Module linux

This module contains specializations that can offload io::copy() operations on file descriptor containing types (File, TcpStream, etc.) to more efficient syscalls than read(2) and write(2).

Specialization is only applied to wholly std-owned types so that user code can't observe that the Read and Write traits are not used.

Since a copy operation involves a reader and writer side where each can consist of different types and also involve generic wrappers (e.g. Take, BufReader) it is not practical to specialize a single method on all possible combinations.

Instead readers and writers are handled separately by the CopyRead and CopyWrite specialization traits and then specialized on by the Copier::copy method.

Copier uses the specialization traits to unpack the underlying file descriptors and additional prerequisites and constraints imposed by the wrapper types.

Once it has obtained all necessary pieces and brought any wrapper types into a state where they can be safely bypassed it will attempt to use the copy_file_range(2), sendfile(2) or splice(2) syscalls to move data directly between file descriptors. Since those syscalls have requirements that cannot be fully checked in advance it attempts to use them one after another (guided by hints) to figure out which one works and falls back to the generic read-write copy loop if none of them does. Once a working syscall is found for a pair of file descriptors it will be called in a loop until the copy operation is completed.

Advantages of using these syscalls:

• fewer context switches since reads and writes are coalesced into a single syscall and more bytes are transferred per syscall. This translates to higher throughput and fewer CPU cycles, at least for sufficiently large transfers to amortize the initial probing.
• copy_file_range creates reflink copies on CoW filesystems, thus moving less data and consuming less disk space
• sendfile and splice can perform zero-copy IO under some circumstances while a naive copy loop would move every byte through the CPU.

Drawbacks:

• copy operations smaller than the default buffer size can under some circumstances, especially on older kernels, incur more syscalls than the naive approach would. As mentioned above the syscall selection is guided by hints to minimize this possibility but they are not perfect.
• optimizations only apply to std types. If a user adds a custom wrapper type, e.g. to report progress, they can hit a performance cliff.
• complexity