Add mkl_path preference to support using "system" MKL

Project.toml

@@ -7,10 +7,12 @@ Artifacts = "56f22d72-fd6d-98f1-02f0-08ddc0907c33"
 Libdl = "8f399da3-3557-5675-b5ff-fb832c97cbdb"
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"
 MKL_jll = "856f044c-d86e-5d09-b602-aeab76dc8ba7"
+Preferences = "21216c6a-2e73-6563-6e65-726566657250"
 
 [compat]
 MKL_jll = "2022.1"
 julia = "1.8"
+Preferences = "1.4"
 
 [extras]
 Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"

README.md

@@ -4,20 +4,23 @@
 
 MKL.jl is a Julia package that allows users to use the Intel MKL library for Julia's underlying BLAS and LAPACK, instead of OpenBLAS, which Julia ships with by default. Julia includes [libblastrampoline](https://github.com/staticfloat/libblastrampoline), which enables picking a BLAS and LAPACK library at runtime. A [JuliaCon 2021 talk](https://www.youtube.com/watch?v=t6hptekOR7s) provides details on this mechanism. 
 
-This package requires Julia 1.7+
+This package requires Julia 1.7+.
 
-## Usage
-
-If you want to use `MKL.jl` in your project, make sure it is the first package you load before any other package. It is essential that MKL be loaded before other packages so that it can find the Intel OMP library and avoid [issues resulting out of GNU OMP being loaded first](https://github.com/JuliaPackaging/BinaryBuilder.jl/issues/700).
+## Installation
 
-## To Install:
+To install the package execute
 
-Adding the package will replace the system BLAS and LAPACK with MKL provided ones at runtime. Note that the MKL package has to be loaded in every new Julia process. Upon quitting and restarting, Julia will start with the default OpenBLAS.
 ```julia
 julia> using Pkg; Pkg.add("MKL")
 ```
 
-## To Check Installation:
+## Usage
+
+Loading the package (`using MKL`) will replace the system BLAS and LAPACK with MKL provided ones at runtime. Make sure it is the first package you load before any other package. It is essential that MKL be loaded before other packages so that it can find the Intel OMP library and avoid [issues resulting out of GNU OMP being loaded first](https://github.com/JuliaPackaging/BinaryBuilder.jl/issues/700).
+
+Note that the MKL package has to be loaded in every new Julia process. Upon quitting and restarting, Julia will start with the default OpenBLAS.
+
+## Check
 
 ```julia
 julia> using LinearAlgebra
@@ -35,6 +38,16 @@ Libraries:
 └ [ILP64] libmkl_rt.1.dylib
 ```
 
+Note that you can put `using MKL` into your `startup.jl` to make Julia automatically use Intel MKL in every session.
+
 ## Using the 64-bit vs 32-bit version of MKL
 
 We use ILP64 by default on 64-bit systems, and LP64 on 32-bit systems.
+
+## Using a system-provided Intel MKL
+
+If you want to use a system-provided Intel MKL installation, you can set the [preference](https://github.com/JuliaPackaging/Preferences.jl) `mkl_path` to hint MKL.jl to the corresponding `libmkl_rt` library. Specifically, the options are:
+
+* `mkl_jll` (default): Download and install MKL via [MKL_jll.jl](https://github.com/JuliaBinaryWrappers/MKL_jll.jl).
+* `system`: The package will try to automatically locate the system-provided libmkl_rt library (i.e. find it on the linker search path).
+* `path/to/my/libmkl_rt.<EXT>`: Explicit path to the `libmkl_rt.<EXT>` where `<EXT>` is the shared library extension of the system at hand (e.g. `.so`, `.dll`, `.dylib`)

src/MKL.jl

@@ -1,6 +1,41 @@
 module MKL
 
-using MKL_jll
+using Preferences
+using Libdl
+using LinearAlgebra
+
+# Choose an MKL path; taking an explicit preference as the first choice,
+# but if nothing is set as a preference, fall back to the default choice of `MKL_jll`.
+const mkl_path = something(
+    @load_preference("mkl_path", nothing),
+    "mkl_jll",
+)::String
+
+if lowercase(mkl_path) == "mkl_jll"
+    # Only load MKL_jll if we are suppoed to use it as the MKL source
+    # to avoid an unnecessary download of the (lazy) artifact.
+    import MKL_jll
+    const libmkl_rt = MKL_jll.libmkl_rt
+elseif lowercase(mkl_path) == "system"
+    # We expect the "system" MKL to already be loaded,
+    # or be on our linker search path.
+    libname = string("libmkl_rt", ".", Libdl.dlext)
+    const libmkl_rt = find_library(libname, [""])
+    libmkl_rt == "" && error("Couldn't find $libname. Try to specify the path to `libmkl_rt` explicitly.")
+else
+    # mkl_path should be a valid path to libmkl_rt.
+    const libmkl_rt = mkl_path
+    isfile(libmkl_rt) || error("Couldn't find MKL library at $libmkl_rt.")
+end
+
+# Changing the MKL provider/path preference
+function set_mkl_path(path)
+    if lowercase(path) ∉ ("mkl_jll", "system")
+        isfile(path) || error("The provided argument $path doesn't seem to be a valid path to libmkl_rt.")
+    end
+    @set_preferences!("mkl_path" => path)
+    @info("New MKL preference set; please restart Julia to see this take effect", path)
+end
 
 using LinearAlgebra
 
@@ -37,7 +72,7 @@ function set_interface_layer(interface::Interface = INTERFACE_LP64)
 end
 
 function __init__()
-    if MKL_jll.is_available()
+    if !(mkl_path == "mkl_jll" && !MKL_jll.is_available())
         if Sys.isapple()
             set_threading_layer(THREADING_SEQUENTIAL)
         end