Forster Brereton reports about his first steps to build a hybrid mutex.
by Foster Brereton
From the article
A blocking mutex will halt the thread until it acquisition. It is useful because it consumes negligible computer resources while blocked. This leaves the CPU free to perform other tasks, including whatever other task currently owns the mutex. All this goodness is not cheap, however: it takes a decent amount of time to block thread. If your critical section is brief, you could be spending a disproportionate amount of time protecting it instead of running it.
Generally, blocking mutexes should be used when your critical section will take a while, such as I/O operations, calling out to the OS, or doing laundry in a collegiate dorm.
A spinning mutex will enter into an infinite loop (spin) until acquisition. It is useful because it can resume very quickly once the lock has been obtained, resulting in minimal overhead while protecting a critical section. However, since the thread remains active on the CPU, it can reduce (or eliminate!) the ability of the CPU to do other work††. If your critical section is long, you could be spending a disproportionate amount of time protecting it instead of running it.
Generally, spin mutexes should be used when your critical section is brief, such as reading or writing a memory-resident data structure.
Finding a middle ground
The dichotomy between the two mutex behaviors has left me stuck more than once. What if I was trying to protect a global resource that occasionally required a call to the OS? In those cases a blocking mutex is not a good fit, as modifying the memory-resident structure is pretty quick. However a spin mutex would be equally bad, because I do need to go to the OS time and again, and it would be a pessimization to spike a CPU while doing so.