Go EP20: sync.Cond, the Most Overlooked Sync Mechanism
In Go, sync.Cond is a synchronization primitive, though it’s not as commonly used as its siblings like sync.Mutex or sync.WaitGroup. You’ll rarely see it in most projects or even in the standard libra
Here are the key takeaways from: https://victoriametrics.com/blog/go-sync-cond/
While most Go developers are familiar with channels and mutexes, sync.Cond is an often-overlooked synchronization tool that's particularly useful when multiple goroutines need to wait for a specific condition to occur.
Instead of wastefully using busy-waiting loops or time.Sleep(), sync.Cond provides batter way to put goroutines to sleep and wake them up precisely when needed.
The relationship between sync.Cond and mutexes is quite clever:
When a goroutine calls Wait(), it automatically releases the mutex lock before going to sleep and re-acquires it when waking up
This also means you must always hold the lock before calling Wait(), or your program will panic.
There's a fascinating race condition that can occur with sync.Cond's notification system.
When a goroutine wakes up from Wait(), the condition it was waiting for might no longer be true because other goroutines could have modified the shared state in the meantime.
This is why you should always use Wait() inside a for loop that checks the condition, rather than using a simple if statement.
for !checkSomeSharedState() {
cond.Wait()
}
The internal implementation uses a ticket-based system, similar to how a deli counter works. Each waiting goroutine gets a unique ticket number, but interestingly, the order in which goroutines are added to the waiting list might not match their ticket numbers.
To sum up, while channels are often the go-to solution for goroutine communication in Go, sync.Cond has a unique advantage: it can repeatedly broadcast to multiple goroutines without needing to create new channels.
If you're interested in diving deeper into the content, you can follow the links https://victoriametrics.com/blog/go-sync-waitgroup/ to read the full posts.