[SOLVED] How to use -j option the right way when compiling with make

George528 · 05-29-2021, 10:37 AM

I am reading "O'Reilly Linux Kernel in a Nutshell" book and came across this:
https://i.imgur.com/uVJhpae.png
they say that the -j option should be the double of your cpu cores.
can you please confirm if this information is true?

In LFS book, they said that the -j number should be the number of cores of your CPU.
https://i.imgur.com/tqoVxHt.png

In gentoo handbook they say that it should one unit should be 2GB of ram, so for example if you have 4GB of ram, you should set -j2, for 8GB, -j4 and so on.
https://i.imgur.com/qRs6uXL.png

Which information is true? i am so confused.

shruggy · 05-29-2021, 10:43 AM

Those are recommendations. All of them are sensible. The definitive documentation for GNU make is the GNU Make Manual. It has following to say:

Quote:

When the system is heavily loaded, you will probably want to run fewer jobs than when it is lightly loaded. You can use the ‘-l’ option to tell make to limit the number of jobs to run at once, based on the load average.

George528 · 05-29-2021, 11:04 AM

Quote:

Originally Posted by shruggy

Those are recommendations. All of them are sensible. The definitive documentation for GNU make is the GNU Make Manual. It has following to say:

i see, thanks for clearing this out

pan64 · 05-29-2021, 01:37 PM

It depends on the code you want to compile (and how do you want to compile).
In general as a very first step you can say -j number should be the number of cores of your CPU. But that is only a very rough estimate.
Probably better to choose something around 2*N (n=number of cores), that will give usually faster result (and also bigger numbers will not help any more), but as it was mentioned it may depend on the available memory too (and the code you want to compile and the flags you use for compilation).
From the other hand your can safely try to increase that number, your box will never use more than the available RAM.
And also you can decide, if you still want to use your box (during compilation) or you want to complete that make as fast as it can do.

George528 · 05-30-2021, 02:56 AM

Quote:

Originally Posted by pan64

It depends on the code you want to compile (and how do you want to compile).
In general as a very first step you can say -j number should be the number of cores of your CPU. But that is only a very rough estimate.
Probably better to choose something around 2*N (n=number of cores), that will give usually faster result (and also bigger numbers will not help any more), but as it was mentioned it may depend on the available memory too (and the code you want to compile and the flags you use for compilation).
From the other hand your can safely try to increase that number, your box will never use more than the available RAM.
And also you can decide, if you still want to use your box (during compilation) or you want to complete that make as fast as it can do.

thanks! i will mark this solved