为什么sync.Mutex存在？

Question

Why sync.Mutex exists while we have sync.RWMutex? I can lock/unlock rw mutex. What is the main difference between them?

Answer 1

It's true that you could use a sync.RWMutex whenever you need a sync.Mutex.

I think both exist because there are a lot of cases when a sync.Mutex is enough (you don't need read and write level locking), and the implementation of sync.Mutex is simpler: requires much less memory and is most likely faster.

sync.Mutex is just 8 bytes:

type Mutex struct {
    state int32
    sema  uint32
}

While sync.RWMutex is 8 + 16 = 24 bytes (it includes a sync.Mutex):

type RWMutex struct {
    w           Mutex  // held if there are pending writers
    writerSem   uint32 // semaphore for writers to wait for completing readers
    readerSem   uint32 // semaphore for readers to wait for completing writers
    readerCount int32  // number of pending readers
    readerWait  int32  // number of departing readers
}

Yes, you could say 8 or 24 bytes should not matter. And it doesn't as long as you only have a few mutexes.

But it's not uncommon to put the mutex into the struct it's ought to protect (either embed or a regular, named field). Now if you have a slice of these struct values, maybe even thousands of them, then yes, it will make a noticeable difference.

Also, if you just need a mutex, sync.Mutex gives you less chance of misusing it (you can't accidentally call RLock() because it doesn't have that method).