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Concurrency Hazards: False Sharing

, 10 Jan 2010 CPOL
False sharing and how to detect and avoid it.
FalseSharing_bin.zip
FalseSharingDemo.exe
ZedGraph.dll
FalseSharing_src.zip
FalseSharing
FalseSharingDemo
Properties
Settings.settings
ZedGraph.dll
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Threading.Tasks;
using System.Threading;
using System.Runtime.InteropServices;

namespace FalseSharingDemo
{
	class Worker
	{
		const int ITERS = ( int ) 1e7;

		int _ThreadCount = 0;
		bool _OneWriter = false;
		int _Padding = 0;
		int _Spacing = 0;
		int _Affinity = 0;

		public Worker( int threadCount, bool oneWriter, int padding, int spacing, int affinity )
		{
			_ThreadCount = threadCount;
			_OneWriter = oneWriter;
			_Padding = padding;
			_Spacing = spacing;
			_Affinity = affinity;
		}

		[DllImport( "kernel32.dll" )]
		static extern IntPtr GetCurrentThread();

		[DllImport( "kernel32.dll" )]
		static extern UIntPtr SetThreadAffinityMask( IntPtr hThread, UIntPtr dwThreadAffinityMask );

		public TimeSpan Work()
		{
			var data = new int[ _Padding + ( _ThreadCount * _Spacing ) ];
			Array.Clear( data, 0, data.Length );

			var iters = ITERS / _ThreadCount;
			//var iters = ITERS;

			using ( var mre = new ManualResetEvent( false ) )
			using ( var countdown = new CountdownEvent( _ThreadCount ) )
			{
				TimeSpan[] tss = new TimeSpan[ Environment.ProcessorCount ];

				for ( int i = 0 ; i < _ThreadCount ; i++ )
				{
					int iThread = i;

					if ( !_OneWriter || iThread == 0 )
					{
						new Thread( () =>
						{
							SetThreadAffinityMask( GetCurrentThread(), new UIntPtr( 1u << ( iThread * _Affinity ) ) );

							var offset = _Padding + ( iThread * _Spacing );

							mre.WaitOne();

							for ( int x = 0 ; x < iters ; x++ ) data[ offset ]++;

							countdown.Signal();

						} ) { IsBackground = true, Priority = ThreadPriority.Highest }.Start();
					}
					else
					{
						new Thread( () =>
						{
							SetThreadAffinityMask( GetCurrentThread(), new UIntPtr( 1u << ( iThread * _Affinity ) ) );

							var offset = _Padding + ( iThread * _Spacing );
							int dummy = 0;

							mre.WaitOne();

							for ( int x = 0 ; x < iters ; x++ ) dummy = data[ offset ];

							countdown.Signal();

						} ) { IsBackground = true, Priority = ThreadPriority.Highest }.Start();
					}
				}

				Thread.Sleep( 100 );

				var sw = Stopwatch.StartNew();

				mre.Set();

				countdown.Wait();

				var ts = sw.Elapsed;

				Trace.WriteLine( "False : " + data.Sum( i => ( long ) i ).ToString( "N0" ) + " in " + ts.TotalSeconds + " secs" );

				return ts;
			}
		}
	}
}

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This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Nicholas Butler

United Kingdom United Kingdom

I built my first computer, a Sinclair ZX80, on my 11th birthday in 1980.
In 1992, I completed my Computer Science degree and built my first PC.
I discovered C# and .NET 1.0 Beta 1 in late 2000 and loved them immediately.
I have been writing concurrent software professionally, using multi-processor machines, since 1995.
 
In real life, I have spent 3 years travelling abroad,
I have held a UK Private Pilots Licence for 20 years,
and I am a PADI Divemaster.
 
I now live near idyllic Bournemouth in England.
 
If you would like help with multithreading, please contact me via my website:
 
 
I can work 'virtually' anywhere!

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