golang标准库中sync.Mutex性能测试

古城微笑少年丶 2022-02-22 15:18 275阅读 0赞

笔者原来是使用C++开发的，一直以为互斥锁时要是进程间通信中信号量的一个特例。在使用golang开发的时候，一直都很谨慎使用互斥锁，能用原子变量实现的就用原子变量实现。直到今天，测试了下golang标准库中sync.Mutex的性能以后，彻底的感觉到自己以前的无知和自以为是。经过笔者测试，两者的性能几乎没差异，有差异估计就是一点点封装消耗的性能成本。查看标准包中sync.Mutex实现的源码发现是以原子变量为基础实现的  
测试代码如下：

type Number struct {
    	number int64
    	atomicmutx int64
    	mutx sync.Mutex
    
    }
    
    func AddNumberMutex() {
    	testlock.mutx.Lock()
    	testlock.number = testlock.number + 1
    	testlock.mutx.Unlock()
    }
    
    func AddNumberAtomic() {
    	for {
    		if atomic.CompareAndSwapInt64(&testlock.number, 0, 1) {
    			testlock.number = testlock.number + 1
    			atomic.StoreInt64(&testlock.number, 0)
    			break
    		}
    	}
    }
    
    func BenchmarkMutex(b *testing.B) {
    	for i := 0; i < b.N; i++ {
    		AddNumberMutex()
    	}
    }
    
    func BenchmarkAtomic(b *testing.B) {
    	atomic.StoreInt64(&testlock.number, 0)
    	for i := 0; i < b.N; i++ {
    		AddNumberAtomic()
    	}
    }

运行结果如下：

goos: linux
    goarch: amd64
    BenchmarkMutex  	100000000	        20.2 ns/op
    BenchmarkAtomic 	100000000	        20.9 ns/op

并发测试代码：

func BenchmarkMutexParallel(b *testing.B) {
    	atomic.StoreInt64(&testlock.number, 0)
    	b.RunParallel(func(pb *testing.PB){
    		for pb.Next() {
    			AddNumberMutex()
    		}
    	})
    }
    
    
    func BenchmarkAtomicParallel(b *testing.B) {
    	atomic.StoreInt64(&testlock.number, 0)
    	b.RunParallel(func(pb *testing.PB){
    		for pb.Next() {
    			AddNumberAtomic()
    		}
    	})
    }

并发测试结果：

BenchmarkMutexParallel  	100000000	        24.3 ns/op
    BenchmarkAtomicParallel 	100000000	        21.2 ns/op

查看Mutex.Lock()接口代码如下，这里不做代码说明，但是确实一看就可以知道是以原子变量为基础实现的：

// Lock locks m.
    // If the lock is already in use, the calling goroutine
    // blocks until the mutex is available.
    func (m *Mutex) Lock() {
    	// Fast path: grab unlocked mutex.
    	if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
    		if race.Enabled {
    			race.Acquire(unsafe.Pointer(m))
    		}
    		return
    	}
    
    	var waitStartTime int64
    	starving := false
    	awoke := false
    	iter := 0
    	old := m.state
    	for {
    		// Don't spin in starvation mode, ownership is handed off to waiters
    		// so we won't be able to acquire the mutex anyway.
    		if old&(mutexLocked|mutexStarving) == mutexLocked && runtime_canSpin(iter) {
    			// Active spinning makes sense.
    			// Try to set mutexWoken flag to inform Unlock
    			// to not wake other blocked goroutines.
    			if !awoke && old&mutexWoken == 0 && old>>mutexWaiterShift != 0 &&
    				atomic.CompareAndSwapInt32(&m.state, old, old|mutexWoken) {
    				awoke = true
    			}
    			runtime_doSpin()
    			iter++
    			old = m.state
    			continue
    		}
    		new := old
    		// Don't try to acquire starving mutex, new arriving goroutines must queue.
    		if old&mutexStarving == 0 {
    			new |= mutexLocked
    		}
    		if old&(mutexLocked|mutexStarving) != 0 {
    			new += 1 << mutexWaiterShift
    		}
    		// The current goroutine switches mutex to starvation mode.
    		// But if the mutex is currently unlocked, don't do the switch.
    		// Unlock expects that starving mutex has waiters, which will not
    		// be true in this case.
    		if starving && old&mutexLocked != 0 {
    			new |= mutexStarving
    		}
    		if awoke {
    			// The goroutine has been woken from sleep,
    			// so we need to reset the flag in either case.
    			if new&mutexWoken == 0 {
    				panic("sync: inconsistent mutex state")
    			}
    			new &^= mutexWoken
    		}
    		if atomic.CompareAndSwapInt32(&m.state, old, new) {
    			if old&(mutexLocked|mutexStarving) == 0 {
    				break // locked the mutex with CAS
    			}
    			// If we were already waiting before, queue at the front of the queue.
    			queueLifo := waitStartTime != 0
    			if waitStartTime == 0 {
    				waitStartTime = runtime_nanotime()
    			}
    			runtime_SemacquireMutex(&m.sema, queueLifo)
    			starving = starving || runtime_nanotime()-waitStartTime > starvationThresholdNs
    			old = m.state
    			if old&mutexStarving != 0 {
    				// If this goroutine was woken and mutex is in starvation mode,
    				// ownership was handed off to us but mutex is in somewhat
    				// inconsistent state: mutexLocked is not set and we are still
    				// accounted as waiter. Fix that.
    				if old&(mutexLocked|mutexWoken) != 0 || old>>mutexWaiterShift == 0 {
    					panic("sync: inconsistent mutex state")
    				}
    				delta := int32(mutexLocked - 1<<mutexWaiterShift)
    				if !starving || old>>mutexWaiterShift == 1 {
    					// Exit starvation mode.
    					// Critical to do it here and consider wait time.
    					// Starvation mode is so inefficient, that two goroutines
    					// can go lock-step infinitely once they switch mutex
    					// to starvation mode.
    					delta -= mutexStarving
    				}
    				atomic.AddInt32(&m.state, delta)
    				break
    			}
    			awoke = true
    			iter = 0
    		} else {
    			old = m.state
    		}
    	}
    
    	if race.Enabled {
    		race.Acquire(unsafe.Pointer(m))
    	}
    }