title: multicore with plyr, MC
body: <p>Hi I am trying to use <code>ddply</code> in the <code>plyr</code> library in R, with the <code>MC</code> package.  It doesn't seem to be speeding up the computation.  This is the code I run:  </p>
  
  <pre><code>require(doMC)
  registerDoMC(4)
  getDoParWorkers()
  ##&gt; 4
  test &lt;- data.frame(x=1:10000, y=rep(c(1:20), 500))
  system.time(ddply(test, "y", mean))
    # user  system elapsed 
    # 0.015   0.000   0.015
  system.time(ddply(test, "y", mean, .parallel=TRUE))
    # user  system elapsed 
    # 223.062   2.825   1.093 
  </code></pre>
  
  <p>Any ideas?</p>