[loeg]

If we ignore POSIX for a moment, the kernel could avoid contending on the one-per-process fd map by sharding the integers into distinct allocation ranges per thread. This would eliminate a source of contention between threads.

In addition to violating POSIX’ lowest hole rule, it would break select(2) (more than it’s already broken).

[kennylevinsen]

This sounds like premature optimization. FD availability is tracked in a bitmask, and finding the next available slot is a matter of scanning for the first unset bit under a spinlock. This is going to be extremely fast.

While you could shard the file descriptor tables for CLONE_FILES processes such as threads, you would likely complicate file descriptor table management and harm the much more important read performance (which is currently just a plain array index and pretty hard to beat).

You could also juts create your processes (or threads) without CLONE_FILES so that they get their own file descriptor table. ------

The fdtable can be seen here: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin..., and alloc_fd and __fget can be found here: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin....