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POwhat is the fastest way to notify another thread that data is available? any alternativies to spinning?
 

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  1. POwhat is the fastest way to notify another thread that data is available? any alternativies to spinning?
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    <p>One my thread writes data to circular-buffer and another thread need to process this data ASAP. I was thinking to write such simple <code>spin</code>. Pseudo-code!</p> <pre><code> while (true) { while (!a[i]) { /* do nothing - just keep checking over and over */ } // process b[i] i++; if (i &gt;= MAX_LENGTH) { i = 0; } } </code></pre> <p>Above I'm using <code>a</code> to indicate that data stored in <code>b</code> is available for processing. Probaly I should also set thread afinity for such "hot" process. Of course such spin is very expensive in terms of CPU but it's OK for me as my primary requirement is <strong>latency</strong>.</p> <p>The question is - am I should really write something like that or <code>boost</code> or <code>stl</code> allows something that:</p> <ol> <li>Easier to use.</li> <li>Has roughly the same (or even better?) <strong>latency</strong> at the same time occupying less CPU resources?</li> </ol> <p>I think that my pattern is so general that there should be some good implementation somewhere.</p> <p><strong>upd</strong> It seems my question is still too complicated. Let's just consider the case when i need to write some items to array in arbitrary order and another thread should read them in right order as items are available, how to do that?</p> <p><strong>upd2</strong></p> <p>I'm adding test program to demonstrate what and how I want to achive. At least on my machine it happens to work. I'm using <code>rand</code> to show you that I can not use general <code>queue</code> and I need to use <code>array-based</code> structure:</p> <pre><code>#include "stdafx.h" #include &lt;string&gt; #include &lt;boost/thread.hpp&gt; #include "windows.h" // for Sleep const int BUFFER_LENGTH = 10; int buffer[BUFFER_LENGTH]; short flags[BUFFER_LENGTH]; void ProcessorThread() { for (int i = 0; i &lt; BUFFER_LENGTH; i++) { while (flags[i] == 0); printf("item %i received, value = %i\n", i, buffer[i]); } } int _tmain(int argc, _TCHAR* argv[]) { memset(flags, 0, sizeof(flags)); boost::thread processor = boost::thread(&amp;ProcessorThread); for (int i = 0; i &lt; BUFFER_LENGTH * 10; i++) { int x = rand() % BUFFER_LENGTH; buffer[x] = x; flags[x] = 1; Sleep(100); } processor.join(); return 0; } </code></pre> <p>Output:</p> <pre><code>item 0 received, value = 0 item 1 received, value = 1 item 2 received, value = 2 item 3 received, value = 3 item 4 received, value = 4 item 5 received, value = 5 item 6 received, value = 6 item 7 received, value = 7 item 8 received, value = 8 item 9 received, value = 9 </code></pre> <p>Is my program guaranteed to work? How would you redesign it, probably using some of existent structures from boost/stl instead of array? Is it possible to get rid of "spin" without affecting latency?</p>
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    1. USjavapowered
    1. USjavapowered
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      1. POwhat is the fastest way to notify another thread that data is available? any alternativies to spinning?
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    1. COWhy not just use a blocking bounded queue to exchange data between the threads? Every half-decent concurrency library should have one. (The corollary is: use a half-decent library of concurrency collections and utilities when writing multithreaded code.)
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    2. COAre you sure you need threads if you're concerned about latency?
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      1. USAlexey Frunze
    3. COHow sensitive is the latency - what is your upper limit where your code stops working? How long does it take to process, and how much time do you spend waiting (as a proportion and max/min of "per message")? If you say "don't know", then you need to do some research before even asking this question.
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      1. USMats Petersson
 

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