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copied!<p>This is actually a hard set of questions to which there are no absolute answers, but the following should serve as decent approximations:</p> <ol> <li><p>It is a function of your application behavior and your runtime environment, and can only be deduced by experimentation. There is usually a threshold after which your performance actually degrades as you increase the number of threads.</p></li> <li><p>Usually, after you find your limits, you have to figure out how to redesign your application such that the cost-per-thread is not as high. (Note that for some domains, you can get better performance by redesigning your algorithm and reducing the number of threads.) </p></li> <li><p>There is no general "ideal" number of threads, but you can <em>sometimes</em> find the optimal number of threads for an application on a specific runtime environment. This is usually done by experimentation, and graphing the results of benchmarks while varying the following:</p> <ul> <li>Number of threads.</li> <li>Buffer sizes (if the data is not in RAM) incrementing at some reasonable value (e.g., block size, packet size, cache size, etc.)</li> <li>Varying chunk sizes (if you can process the data incrementally).</li> <li>Various tuning knobs for the OS or language runtime.</li> <li>Pinning threads to CPUs to improve locality.</li> </ul></li> <li><p>There are many factors that affect thread limits, but the most common ones are:</p> <ul> <li>Per-thread memory usage (the more memory each thread uses, the fewer threads you can spawn)</li> <li>Context-switching cost (the more threads you use, the more CPU-time is spent switching).</li> <li>Lock contention (if you rely on a lot of coarse grained locking, the increasing the number of threads simply increases the contention.)</li> <li>The threading model of the OS (How does it manage the threads? What are the per-thread costs?)</li> <li>The threading model of the language runtime. (Coroutines, green-threads, OS threads, sparks, etc.)</li> <li>The hardware. (How many CPUs/cores? Is it hyperthreaded? Does the OS loadbalance the threads appropriately, etc.)</li> <li>Etc. (there are many more, but the above are the most important ones.)</li> </ul></li> </ol>

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