low-level interface macro Field.

This work

This library provides a polymorphic interface mocking a subset of the Array interface to work with contiguous piece of memory. Using the library should be as easy as adding module Array = Carray.

A C macro Carray_val is also provided for developers writing bindings and requires to simply cast in the underlying C type.

It has also been observed sub arrays are sometimes used for read-only operations. However, Array.sub allocates a fresh copy of the requested sub part. Carray leverages this memory cost by providing noalloc variants, like sub_noalloc.

Performances

The concept has been tested and used in real world applications like the polynomial library used by Nomadic Labs to implement zk-rollups. A speed up of around 50% has been observed when using contiguous arrays compared to OCaml arrays to compute NTT/FFT.

Usage

This library is experimental. Use this library with caution. The interface might change in the future.

opam install carray.0.0.1

Links

• Repository: https://gitlab.com/dannywillems/ocaml-carray
• License: MIT
• Release: 0.0.1
• Documentation: https://dannywillems.gitlab.io/ocaml-carray/carray/index.html
• Nomadic Labs website: https://nomadic-labs.com
• Tezos ZK-rollups repository: https://gitlab.com/nomadic-labs/privacy-team