How and when to use /dev/shm for efficiency?

Question

How is /dev/shm more efficient than writing the file on the regular file system? As far as I know, /dev/shm is also a space on the HDD so the read/write speeds are the same.

My problem is, I have a 96GB file and only 64GB RAM (+ 64GB swap). Then, multiple threads from the same process need to read small random chunks of the file (about 1.5MB).

Is /dev/shm a good use case for this?
Would it be faster than opening the file in read-only mode from /home and then passing over to the threads to do the reading the required random chunks?

Answer 1

You don't use /dev/shm. It exists so that the POSIX C library can provide shared memory support via the POSIX API. Not so you can poke at stuff in there.

If you want an in-memory filesystem of your very own, you can mount one wherever you want it.

mount -t tmpfs tmpfs /mnt/tmp, for example.

A Linux tmpfs is a temporary filesystem that only exists in RAM. It is implemented by having a file cache without any disk storage behind it. It will write its contents into the swap file under memory pressure. If you didn't want the swapfile you can use a ramfs.

I don't know where you got the idea of using /dev/shm for efficiency in reading files, because that isn't what it does at all.

Maybe you were thinking of using memory mapping, via the mmap system call?