Assume a program has 8 threads and has no race conditions. It is running on a 4-core CPU (i.e. 4-threads can run simultaneously). The number of threads running in a critical section of this program would be

Question

BlackTom AI · Accepted Answer

A critical section guarded by mutual exclusion allows only one thread at a time to execute the protected code. Key concepts: (1) Critical section — a region of code that must not be concurrently executed by more than one thread to prevent data races; (2) Mutual exclusion — enforcement mechanism (mutex, semaphore, lock) that ensures exclusive access; (3) Number of hardware cores affects parallelism but does not change the mutual-exclusion guarantee.

Step-by-step reasoning:
1. The program has 8 threads total. That is how many threads exist that could potentially enter the critical section.
2. The CPU has 4 cores, so up to 4 threads can run simultaneously at the hardware level. Hardware concurrency determines how many threads may be scheduled to run at the same time, but it does not override synchronization constraints in software.
3. The problem states the program has no race conditions, which implies correct synchronization (for example, every critical section is protected by a mutex or equivalent). Correct synchronization enforces mutual exclusion: when one thread enters the critical section it acquires the lock; any other thread attempting to enter will be blocked (or put to wait) until the lock is released.
4. Therefore, even though multiple threads exist and multiple cores are available, at most one thread can be executing inside the critical section at any time.

Why the correct answer is correct: Mutual exclusion is the defining property of a correctly synchronized critical section. It guarantees exclusive access, so the count of threads simultaneously inside must be 1.

Why other naive answers would be wrong: Assuming 4 threads (one per core) ignores the synchronization mechanism; assuming 8 threads ignores both hardware limits and synchronization. Both misunderstand that mutual exclusion is a software constraint independent of core count.

Conclusion: The number of threads running in the critical section is 1, matching the provided answer.

Assume a program has 8 threads and has no race conditions. It is running on a 4-core CPU (i.e. 4-threads can run simultaneously). The number of threads running in a critical section of this program would be单项选择题

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