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    <p><code>flush</code> is not a function - it is an OpenMP <em>compiler directive</em>. It affects the way the compiler generates the executable code and instructs it to synchronise the values of all optimised variables (stored in CPU registers or other explicitly programmable cache / thread-local memory) in the flush-set. This is similar to the effect that the <code>volatile</code> storage modifier has on code generation, but has more limited point-local effect.</p> <p>How does it work? While parsing the source code, the compiler analyses the flow of statements and the data (variables) that gets affected by those statements. Consequently the compiler builds an execution graph and a data dependency graph from the code. It knows exactly where and how the value of each variable is being used and the execution of which code block affects which variables. Then the compiler tries to optimise the code by simplifying the graph and to reduce the number of expensive memory operations by either using CPU registers to store intermediate values or by using another for of faster thread-addressable local memory. The <code>flush</code> directive adds special points in the execution graph, where the compiler must explicitly synchronise the memory view of the thread (register variables and local-memory variables) with the global shared memory. Since the compiler has built the dependency graph in the first place, it knows exactly which variables in the flush-set were modified and hence have to be written to the shared memory; all other variables in the flush-set have to be read from the shared memory.</p> <p>So the answer to your question is that it is usually the compiler who processes the <code>flush</code> directive, not the OS, although the compiler might call into the OS to actually implement the flush, e.g. on systems with explicitly programmable caches/local memories. But one should also note that OpenMP is an <em>abstract standard</em>, which can be implemented on many different hardware platforms and that some of those platforms provide certain hardware that can help with implementing the OpenMP abstractions more efficiently (e.g. the CPU ASIC in IBM's Blue Gene/Q provides many such features).</p>
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