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POIs it possible to check in Java if the CPU is hyper threading?
 

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  1. POIs it possible to check in Java if the CPU is hyper threading?
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    <p>I would like to know the optimal number of threads I can run. Normally, this equals to <code>Runtime.getRuntime().availableProcessors()</code>. </p> <p>However, the returned number is twice as high on a CPU supporting hyper threading. Now, for some tasks hyper threading is good, but for others it does nothing. In my case, I suspect, it does nothing and so I wish to know whether I have to divide the number returned by <code>Runtime.getRuntime().availableProcessors()</code> in two. </p> <p>For that I have to deduce whether the CPU is hyper threading. Hence my question - how can I do it in Java?</p> <p>Thanks.</p> <p><strong>EDIT</strong></p> <p>OK, I have benchmarked my code. Here is my environment:</p> <ul> <li>Lenovo ThinkPad W510 (i.e. i7 CPU with 4 cores and hyperthreading), 16G of RAM</li> <li>Windows 7</li> <li>84 zipped CSV files with zipped sizes ranging from 105M to 16M</li> <li>All the files are read one by one in the main thread - no multithreading access to the HD.</li> <li>Each CSV file row contains some data, which is parsed and a fast context-free test determines whether the row is relevant.</li> <li>Each relevant row contains two doubles (representing longitude and latitude, for the curious), which are coerced into a single <code>Long</code>, which is then stored in a shared hash set.</li> </ul> <p>Thus the worker threads do not read anything from the HD, but they do occupy themselves with unzipping and parsing the contents (using the <a href="http://opencsv.sourceforge.net/" rel="noreferrer">opencsv</a> library).</p> <p>Below is the code, w/o the boring details:</p> <pre><code>public void work(File dir) throws IOException, InterruptedException { Set&lt;Long&gt; allCoordinates = Collections.newSetFromMap(new ConcurrentHashMap&lt;Long, Boolean&gt;()); int n = 6; // NO WAITING QUEUE ! ThreadPoolExecutor exec = new ThreadPoolExecutor(n, n, 0L, TimeUnit.MILLISECONDS, new SynchronousQueue&lt;Runnable&gt;()); StopWatch sw1 = new StopWatch(); StopWatch sw2 = new StopWatch(); sw1.start(); sw2.start(); sw2.suspend(); for (WorkItem wi : m_workItems) { for (File file : dir.listFiles(wi.fileNameFilter)) { MyTask task; try { sw2.resume(); // The only reading from the HD occurs here: task = new MyTask(file, m_coordinateCollector, allCoordinates, wi.headerClass, wi.rowClass); sw2.suspend(); } catch (IOException exc) { System.err.println(String.format("Failed to read %s - %s", file.getName(), exc.getMessage())); continue; } boolean retry = true; while (retry) { int count = exec.getActiveCount(); try { // Fails if the maximum of the worker threads was created and all are busy. // This prevents us from loading all the files in memory and getting the OOM exception. exec.submit(task); retry = false; } catch (RejectedExecutionException exc) { // Wait for any worker thread to finish while (exec.getActiveCount() == count) { Thread.sleep(100); } } } } } exec.shutdown(); exec.awaitTermination(1, TimeUnit.HOURS); sw1.stop(); sw2.stop(); System.out.println(String.format("Max concurrent threads = %d", n)); System.out.println(String.format("Total file count = %d", m_stats.getFileCount())); System.out.println(String.format("Total lines = %d", m_stats.getTotalLineCount())); System.out.println(String.format("Total good lines = %d", m_stats.getGoodLineCount())); System.out.println(String.format("Total coordinates = %d", allCoordinates.size())); System.out.println(String.format("Overall elapsed time = %d sec, excluding I/O = %d sec", sw1.getTime() / 1000, (sw1.getTime() - sw2.getTime()) / 1000)); } public class MyTask&lt;H extends CsvFileHeader, R extends CsvFileRow&lt;H&gt;&gt; implements Runnable { private final byte[] m_buffer; private final String m_name; private final CoordinateCollector m_coordinateCollector; private final Set&lt;Long&gt; m_allCoordinates; private final Class&lt;H&gt; m_headerClass; private final Class&lt;R&gt; m_rowClass; public MyTask(File file, CoordinateCollector coordinateCollector, Set&lt;Long&gt; allCoordinates, Class&lt;H&gt; headerClass, Class&lt;R&gt; rowClass) throws IOException { m_coordinateCollector = coordinateCollector; m_allCoordinates = allCoordinates; m_headerClass = headerClass; m_rowClass = rowClass; m_name = file.getName(); m_buffer = Files.toByteArray(file); } @Override public void run() { try { m_coordinateCollector.collect(m_name, m_buffer, m_allCoordinates, m_headerClass, m_rowClass); } catch (IOException e) { e.printStackTrace(); //To change body of catch statement use File | Settings | File Templates. } } } </code></pre> <p>Please, find below the results (I have slightly changed the output to omit the repeating parts):</p> <pre><code>Max concurrent threads = 4 Total file count = 84 Total lines = 56395333 Total good lines = 35119231 Total coordinates = 987045 Overall elapsed time = 274 sec, excluding I/O = 266 sec Max concurrent threads = 6 Overall elapsed time = 218 sec, excluding I/O = 209 sec Max concurrent threads = 7 Overall elapsed time = 209 sec, excluding I/O = 199 sec Max concurrent threads = 8 Overall elapsed time = 201 sec, excluding I/O = 192 sec Max concurrent threads = 9 Overall elapsed time = 198 sec, excluding I/O = 186 sec </code></pre> <p>You are free to draw your own conclusions, but mine is that hyperthreading does improve the performance in my concrete case. Also, having 6 worker threads seems to be the right choice for this task and my machine. </p>
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    1. COInteresting question. +1. I found something which may interest you although it may not answer your question. http://stackoverflow.com/questions/215236/threads-per-processor
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    2. COHyperthreading adds "logical processors", you can turn off Hyperthreading if you don't want to use it.
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    3. COWill it make any noticeable, or significant, difference to your app performance if, on some machines, you have twice as many threads as cores?
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      1. USMartin James
 

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