Performance testing: One core vs multiple - multithreading

Question

I'm trying to understand one problem that I encountered recently in my project. I'm using Aurigma library to resize images. It is used in the single thread mode and produce only one thread during calculation. Lately I decided to move to ImageMagick project, because it is free and open source. I've built IM in the single thread mode and started to test. At first I wanted to compare their performance without interruptions, so I created a test that has high priorities for a process and its thread. Also, I set affinity to one core. I got that IM faster than Aurigma on ~25%. But than more threads I added than less IM had advantage against Aurigma.
My project is a windows service that starts about 7-10 child processes. Each process has 2 threads to process images. When I run my test as two different processes with 2 threads each, I noticed that IM worked worse than Aurigma on about 5%.
Maybe my question is not very detailed, but this scope is a little new for me and I would be glad to get direction for further investigation. How can it be that one program works faster if it is run on one thread in one process, but worse if it is run in multiple processes at the same time.
Fro example,
Au: 8 processes x 2Th(20 tasks per thread) = 320 tasks for 245 secs
IM: 8 processes x 2Th(20 tasks per thread) = 320 tasks for 280 secs
Au: 4 processes x 2Th(20 tasks per thread) = 160 tasks for 121 secs
IM: 4 processes x 2Th(20 tasks per thread) = 160 tasks for 141 secs
We can see that Au works better if we have more that 1 process, but in single process mode: Au process one task for 2,2 sec, IM for 1,4 sec and the sum time is better for IM
private static void ThreadRunner(
Action testFunc,
int repeatCount,
int threadCount
)
{
WaitHandle[] handles = new WaitHandle[threadCount];
var stopwatch = new Stopwatch();
// warmup
stopwatch.Start();
for (int h = 0; h < threadCount; h++)
{
var handle = new ManualResetEvent(false);
handles[h] = handle;
var thread = new Thread(() =>
{
Runner(testFunc, repeatCount);
handle.Set();
});
thread.Name = "Thread id" + h;
thread.IsBackground = true;
thread.Priority = ThreadPriority.Normal;
thread.Start();
}
WaitHandle.WaitAll(handles);
stopwatch.Stop();
Console.WriteLine("All Threads Total time taken " + stopwatch.ElapsedMilliseconds);
}
private static void Runner(
Action testFunc,
int count
)
{
//Process.GetCurrentProcess().ProcessorAffinity = new IntPtr(2); // Use only the second core
Process.GetCurrentProcess().PriorityClass = ProcessPriorityClass.BelowNormal;
Process.GetCurrentProcess().PriorityBoostEnabled = false;
Thread.CurrentThread.Priority = ThreadPriority.Normal;
var stopwatch = new Stopwatch();
// warmup
stopwatch.Start();
while(stopwatch.ElapsedMilliseconds < 10000)
testFunc();
stopwatch.Stop();
long elmsec = 0;
for (int i = 0; i < count; i++)
{
stopwatch.Reset();
stopwatch.Start();
testFunc();
stopwatch.Stop();
elmsec += stopwatch.ElapsedMilliseconds;
Console.WriteLine("Ticks: " + stopwatch.ElapsedTicks +
" mS: " + stopwatch.ElapsedMilliseconds + " Thread name: " + Thread.CurrentThread.Name);
}
Console.WriteLine("Total time taken " + elmsec + " Thread name: " + Thread.CurrentThread.Name);
}
/// <summary>
/// Entry point
/// </summary>
/// <param name="args"></param>
private static void Main(string[] args)
{
var files = GetFiles(args.FirstOrDefault());
if (!files.Any())
{
Console.WriteLine("Source files were not found.");
goto End;
}
//// Run tests
Console.WriteLine("ImageMagick run... Resize");
Runner(() => PerformanceTests.RunResizeImageMagickTest(files[0]), 20);
Console.WriteLine("Aurigma run... Resize");
Runner(() => PerformanceTests.RunResizeAurigmaTest(files[0]), 20);
Console.WriteLine("ImageMagick run... multi Resize");
ThreadRunner(() => PerformanceTests.RunResizeImageMagickTest(files[0]), 20, 2);
Console.WriteLine("Aurigma run... multi Resize");
ThreadRunner(() => PerformanceTests.RunResizeAurigmaTest(files[0]), 20, 2);
End:
Console.WriteLine("Done");
Console.ReadKey();
}
public static void RunResizeImageMagickTest(string source)
{
float[] ratios = { 0.25f, 0.8f, 1.4f };
// load the source bitmap
using (MagickImage bitmap = new MagickImage(source))
{
foreach (float ratio in ratios)
{
// determine the target image size
var size = new Size((int)Math.Round(bitmap.Width * ratio), (int)Math.Round(bitmap.Height * ratio));
MagickImage thumbnail = null;
try
{
thumbnail = new MagickImage(bitmap);
// scale the image down
thumbnail.Resize(size.Width, size.Height);
}
finally
{
if (thumbnail != null && thumbnail != bitmap)
{
thumbnail.Dispose();
}
}
}
}
}
public static void RunResizeAurigmaTest(string source)
{
float[] ratios = { 0.25f, 0.8f, 1.4f };
//// load the source bitmap
using (ABitmap bitmap = new ABitmap(source))
{
foreach (float ratio in ratios)
{
// determine the target image size
var size = new Size((int)Math.Round(bitmap.Width * ratio), (int)Math.Round(bitmap.Height * ratio));
ABitmap thumbnail = null;
try
{
thumbnail = new ABitmap();
// scale the image down
using (var resize = new Resize(size, InterpolationMode.HighQuality))
{
resize.ApplyTransform(bitmap, thumbnail);
}
}
finally
{
if (thumbnail != null && thumbnail != bitmap)
{
thumbnail.Dispose();
}
}
}
}
}
Code for testing is added. I use C#/.NET, ImageMagick works through ImageMagick.Net library, for Aurigma there is one too. For IM .net lib is written on C++/CLI, IM is C. A lot of languages are used.
OpenMP for IM is off.

Answer 1

Could be a memory cache issue. It is possible that multiple threads utilizing memory in a certain way create a scenario that one thread invalidates cache memory that another thread was using, causing a stall.
Programs that are not purely number crunching, but rely on a lot of IO (CPU<->Memory) are more difficult to analyze.