IO (part 1)

0. Pitch

• demo: /proc/<PID>/io
• demo: fwrite() vs write() vs write() with O_DIRECT vs write() with O_DIRECT | O_SYNC

1. Processes and the Outside World

• process uses threads and virtual address space for compute (CPU + memory)
• data is read from and written to I/O
• diagram: I/O channels: read / write
• diagram: types of I/O channels: socket-like (connect to another process) and file-like (connect to a device)
• demo: lsof

2. The Process I/O Interface

• working with I/O channels
• diagram: create, open, read, write, stat, ioctl, close, delete
• file descriptor
• demo: I/O operations on a file
• demo: I/O operations on a FIFO

3. File Descriptor table

• per-process descriptor table
• I/O interface for processes
• diagram: pointers for channel structures
• creating new entries
• demo: file descriptor table size

4. Types of I/O Channels

• socket-like vs file-like
• identifying a channel
  • filesystem entry
  • port
  • inherited file descriptor
• demo: sample channels:
  • socket-like (IPC: process-to-process)
    • pipe
    • FIFO
    • Unix-domain socket
    • network socket
  • file-like (process-to-file / device)
    • file

5. File Channel Internals

• open permissions
• file pointer
• reference to actual file information
• reference counter
• demo: file permissions

6. File Channel Operations

• diagram: logical view of file: array of bytes, size
• demo: read, write, seek, stat
• size vs actual size
• demo: truncate

Conclusion and Takeaways

• Processes use I/O channels to interact with the outside world (outside the process address space).
• The operating system exposes an I/O interface: file descriptors and operations with file descriptors.
• There are two large types of I/O channels: socket-like - process-to-process, and file-like: process to device
• For file-like I/O channels, the channel structure stores open permissions, the file cursor, a reference counter and the reference to the actual file metadata.
• Typical file operations modify contents, size and file cursor / pointer.
• A file descriptor is an index into the file descriptor table, that contains pointers to a channel structure.