title: external sort with k-way merging vs quick sort
body: <p>Which one is better?
  Say 1GB memory and 100GB file to sort.</p>
  
  <p>One instance of 10-way merging needs:
        - 100 1GB loads followed by 10*10 + 10*100 100MB loads (for 10-way followed by 10-way merging)</p>
  
  <p>Quicksort needs 100*7*2 (nlogn) 1GB loads?</p>