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POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
 

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  1. POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
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    <p>I'm experiencing a weird behaviour in my computer when distributing processes among its cores using doMC and foreach. Does someone knows why using single core I got better performance than using 2 cores? As you can see, processing the same code without register any core (which supposedly use only 1 core) yields to a much more time-efficiency processing. While %do% seems to perform better than %dopar%, registering 2 cores out of 4 yield to more time consuming. </p> <pre><code>require(foreach) require(doMC) # 1-core &gt; system.time(m &lt;- foreach(i=1:100) %dopar% + matrix(rnorm(1000*1000), ncol=5000) ) user system elapsed 9.285 1.895 11.083 &gt; system.time(m &lt;- foreach(i=1:100) %do% + matrix(rnorm(1000*1000), ncol=5000) ) user system elapsed 9.139 1.879 10.979 # 2-core &gt; registerDoMC(cores=2) &gt; system.time(m &lt;- foreach(i=1:100) %dopar% + matrix(rnorm(1000*1000), ncol=5000) ) user system elapsed 3.322 3.737 132.027 &gt; system.time(m &lt;- foreach(i=1:100) %do% + matrix(rnorm(1000*1000), ncol=5000) ) user system elapsed 9.744 2.054 11.740 </code></pre> <p>Using 4 cores in few trials yield to very different outcomes: </p> <pre><code>&gt; registerDoMC(cores=4) &gt; system.time(m &lt;- foreach(i=1:100) %dopar% { matrix(rnorm(1000*1000), ncol=5000) } ) user system elapsed 11.522 4.082 24.444 &gt; system.time(m &lt;- foreach(i=1:100) %dopar% { matrix(rnorm(1000*1000), ncol=5000) } ) user system elapsed 21.388 6.299 25.437 &gt; system.time(m &lt;- foreach(i=1:100) %dopar% { matrix(rnorm(1000*1000), ncol=5000) } ) user system elapsed 17.439 5.250 9.300 &gt; system.time(m &lt;- foreach(i=1:100) %dopar% { matrix(rnorm(1000*1000), ncol=5000) } ) user system elapsed 17.480 5.264 9.170 </code></pre>
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    1. USSteve Weston
    1. USdaniel
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      1. POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
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      1. POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
      1. USRicardo Saporta
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    1. CO1. Producing a single matrix is not (in general) done in parallel unless you define how it is to be done, and you have not. You should expect worse results when using more than one core. 2. Does `registerDoMC` affect `%do%`? The results are similar.
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      1. POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
      1. USMatthew Lundberg
    2. CO@MatthewLundberg: Please `%do%` check the documentation :-)
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      1. POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
      1. USkrlmlr
    3. COUnless I am reading the output wrong, I am not seeing _"much more time-efficiency processing"_, it looks like a negligble difference of barely 1.5%
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      1. POParallelization in R: %dopar% vs %do%. Why using a single core yields to better performance?
      1. USRicardo Saporta
 

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