Read file without disk caching in Linux

Question

I have a C program that runs only weekly, and reads a large amount of files only once. Since Linux also caches everything that's read, they fill up the cache needlessly and this slows down the system a lot unless it has an SSD drive.

So how do I open and read from a file without filling up the disk cache?

Note:

By disk caching I mean that when you read a file twice, the second time it's read from RAM, not from disk. I.e. data once read from the disk is left in RAM, so subsequent reads of the same file will not need to reread the data from disk.

Answer 1

I believe passing O_DIRECT to open() should help:

O_DIRECT (Since Linux 2.4.10)

Try to minimize cache effects of the I/O to and from this file. In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching. File I/O is done directly to/from user space buffers. The O_DIRECT flag on its own makes at an effort to transfer data synchronously, but does not give the guarantees of the O_SYNC that data and necessary metadata are transferred. To guarantee synchronous I/O the O_SYNC must be used in addition to O_DIRECT.

There are further detailed notes on O_DIRECT towards the bottom of the man page, including a fun quote from Linus.

Answer 2

You can use posix_fadvise() with the POSIX_FADV_DONTNEED advice to request that the system free the pages you've already read.