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    <p>False sharing between threads is when 2 or more threads use the same cache line.</p> <p>E.g. :</p> <pre><code>struct Work { Work( int&amp; d) : data( d ) {} void operator()() { ++data; } int&amp; data; }; int main() { int false_sharing[10] = { 0 }; boost::thread_group threads; for (int i = 0; i &lt; 10; ++i) threads.create_thread(Work(false_sharing[i])); threads.join_all(); int no_false_sharing[10 * CACHELINE_SIZE_INTS] = { 0 }; for (int i = 0; i &lt; 10; ++i) threads.create_thread(Work(no_false_sharing[i * CACHELINE_SIZE_INTS])); threads.join_all(); } </code></pre> <p>The threads in the first block do suffer from false sharing. The threads in the second block do not (thanks to <code>CACHELINE_SIZE</code>).</p> <p>Data on the stack is always 'far' away from other threads. (E.g. under windows, at least a couple of pages).</p> <p>With your definition of a function object, false sharing can appear, because the instances of <code>Work</code> get created on the heap and this heap space is used inside the thread.</p> <p>This may lead to several <code>Work</code> instances to be adjacent and so may incur sharing of cache lines.</p> <p>But ... your sample does not make sense, because data is never touched outside and so false sharing is induced needlessly.</p> <p>The easiest way, to prevent problems like this, is to copy your 'shared' data locally on tho the stack, and then work on the stack copy. When your work is finished copy it back to the output var.</p> <p>E.g: </p> <pre><code>struct Work { Work( int&amp; d) : data( d ) {} void operator()() { int tmp = data; for( int i = 0; i &lt; lengthy_op; ++i ) ++tmp; data = tmp; } int&amp; data; }; </code></pre> <p>This prevents all problems with sharing.</p>
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    1. POFalse sharing and stack variables
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    1. POFalse sharing and stack variables
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    1. COAre you saying that data can be affected by false sharing? In my case copying it into the function's stack won't help, because the function itself must be called frequently and uses data only once per call.
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    2. COWhen the function must be called very frequently, it doesn't make sense to create a thread every time. Either you do much work in a new thread, or you just burn cycles for thread creation/destruction. And in the later case, you overshadow the cost of false sharing by the enormous costs of the threads.
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    3. CONevertheless. If you cannot copy data onto the stack for your operation, then just make 'Work' large enough to be at least CACHLINE_SIZE long. You lose a couple of bytes, but you can really be sure to never run into false sharing problems.
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