ReaderWriterLockSlim vs. Monitor

Question

I have an IDictionary<TKey,TValue> implementation that internally holds n other Dictionary<TKey, TValue> and distributes that insertions by the HashCode of the key to the invidual sub-dictionaries. With 16 sub-dictionaries, the number of collisions is pretty low on a 4-core machine.

For parallel insertions, i locked the Add-method with a ReaderWriterLockSlim, locking only the individual sub-dictionary:

  public void Add(TKey key, TValue value)
        {
            int poolIndex = GetPoolIndex(key);
            this.locks[poolIndex].EnterWriteLock();
            try
            {
                this.pools[poolIndex].Add(key, value);
            }
            finally
            {
                this.locks[poolIndex].ExitWriteLock();
            }
        }

When inserting items with four threads, i only got about 32% cpu usage and bad performance. So i replaced the ReaderWriterLockSlim by a Monitor (i.e., the lock keyword). CPU usage was now at nearly 100% and the performance was more than doubled.

My question is: Why did the CPU usage increase? The number of collisions should not have changed. What makes ReaderWriterLock.EnterWriteLock wait so many times?

Answer 1

For write-only load the Monitor is cheaper than ReaderWriterLockSlim, however, if you simulate read + write load where read is much greater than write, then ReaderWriterLockSlim should out perform Monitor.