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Infinite Loops Best Practice (Not a real best practice)

[Updated: This post is too old to be correct. Even #SNMP starts to use a completely different way to handle incoming packets http://code.google.com/p/sharpsnmplib/source/browse/SharpSnmpLib/Messaging/ListenerBinding.cs. So please read it carefully.]

In .NET (especially when you program with socket), sometimes you have to write an infinite loop. In #SNMP Library, this loop exists in TrapListener.Worker_DoWork method. As a UDP server that monitors incoming diagrams, this loop is unavoidable. However, if it is not correctly and efficiently implemented, you mess things up.

The Real Infinite Loop

At the very beginning, I used the following loop.

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while (!((BackgroundWorker)sender).CancellationPending)
{
    int number = _watcher.Available;
    if (number == 0)
    {
        continue;
    }
    // handle diagrams here
}

Unfortunately my machine is Intel Core 2 Duo powered, so I did not find any issue until a user complained that it ate up all CPU resources. Yes, this user uses a single core CPU.

The Sleeping Loop

A quick fix for the infinite loop issue is to add Thread.Sleep. Now the code looks like this,

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while (!((BackgroundWorker)sender).CancellationPending)
{
    int number = _watcher.Available;
    if (number == 0)
    {
        Thread.Sleep(100);
        continue;
    }
    // handle diagrams
}

Although it is correct, how efficient is it? It depends on the number used in Sleep, and it does not perform well on multi-core CPU because Sleep always triggers a context switch which consumes extra resources.

The Composite Loop

It was a post of CSDN about how to use SpinWait that reminded me of a new approach. By Googling I found this wonderful post.

http://www.bluebytesoftware.com/blog/2006/08/23/PriorityinducedStarvationWhySleep1IsBetterThanSleep0AndTheWindowsBalanceSetManager.aspx

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while (!((BackgroundWorker)sender).CancellationPending)
{
    int number = _watcher.Available;
    if (number == 0)
    {
        if (Environment.ProcessorCount == 1 || unchecked(++loops % 100) == 0)
        {
            Thread.Sleep(1);
        }
        else
        {
            Thread.SpinWait(20);
        }

        continue;
    }
    // handle diagrams here
}

Oh, isn’t it a wonder? On single core CPUs, this loop falls back to the sleeping only approach, while SpinWait is used to optimize performance on multi-core CPUs.

This composite loop is already available in #SNMP repository.

This post is licensed under CC BY 4.0 by the author.
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