Threadsafe golang Map - Efficient safe read-write map than sync.Map or atomic.Value CopyOnWrite Map.

See Implementation: https://github.com/niktri/go-efficientmap/blob/main/efficient_test.go

EfficientMap is a threadsafe CopyOnWrite map, allowing safe reads & writes concurrently.

We exploit the fact that golang assignments are atomic. Cool down! Yes they are not officially, but read on...

• Keeping a Config settings/Cache, which hardly changes but read on every request. Too frequent Get and too infrequent Put for keys. getToPutRatio >>> 1000.
• Keys may or may not be disjoint.

• Concurrent reads are safe.
• It is a runtime panic if multiple go-routines are reading & writing.
• It is still a panic even if single go-routine is writing protected by mutex, if other go-routines are reading unprotected.

• golang sync.Map is drop-in placement for this usecase. Get and Store operations are already Threadsafe.
• Already threadsafe. No mutex required in client code.
• sync.Map is however better suited for disjoint set of keys.

    type SyncMap struct {m sync.Map}
    func NewSyncMap() *SyncMap { return &SyncMap{sync.Map{}} }
    func (sm *SyncMap) Get(key string) (interface{}, bool) {return sm.m.Load(key)}
    func (sm *SyncMap) Put(key string, val interface{}) {sm.m.Store(key, val)}

• golang atomic.Value provides an atomic way to load/store anything. We can implement CopyOnWrite as follows:
• Get operation Load our map from atomic.Value. Now we can read this without any lock.
• Put operation Lock mutex. Load our map. Copy it. Update copy with new value. Atomically store into atomic.Value. Unlock mutex.

    type AtomicMap struct {av    *atomic.Value;mutex sync.Mutex}
    func (am *AtomicMap) Get(key string) (interface{}, bool) {
        ret, ok := am.av.Load().(map[string]interface{})[key]
        return ret, ok
    }
    func (am *AtomicMap) Put(key string, val interface{}) {
        am.mutex.Lock()
        defer am.mutex.Unlock()
        m := am.av.Load().(map[string]interface{})
        copy := make(map[string]interface{}, len(m))
        for k, v := range m {
            copy[k] = v
        }
        copy[key] = val
        am.av.Store(copy)
    }

• Since golang map assignments are atomic, we can implement above CopyOnWrite Solution without using atomic.Value.

    type EfficientMap struct {m     map[string]interface{};mutex sync.Mutex} // Keep plain map
    func (em *EfficientMap) Get(key string) (interface{}, bool) {
        ret, ok := em.m[key] // map read exploit !
        return ret, ok
    }
    func (em *EfficientMap) Put(key string, val interface{}) {
        em.mutex.Lock()
        defer em.mutex.Unlock()
        copy := make(map[string]interface{}, len(em.m))
        for k, v := range em.m {
            copy[k] = v
        }
        copy[key] = val
        em.m = copy // map assignment exploit !
    }

• Go memory model does not guarantee any assignments to be atomic. But..
• Here Russ Cox happens to know a var assignment to be atomic.
• As of go1.15 it seems(to me) that int family, map & pointers assignments are atomic. This may not be true for int64 on 32 or 16 bit systems. -race would ofcourse complain this. So far never found any concurrency panics or wrong values. Please let me know if you find one.
• Interface, string, slices, structs are of course not atomic.
• This may change in future and it is dangerous to rely on this atomicity.
• As per the go memory model, becoming too clever isn't a good idea.

BenchmarkAtomicMap-12       	  149994	      8505 ns/op	     207 B/op	      24 allocs/op
BenchmarkEfficientMap-12    	  148770	      8350 ns/op	     207 B/op	      24 allocs/op
BenchmarkSyncMap-12         	  137811	      8651 ns/op	     203 B/op	      24 allocs/op

Occassionally Our Efficient map is about 1%-2% efficient than AtomicMap which is 2%-5% efficient than sync.SyncMap.

• Results are not very consistent. We seldom achieved >2% performance gain.
• Also we assumed getToPutRatio >> 1000. For lower ratios sync.Map is best, especially if keys are disjoint.
• If map size is big CopyOnWrite is waste.

Our Efficient Map is good for academic interest. In practice, always use plain map with mutex or sync.Map.