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Busy-waiting itself can be made much less wasteful by using a delay function (e.g., sleep()) found in most operating systems. This puts a thread to sleep for a specified time, during which the thread will waste no CPU time. If the loop is checking something simple then it will spend most of its time asleep and will waste very little CPU time.
A few multi-core processors have a "power-conscious spin-lock" instruction that puts a processor to sleep, then wakes it up on the next cycle after the lock is freed. A spin-lock using such instructions is more efficient and uses less energy than spin locks with or without a back-off loop. [6]
lock contention: this occurs whenever one process or thread attempts to acquire a lock held by another process or thread. The more fine-grained the available locks, the less likely one process/thread will request a lock held by the other. (For example, locking a row rather than the entire table, or locking a cell rather than the entire row);
Since 7 October 2024, Python 3.13 is the latest stable release, and it and, for few more months, 3.12 are the only releases with active support including for bug fixes (as opposed to just for security) and Python 3.9, [55] is the oldest supported version of Python (albeit in the 'security support' phase), due to Python 3.8 reaching end-of-life.
Once the previous lock value // was 0, however, then it indicates the lock was **not** locked before we // locked it, but now it **is** locked because we locked it, indicating // we own the lock. while (test_and_set (& lock) == 1); critical section // only one process can be in this section at a time lock = 0; // release lock when finished with ...
As soon as the first thread in question is switched back to, its program counter will be at step 1c, and it will sleep and be unable to be woken up again, violating the invariant that it should have been on c's sleep-queue when it slept. Other race conditions depend on the ordering of steps 1a and 1b, and depend on where a context switch occurs.
Alternatively an RW lock can be implemented in terms of a condition variable, cond, an ordinary (mutex) lock, g, and various counters and flags describing the threads that are currently active or waiting. [7] [8] [9] For a write-preferring RW lock one can use two integer counters and one Boolean flag:
Specifically, to acquire a lock, process i executes [4]: 22 i ← ProcessNo for ℓ from 0 to N − 1 exclusive level[i] ← ℓ last_to_enter[ℓ] ← i while last_to_enter[ℓ] = i and there exists k ≠ i, such that level[k] ≥ ℓ wait. To release the lock upon exiting the critical section, process i sets level[i] to −1.