Overlays

This chapter describes how to extend and change Nixpkgs using overlays. Overlays are used to add layers in the fixed-point used by Nixpkgs to compose the set of all packages.

Nixpkgs can be configured with a list of overlays, which are applied in order. This means that the order of the overlays can be significant if multiple layers override the same package.

Installing overlays

The list of overlays can be set either explicitly in a Nix expression, or through <nixpkgs-overlays> or user configuration files.

Set overlays in NixOS or Nix expressions

On a NixOS system the value of the nixpkgs.overlays option, if present, is passed to the system Nixpkgs directly as an argument. Note that this does not affect the overlays for non-NixOS operations (e.g. nix-env), which are looked up independently.

The list of overlays can be passed explicitly when importing nixpkgs, for example import <nixpkgs> { overlays = [ overlay1 overlay2 ]; }.

NOTE: DO NOT USE THIS in nixpkgs. Further overlays can be added by calling the pkgs.extend or pkgs.appendOverlays, although it is often preferable to avoid these functions, because they recompute the Nixpkgs fixpoint, which is somewhat expensive to do.

Install overlays via configuration lookup

The list of overlays is determined as follows.

First, if an overlays argument to the Nixpkgs function itself is given, then that is used and no path lookup will be performed.
Otherwise, if the Nix path entry <nixpkgs-overlays> exists, we look for overlays at that path, as described below.

See the section on NIX_PATH in the Nix manual for more details on how to set a value for <nixpkgs-overlays>.
If one of ~/.config/nixpkgs/overlays.nix and ~/.config/nixpkgs/overlays/ exists, then we look for overlays at that path, as described below. It is an error if both exist.

If we are looking for overlays at a path, then there are two cases:

If the path is a file, then the file is imported as a Nix expression and used as the list of overlays.
If the path is a directory, then we take the content of the directory, order it lexicographically, and attempt to interpret each as an overlay by:
- Importing the file, if it is a .nix file.
- Importing a top-level default.nix file, if it is a directory.

Because overlays that are set in NixOS configuration do not affect non-NixOS operations such as nix-env, the overlays.nix option provides a convenient way to use the same overlays for a NixOS system configuration and user configuration: the same file can be used as overlays.nix and imported as the value of nixpkgs.overlays.

Defining overlays

Overlays are Nix functions which accept two arguments, conventionally called self and super, and return a set of packages. For example, the following is a valid overlay.

self: super:

{
  boost = super.boost.override {
    python = self.python3;
  };
  rr = super.callPackage ./pkgs/rr {
    stdenv = self.stdenv_32bit;
  };
}

The first argument (self) corresponds to the final package set. You should use this set for the dependencies of all packages specified in your overlay. For example, all the dependencies of rr in the example above come from self, as well as the overridden dependencies used in the boost override.

The second argument (super) corresponds to the result of the evaluation of the previous stages of Nixpkgs. It does not contain any of the packages added by the current overlay, nor any of the following overlays. This set should be used either to refer to packages you wish to override, or to access functions defined in Nixpkgs. For example, the original recipe of boost in the above example, comes from super, as well as the callPackage function.

The value returned by this function should be a set similar to pkgs/top-level/all-packages.nix, containing overridden and/or new packages.

Overlays are similar to other methods for customizing Nixpkgs, in particular the packageOverrides attribute described in . Indeed, packageOverrides acts as an overlay with only the super argument. It is therefore appropriate for basic use, but overlays are more powerful and easier to distribute.

Using overlays to configure alternatives

Certain software packages have different implementations of the same interface. Other distributions have functionality to switch between these. For example, Debian provides DebianAlternatives. Nixpkgs has what we call alternatives, which are configured through overlays.

BLAS/LAPACK

In Nixpkgs, we have multiple implementations of the BLAS/LAPACK numerical linear algebra interfaces. They are:

OpenBLAS

The Nixpkgs attribute is openblas for ILP64 (integer width = 64 bits) and openblasCompat for LP64 (integer width = 32 bits). openblasCompat is the default.
LAPACK reference (also provides BLAS)

The Nixpkgs attribute is lapack-reference.
Intel MKL (only works on the x86_64 architecture, unfree)

The Nixpkgs attribute is mkl.
BLIS

BLIS, available through the attribute blis, is a framework for linear algebra kernels. In addition, it implements the BLAS interface.
AMD BLIS/LIBFLAME (optimized for modern AMD x86_64 CPUs)

The AMD fork of the BLIS library, with attribute amd-blis, extends BLIS with optimizations for modern AMD CPUs. The changes are usually submitted to the upstream BLIS project after some time. However, AMD BLIS typically provides some performance improvements on AMD Zen CPUs. The complementary AMD LIBFLAME library, with attribute amd-libflame, provides a LAPACK implementation.

Introduced in PR #83888, we are able to override the blas and lapack packages to use different implementations, through the blasProvider and lapackProvider argument. This can be used to select a different provider. BLAS providers will have symlinks in $out/lib/libblas.so.3 and $out/lib/libcblas.so.3 to their respective BLAS libraries. Likewise, LAPACK providers will have symlinks in $out/lib/liblapack.so.3 and $out/lib/liblapacke.so.3 to their respective LAPACK libraries. For example, Intel MKL is both a BLAS and LAPACK provider. An overlay can be created to use Intel MKL that looks like:

self: super:

{
  blas = super.blas.override {
    blasProvider = self.mkl;
  };

  lapack = super.lapack.override {
    lapackProvider = self.mkl;
  };
}

This overlay uses Intel's MKL library for both BLAS and LAPACK interfaces. Note that the same can be accomplished at runtime using LD_LIBRARY_PATH of libblas.so.3 and liblapack.so.3. For instance:

$ LD_LIBRARY_PATH=$(nix-build -A mkl)/lib:$LD_LIBRARY_PATH nix-shell -p octave --run octave

Intel MKL requires an openmp implementation when running with multiple processors. By default, mkl will use Intel's iomp implementation if no other is specified, but this is a runtime-only dependency and binary compatible with the LLVM implementation. To use that one instead, Intel recommends users set it with LD_PRELOAD. Note that mkl is only available on x86_64-linux and x86_64-darwin. Moreover, Hydra is not building and distributing pre-compiled binaries using it.

For BLAS/LAPACK switching to work correctly, all packages must depend on blas or lapack. This ensures that only one BLAS/LAPACK library is used at one time. There are two versions of BLAS/LAPACK currently in the wild, LP64 (integer size = 32 bits) and ILP64 (integer size = 64 bits). Some software needs special flags or patches to work with ILP64. You can check if ILP64 is used in Nixpkgs with blas.isILP64 and lapack.isILP64. Some software does NOT work with ILP64, and derivations need to specify an assertion to prevent this. You can prevent ILP64 from being used with the following:

{ stdenv, blas, lapack, ... }:

assert (!blas.isILP64) && (!lapack.isILP64);

stdenv.mkDerivation {
  ...
}

Switching the MPI implementation

All programs that are built with MPI support use the generic attribute mpi as an input. At the moment Nixpkgs natively provides two different MPI implementations:

Open MPI (default), attribute name openmpi
MPICH, attribute name mpich

To provide MPI enabled applications that use MPICH, instead of the default Open MPI, simply use the following overlay:

self: super:

{
  mpi = self.mpich;
}

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