use madvise(MADV_RANDOM) when torrenting mmaped files to reduce disk readahead

[alanswanson]

For #7405. When torrenting (random_access) files, reduce excessive disk IO readahead with mmaped files by using MADV_RANDOM and hence matching normal file access using POSIX_FADV_RANDOM.

Could cause more IO calls though as unfortunately there's no reduced read ahead option to madvise.

[arvidn]

Have you measured the performance impact of this change?

[arvidn]

reading up on the issue, it sounds like you did not observe a clear improvement. Are you confident the excess reading from disk is caused by read-ahead?
I'm a bit hesitant to throw additional flags in without at least some evidence of it improving things (even though it seems very reasonable to hint random access).

I encountered a windows pitfall a long time ago, relating to a flag like this: https://blog.libtorrent.org/2012/05/windows-disk-cache/

[alanswanson]

No change for my or original reporters database64128 usage. Probably depends on number active torrents, upload speed and free memory.

Obviously shouldn't affect Windows without madvise while on Linux only disables the readahead with no other effects but mmap readahead has a miss counter to backoff readahead anyway. So probably just close and can reconsider in future if anyone else has problems (and they cannot lower number of readahead pages).