How do you implement a high-performance sharded concurrent map in Go?

A single mutex protecting one map is a bottleneck under high concurrency. A sharded map divides the key space across N independent maps, each with its own mutex, reducing contention by approximately N-fold. Goroutines with keys in different shards can operate in parallel without competing.

import (
    "hash/fnv"
    "fmt"
    "sync"
)

const numShards = 32

type ShardedMap[K comparable, V any] struct {
    shards [numShards]struct {
        sync.RWMutex
        m map[K]V
    }
}

func NewShardedMap[K comparable, V any]() *ShardedMap[K, V] {
    sm := &ShardedMap[K, V]{}
    for i := range sm.shards { sm.shards[i].m = make(map[K]V) }
    return sm
}

func (sm *ShardedMap[K, V]) shardFor(key K) *struct {
    sync.RWMutex; m map[K]V
} {
    h := fnv.New32a()
    fmt.Fprint(h, key)
    return &sm.shards[h.Sum32()%numShards]
}

func (sm *ShardedMap[K, V]) Set(key K, val V) {
    s := sm.shardFor(key)
    s.Lock(); defer s.Unlock()
    s.m[key] = val
}

func (sm *ShardedMap[K, V]) Get(key K) (V, bool) {
    s := sm.shardFor(key)
    s.RLock(); defer s.RUnlock()
    v, ok := s.m[key]
    return v, ok
}

// With 32 shards: 32 goroutines with different keys can write simultaneously
// vs a single mutex where all goroutines serialise into one queue

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