madvise: not understood

Question

CONTEXT:

I run on an old laptop. I only just have 128Mo ram free on 512Mo total. No money to buy more ram.

I use mmap to help me circumvent this issue and it works quite well. C code. Debian 64 bits.

PROBLEM:

Besides all my efforts, I am running out of memory pretty quick right know and I would like to know if I could release the mmaped regions I read to free my ram.

I read that madvise could help, especially the option MADV_SEQUENTIAL.

But I don't quite understand the whole picture.

THE NEED:

To be able to free mmaped allocated memory after the region is read so that it doesn't fill my whole ram with large files. I will not read it soon so it is garbage to me. It is pointless to keep it in ram.

Update: I am not done with the file so don't want to call munmap. I have other stuffs to do with it but in another regions of it. Random reads.

Answer 1

For random read/write access to a mmap()ed file, MADV_SEQUENTIAL is probably not very useful (and may in fact cause undesired behavior). MADV_RANDOM or MADV_DONTNEED would be better options in this case. However, be aware that the kernel is free to ignore any madvise() - although in my understanding, Linux currently does not, as it tends to treat madvise() more as a command than an advisory...

Another option would be to mmap() only selected sections of the file as needed, and munmap() them as you're done with them, perhaps maintaining a pool of some small number of currently active mappings (i.e. mapping more than one region at once if needed, but still keeping it limited).

Answer 2

Or course you must free resources when you're done with them in order not to leak them and thus run out of available space too soon.

Not sure what the question is, if you know about mmap() then surely you know about munmap() too? It's right there on the same manual page.