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ECS150-L13

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📗 -> 02/06/26: ECS150-L13

makefile
Sync

🎤 Vocab

❗ Unit and Larger Context

Discussion: Makefile
Lecture: Synchronization

✒️ -> Scratch Notes

Synchronization

Using threads recap:

  • Private process registers
  • Private stack
  • Shared global memory

Can be independent (distinct areas of data [:N/2] and [N/2:] for instance) or cooperative (doing seperate things but both writing to a[i])

Concurrency issues:

Thread (preemptive) scheduling:

  • Indeterministic scheduling (many possible executions of thread orders)
    • Does the order of scheduling affect your program output? very important

Even good code can go bad:

Instruction reordering:

  • The compiler can reorder instructions (it assumes synchronous code?)
  • Hardware can reorder instruction (x86 is an out of order processor for instance)

Multi-word operations:

  • Not an atomic operation, gets decomposed into multiple lines by compiler (x=x+1 turns from a 3 instruction command into a 7 instruction command if int64 instead of normal int)
    • Makes the code even more vulnerable to interrupts

These issues all have a name: race conditions

Race conditions

Def: When the order of operations between concurrent threads leads to undesirable/unexpected results

Concurrency introduces different ‘interleavings’ of code. MANY possible.

  • Most are good
  • Some may be very bad

Solution?

  • Synchronization

Use locks (basically):

# thread1
note1 = 1; 
while (note2 == 1) ; 
if (milk == 0) { milk++; }
note1 = 0;

# thread 2
note2 = 1;
if (note1 == 0) { 
	if (milk == 0) { milk++; } 
} 
note2 = 0;

Safe, but involved busy waiting

Peterson’s Algorithm

  • Share a single resource using only shared memory
  • Symmetric and scalable
  • complex
  • wiki

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