ConsurrentDictionary并发字典知多少? (3)

Volatile读取指定字段时，在读取的内存中插入一个内存屏障，阻止处理器重新排序内存操作，如果在代码中此方法之后出现读取或写入，则处理器无法在此方法之前移动它。

public bool TryGetValue(TKey key, out TValue value) { if (key == null) throw new ArgumentNullException("key"); // We must capture the m_buckets field in a local variable. It is set to a new table on each table resize. Tables tables = m_tables; IEqualityComparer<TKey> comparer = tables.m_comparer; GetBucketAndLockNo(comparer.GetHashCode(key), out var bucketNo, out _, tables.m_buckets.Length, tables.m_locks.Length); // We can get away w/out a lock here. // The Volatile.Read ensures that the load of the fields of 'n' doesn't move before the load from buckets[i]. Node n = Volatile.Read(ref tables.m_buckets[bucketNo]); while (n != null) { if (comparer.Equals(n.m_key, key)) { value = n.m_value; return true; } n = n.m_next; } value = default(TValue); return false; } Remove

Remove方法实现其实也并不复杂,类似我们链表操作中移除某个Node.移除节点的同时,还要对前后节点进行链接,相信一块小伙伴们肯定很好理解.

private bool TryRemoveInternal(TKey key, out TValue value, bool matchValue, TValue oldValue) { while (true) { Tables tables = m_tables; IEqualityComparer<TKey> comparer = tables.m_comparer; int bucketNo, lockNo; GetBucketAndLockNo(comparer.GetHashCode(key), out bucketNo, out lockNo, tables.m_buckets.Length, tables.m_locks.Length); lock (tables.m_locks[lockNo]) { if (tables != m_tables) continue; Node prev = null; for (Node curr = tables.m_buckets[bucketNo]; curr != null; curr = curr.m_next) { if (comparer.Equals(curr.m_key, key)) { if (matchValue) { bool valuesMatch = EqualityComparer<TValue>.Default.Equals(oldValue, curr.m_value); if (!valuesMatch) { value = default(TValue); return false; } } if (prev == null) Volatile.Write(ref tables.m_buckets[bucketNo], curr.m_next); else { prev.m_next = curr.m_next; } value = curr.m_value; tables.m_countPerLock[lockNo]--; return true; } prev = curr; } } value = default(TValue); return false; } } Grow table

当table中任何一个m_countPerLock的数量超过了设定的阈值后,会触发此操作对Table进行扩容.

private void GrowTable(Tables tables, IEqualityComparer<TKey> newComparer, bool regenerateHashKeys, int rehashCount) { int locksAcquired = 0; try { //首先锁住第一个lock进行resize操作. AcquireLocks(0, 1, ref locksAcquired); if (regenerateHashKeys && rehashCount == m_keyRehashCount) { tables = m_tables; } else { if (tables != m_tables) return; long approxCount = 0; for (int i = 0; i < tables.m_countPerLock.Length; i++) { approxCount += tables.m_countPerLock[i]; } //如果bucket数组太空，则将预算加倍，而不是调整表的大小 if (approxCount < tables.m_buckets.Length / 4) { m_budget = 2 * m_budget; if (m_budget < 0) { m_budget = int.MaxValue; } return; } } int newLength = 0; bool maximizeTableSize = false; try { checked { newLength = tables.m_buckets.Length * 2 + 1; while (newLength % 3 == 0 || newLength % 5 == 0 || newLength % 7 == 0) { newLength += 2; } } } catch (OverflowException) { maximizeTableSize = true; } if (maximizeTableSize) { newLength = int.MaxValue; m_budget = int.MaxValue; } AcquireLocks(1, tables.m_locks.Length, ref locksAcquired); object[] newLocks = tables.m_locks; //Add more locks if (m_growLockArray && tables.m_locks.Length < MAX_LOCK_NUMBER) { newLocks = new object[tables.m_locks.Length * 2]; Array.Copy(tables.m_locks, newLocks, tables.m_locks.Length); for (int i = tables.m_locks.Length; i < newLocks.Length; i++) { newLocks[i] = new object(); } } Node[] newBuckets = new Node[newLength]; int[] newCountPerLock = new int[newLocks.Length]; for (int i = 0; i < tables.m_buckets.Length; i++) { Node current = tables.m_buckets[i]; while (current != null) { Node next = current.m_next; int newBucketNo, newLockNo; int nodeHashCode = current.m_hashcode; if (regenerateHashKeys) { //Recompute the hash from the key nodeHashCode = newComparer.GetHashCode(current.m_key); } GetBucketAndLockNo(nodeHashCode, out newBucketNo, out newLockNo, newBuckets.Length, newLocks.Length); newBuckets[newBucketNo] = new Node(current.m_key, current.m_value, nodeHashCode, newBuckets[newBucketNo]); checked { newCountPerLock[newLockNo]++; } current = next; } } if (regenerateHashKeys) { unchecked { m_keyRehashCount++; } } m_budget = Math.Max(1, newBuckets.Length / newLocks.Length); m_tables = new Tables(newBuckets, newLocks, newCountPerLock, newComparer); } finally { ReleaseLocks(0, locksAcquired); } } 学习感悟