Question

A multithreaded web server wishes to keep track of the number of requests it services (known as hits). Consider the two following strategies to prevent a race condition on the variable hits. The first strategy is to use a basic mutex lock when updating hits:int hits;mutex lock hit lock;hit lock.acquire();hits++;hit lock.release();A second strategy is to use an atomic integer:atomic t hits;atomic inc(&hits);Explain which of these two strategies is more efficient.

Answer 1

Atomic integer strategy will be more efficient than a mutex. Technically, the atomic will lock the memory bus on most platforms. However, there are two ameliorating details It is impossible to suspend a thread during the memory bus lock, but it is possible to suspend a thread during a mutex lock. This is what lets you get a lockfree guarentee (which doesn't say anything about not locking