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Cooperating Threads

Cooperating Threads. Andy Wang Operating Systems COP 4610 / CGF 5765. Independent Threads. No states shared with other threads Deterministic computation Output depends on input Reproducible Output does not depend on the order and timing of other threads Scheduling order does not matter.

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Cooperating Threads

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  1. Cooperating Threads Andy Wang Operating Systems COP 4610 / CGF 5765

  2. Independent Threads • No states shared with other threads • Deterministic computation • Output depends on input • Reproducible • Output does not depend on the order and timing of other threads • Scheduling order does not matter

  3. Cooperating Threads • Shared states • Nondeterministic • Nonreproducible • Example: 2 threads sharing the same display Thread A Thread B cout << “ABC”; cout << “123”; • You may get “A12BC3”

  4. So, Why Allow Cooperating Threads? • Shared resources • e.g., a single processor • Speedup • Occurs when threads use different resources at different times • Modularity • An application can be decomposed into threads

  5. Some Concurrent Programs • If threads work on separate data, scheduling does not matter Thread A Thread B x = 1; y = 2;

  6. Some Concurrent Programs • If threads share data, the final values are not as obvious Thread A Thread B x = 1; y = 2; x = y + 1; y = y * 2; • What are the indivisible operations?

  7. Atomic Operations • An atomic operation always runs to completion; it’s all or nothing • e.g., memory loads and stores on most machines • Many operations are not atomic • Double precision floating point store on 32-bit machines

  8. Suppose… • Each C statement is atomic • Let’s revisit the example…

  9. x = 1 y = 2 x = y + 1 y = 2 x = 1 y = y * 2 x = 1 y = 2 x = y + 1 y = y * 2 x = y + 1 y = y * 2 y = y * 2 x = y + 1 All Possible Execution Orders Thread A Thread B x = 1; y = 2; x = y + 1; y = y * 2; A decision tree

  10. All Possible Execution Orders Thread A Thread B x = 1; y = 2; x = y + 1; y = y * 2; (x = ?, y = ?) (x = ?, y = 2) (x = 1, y = ?) x = 1 y = 2 (x = 1, y = 2) (x = ?, y = ?) (x = ?, y = 4) x = y + 1 y = 2 x = 1 y = y * 2 (x = ?, y = 2) (x = 3, y = 2) (x = 1, y = 4) (x = 1, y = 4) x = 1 y = 2 x = y + 1 y = y * 2 x = y + 1 y = y * 2 y = y * 2 x = y + 1 (x = ?, y = 4) (x = 3, y = 4) (x = 5, y = 4) (x = 5, y = 4)

  11. Another Example • Assume each C statement is atomic Thread A Thread B j = 0; j = 0; while (j < 10) { while (j > -10) { ++j; --j; } } cout << “A wins”; cout << “B wins”;

  12. So… • Who wins? • Can the computation go on forever? • Race conditions occur when threads share data, and their results depend on the timing of their executions…

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