When npm installs native node modules it uses node-gyp to compile code. This is the seam node uses for targeting different operating systems, e.g. OS X, linux, Windows etc.
By default node-gyp compiles using one core and if you have more than one you probably want to utilize that power to speed up compile time. The way node-gyp handles this is by using the JOBS environment variable, which sets the jobs variable here. This piece of code then checks the value of jobs to determine how many cores to use.
Note that if the value of JOBS is max then all cores will be used. So lets try this on leveldown. First lets check that JOBS isn't set yet:
lms@ux301|01:34|~/src/leveldb-repos/leveldown (master) $ echo $JOBS
Ok lets install leveldown and time it. Everything has been pre-installed so we aren't measuring the time for downloading the modules:
lms@ux301|01:34|~/src/leveldb-repos/leveldown (master) $ time npm i
> [email protected] install /home/lms/src/leveldb-repos/leveldown
> node-gyp rebuild
make: Entering directory '/home/lms/src/leveldb-repos/leveldown/build'
CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.17.0/db/builder.o
CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.17.0/db/db_impl.o
CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.17.0/db/db_iter.o
..
..
make: Leaving directory '/home/lms/src/leveldb-repos/leveldown/build'
real 0m21.028s
user 0m19.299s
sys 0m1.926s
Now lets set JOBS=max:
lms@ux301|01:35|~/src/leveldb-repos/leveldown (master) $ export JOBS=max
lms@ux301|01:37|~/src/leveldb-repos/leveldown (master) $ echo $JOBS
max
And lets time it again:
lms@ux301|01:37|~/src/leveldb-repos/leveldown (master) $ time npm i
> [email protected] install /home/lms/src/leveldb-repos/leveldown
> node-gyp rebuild
make: Entering directory '/home/lms/src/leveldb-repos/leveldown/build'
CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.17.0/db/builder.o
CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.17.0/db/db_impl.o
CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.17.0/db/db_iter.o
..
..
make: Leaving directory '/home/lms/src/leveldb-repos/leveldown/build'
real 0m10.630s
user 0m32.511s
sys 0m2.808s
I'm using four cores so I expected something slightly better than a 50% improvement, but I'm just happy I can speed it up somewhat.
So, if you're on *nix, make sure to add export JOBS=max to your ~/.bashrc or ~/.profile or whatever you use :)
Magnus
Just a small note, this isn't always a good idea. If you're building a native module like OpenCV on a low spec board like the Raspberry Pi, you will often run into "Segmentation Fault" errors.