How expensive is lock(...) when the lock isn't contended?

Question

While looking into double-checked locking I've seen numerous recommendations to just skip the first check and immediately go for the lock and just check after taking it instead.

This lead me to wonder, is lock (this.padlock) "cheap" when uncontended?

Answer 1

We can test it...

I get:

1000000000; 2164 (no lock) 1000000000; 23258 (lock) 21.094ns per lock 

Code:

using System; using System.Diagnostics;  static class P {     static void Main()     {          Test(1); // for JIT         Test(1000000);     }     static readonly object syncLock = new object();     static void Test(int count)     {         int j = 0;         var watch = Stopwatch.StartNew();         for(int i = 0 ; i < count ; i++)         {             for (int z = 0; z < 1000; z++)                 j++;         }         watch.Stop();         long withoutMillis = watch.ElapsedMilliseconds;         Console.WriteLine("{0}; {1} (no lock)", j, watch.ElapsedMilliseconds);          j = 0;         watch = Stopwatch.StartNew();         for (int i = 0; i < count; i++)         {             for (int z = 0; z < 1000; z++ )                 lock (syncLock)                 {                     j++;                 }         }         watch.Stop();         long withMillis = watch.ElapsedMilliseconds;         Console.WriteLine("{0}; {1} (lock)", j, watch.ElapsedMilliseconds);          long deltaNano = (withMillis - withoutMillis) * 1000000;                 // nano = 1000 micro = 1000000 milli         double perLockNano = deltaNano/(1000.0 * count);         Console.WriteLine("{0}ns per lock", perLockNano);     } } 

Answer 2

According to this source, the overhead to lock and unlock is about 20ns.