Memory & Concurrency

Every address a program uses is a lie the hardware keeps: a page table maps it to a frame, a tiny buffer remembers the last few answers, and a page not in memory is a fault that reads the disk.

Live means reachable from a root. Mark everything the program can still reach, sweep the heap and free the rest, cycles included, then compact if the holes matter.

Two threads each add one to a counter and the result is wrong, because an increment is three instructions and the scheduler may switch between any two. The bug comes and goes with the timing.

A lock fixes the race and opens a new trap: five philosophers each pick up the left fork and wait forever for the right. Deadlock is a cycle of waits, and every cure makes the cycle impossible.

One thread, no races, and still a timer, a promise, and a file read all in flight: the script runs to its end, microtasks drain, then one task at a time. Order comes from the queues, not the clock.

An actor owns its state and handles messages one at a time: nothing shared, so nothing races or locks. Two workers incrementing a counter get six in any order. The price is a moment nobody can see.