Setting up OpenMP on Apple Silicon Mac


Lauren Ilano


January 29, 2024

This blog post is an example of two things:

  1. You don’t know what you don’t know, and
  2. Using a Mac does sometimes make things harder1

I am used to just installing packages in R and having them just work. I honestly did not even pay much attention to the warning I would get when loading fst: “OpenMP was not detected, using single threaded mode.” It wasn’t until someone pointed out that I was only using a single thread that I noticed something was wrong–or performance could’ve been improved if I had paid a bit more attention.

I am not an expert on why the default mac version of the C-compiler does not support OpenMP. I did however get it to work on my M1 mac so I figured I would document the steps here and share. I am also writing this to my future self to help me through setting this up on future computers.

Setting Up OpenMP multi threads for data.table and fst()

  1. In terminal, install xcode (if not already installed)
xcode-select --install
  1. In terminal, install homebrew (if not already installed)
brew install pkg-config
  1. In terminal, using homebrew install openMP library and other dependencies
brew install llvm libomp gcc
  1. In terminal, create a directory called .R and within this directory, create a file called Makevars
mkdir ~/.R
touch ~/.R/Makevars
  1. Edit the Makevars file. Can do this in RStudio by navigating to in the Files pane: Home > .R > Makevars. Open and edit the file as you would an .Reviron file. Type the following in the file and save.
LDFLAGS += -L/opt/homebrew/opt/libomp/lib -lomp
CPPFLAGS += -I/opt/homebrew/opt/libomp/include -Xclang -fopenmp

  1. Back in the terminal, load R and install data.table from source and then quit.
# In terminal
install.packages("data.table", type = "source")

quit() # quit R
  1. Check to make sure data.table is using multiple threads in RStudio by restarting R and loading data.table
getDTthreads() # check # of threads
  1. Go to ~/.R/Makefile and comment out CPPFLAGS line and add additional lines so that makefile looks like this:
LDFLAGS += -L/opt/homebrew/opt/libomp/lib -lomp
# CPPFLAGS += -I/opt/homebrew/opt/libomp/include -Xclang -fopenmp

CXX1X=/opt/homebrew/Cellar/gcc/13.1.0/bin/g++-13 -fopenmp # had to brew install gcc
CXX98=/opt/homebrew/Cellar/gcc/13.1.0/bin/g++-13 -fopenmp
CXX11=/opt/homebrew/Cellar/gcc/13.1.0/bin/g++-13 -fopenmp
CXX14=/opt/homebrew/Cellar/gcc/13.1.0/bin/g++-13 -fopenmp
CXX17=/opt/homebrew/Cellar/gcc/13.1.0/bin/g++-13 -fopenmp

  1. If step 8 doesn’t work, check to make sure the paths are correct for lines 4-8. To do this, you can go in terminal and make sure that the path exists using code below. The output should show 13.1.0.
cd /opt/homebrew/Cellar/gcc
  1. In the terminal, install fst and fstcore from source:
install.packages("fstcore", type = "source")
install.packages("fst", type = "source")
  1. Go back to RStudio and restart R. Load fst and check threads:
[1] 8


  1. and yet, I will continue to stan and it will always be my preferred OS so plz don’t try and change my mind.↩︎