Question

Can anyone explain why in linux when I start gcc or javac after some time of inactivity it takes a while for them to start. Subsequent invocations are way faster. Is there a way to ensure quick startup always? (This requirement may seem strange, but is necessary in my case). Ubuntu by the way.

Answer 1

Most likely, it’s the time it takes for code pages to fault in. There are a few ways to avoid this delay if you really have to. The simplest would be to run gcc periodically. Another would be to install gcc to a RAM disk.

Another approach would be to make a list of which files are involved and then write a simple program to lock all those files into memory. You can use something like:
strace -f gcc *rest of gcc command* 2>&1 | grep open | grep -v -- -1
Use a GCC command line that’s typical of how you are using GCC.

You’ll find libraries and binaries being opened in there. Make a full list in a file. Then write a program that calls mlockall(MCL_FUTURE) then reads in filenames from the file. For each file, mmap it into memory and read each byte. Then have the program just sleep forever (or until killed).

This will have the effect of forcing every page of every file in memory. You should check the total size of all these files and make sure it’s not a significant fraction of the amount of memory you actually have!

By the way, there used to be something called a sticky bit that did something like this. If by some chance your platform supports it, just set it on all the files used. (Although it traditionally caused the files to be saved to swap, which on a modern system won’t make things any faster.)