Process Management

Process Management

Preemptive weighted round-robin scheduler. Fixed process table (64 entries).

See also: Architecture, Syscalls, Userspace, SMP, Memory Management, Shell

Process states

PROC_RUN → PROC_BLOCKED → PROC_ZOMBIE → PROC_EMPTY

Scheduler

• Each process has sched_weight (1–64) and sched_quantum countdown
• PIT fires at 1000 Hz; ticks decrement quantum, switch when quantum hits 0
• Compositor gets weight 4 for responsive GUI
• nice/renice set job weights from the Shell

Kernel threads

Created via create_task(func, stack). Run in ring 0 with kernel PML4.

User processes

Ring 3 with own PML4. Lifecycle: spawn → schedule → exit → reap. See Userspace.

fork()

SYS_FORK (syscall #10) clones the calling ring-3 process via copy-on-write:

• Refcounted frames: page_incref() tracks share count; free_page() only frees at last reference
• COW clone: clone_page_directory_cow() walks user PML4[0..510]; writable leaves downgraded to read-only + COW in both parent & child (refcount bumped), read-only pages (code) shared as-is
• First writer: vm_handle_fault detects COW → allocates private copy, drops shared reference
• Return: child gets fork()==0, parent gets child's PID — both resume in round-robin

Blocking

• kwait(pid): block until child exits
• sleep(ms): block with wake_tick, scheduler wakes on timer tick
• cli;hlt yield pattern ensures atomic check-then-sleep

Preemption safety

preempt_disable/enable guards kmalloc, kfree, EXT2 ops, VFS node pool.

SMP

See SMP for multi-core scheduling (future: per-CPU run queues).