Unexpected results when multithreading
I have a function that needs to be called by a thread initiated by start and end parameters. The function works fine when run on one main thread. However, when I try to use multithreading, the code breaks.
The function is as follows:
static void processThread(long startLimit, long endLimit)
{
long rangeLimit = startLimit;
while (startLimit < endLimit) {
rangeLimit = rangeLimit + 100;
startLimit++;
Console.WriteLine("Processed for " + startLimit + ", " + rangeLimit);
startLimit = rangeLimit;
}
}
I call it from main like ::
int threadCount = 4;
long[] startPoints = new long[threadCount];
long[] endPoints = new long[threadCount];
if ((endLimit / 100) % threadCount == 0)
{
for (int i = 0; i < threadCount; i++)
{
endPoints[i] = endLimit * (i + 1) / threadCount;
}
startPoints[0] = 0;
for (int i = 1; i < threadCount; i++)
{
startPoints[i] = endPoints[i - 1];
}
}
Thread[] threads = new Thread[threadCount];
for (int i = 0; i < threadCount; i++)
{
threads[i] = new Thread(() => processThread(startPoints[i], endPoints[i]));
threads[i].Start();
Console.WriteLine("Started for " + startPoints[i] + ", " + endPoints[i]);
}
Expected Result:
Processed for 1, 100
Processed for 101, 200
Processed for 201, 300
Processed for 301, 400
Processed for 401, 500
Processed for 501, 600
Processed for 601, 700
Processed for 701, 800
.....
And so on ... But I get:
Started for 0, 2500
Started for 2500, 5000
Processed for 5001, 5100
Processed for 5101, 5200
Processed for 5201, 5300
Processed for 5301, 5400
Processed for 5401, 5500
Processed for 5501, 5600
Processed for 5601, 5700
Processed for 5001, 5100
Started for 5000, 7500
Processed for 5001, 5100
Processed for 5101, 5200
Processed for 5201, 5300
Processed for 5301, 5400
Processed for 5401, 5500
Processed for 5501, 5600
Processed for 5601, 5700
Processed for 5701, 5800
Processed for 5801, 5900
Processed for 5901, 6000
Processed for 6001, 6100
Processed for 6101, 6200
Started for 7500, 10000
Processed for 6201, 6300
Processed for 6301, 6400
Processed for 6401, 6500
Processed for 6501, 6600
Processed for 5701, 5800
Which has many duplicate values and none of the 0-2500 range. I also tried with Task.Factory and got the same result.
Any help on this would be greatly appreciated.
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I would not comment on the correct solution to your problem, just to fix the output, you are missing this at the end to block the main thread until all children have finished:
Console.WriteLine("Started for " + startPoints[i] + ", " + endPoints[i]);
}
foreach (var thread in threads)
{
thread.Join();
}
Console.ReadKey();
}
One more thing I fixed in the code:
int count = i;
threads[i] = new Thread(() =>
{
processThread(startPoints[count], endPoints[count]);
});
To prevent potential problems where I am invalid when evaluating from child threads.
I would also say that unless you really need to use the "old" .net streams, parallel library, or tasks, you should use it as suggested by other commenters.
The same was rewritten using the Task approach (I kept it as soon as I replaced Thread with Task for a better illustration).
var factory = new TaskFactory();
var tasks = new List<Task>();
for (int i = 0; i < threadCount; i++)
{
int count = i;
Task task = factory.StartNew(() =>
{
processThread(startPoints[count], endPoints[count]);
});
tasks.Add(task);
Console.WriteLine("Started for " + startPoints[i] + ", " + endPoints[i]);
}
Task.WaitAll(tasks.ToArray());
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I ran your code and actually got an out of bounds index exception when starting threads. This, however, worked for me:
Thread[] threads = new Thread[threadCount];
for (int i = 0; i < threadCount; i++) {
var start = startPoints[i];
var end = endPoints[i];
threads[i] = new Thread( () => processThread( start, end ) );
threads[i].Start();
Console.WriteLine( "Started for " + startPoints[i] + ", " + endPoints[i] );
}
The problem might be that when the thread accesses startPoints[i]
in your code, it i
has already changed in the main thread.
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