Heapsort is useless in real world...

Posted Jan 20, 2012 22:45 UTC (Fri) by Wol (subscriber, #4433)
In reply to: Heapsort is useless in real world... by ekj
Parent article: Denial of service via hash collisions

Except that, is there a pathological case for heap sort? It's actually a very well behaved sort. Setting up the heap is the most unpredictable bit, and the most pathological case (an already-sorted array) isn't that much worse than a random array.

And once the heap is created, it's completely predictable. For a heap of 2^x items, it will require x comparisons and x swaps to get the next value. (Give or take an off-by-one error on my part :-)

Cheers,
Wol

It was already explained...

Posted Jan 22, 2012 0:54 UTC (Sun) by khim (subscriber, #9252) [Link]

Except that, is there a pathological case for heap sort?

Sometimes it's good idea to actually look up further upthread.

It's actually a very well behaved sort.

No, it's absolutely not. It's quite good WRT to comparisons and swaps, but it's absolute disaster WRT to memory access. If you data fit in L1 cache (16K-32K) it works just fine, if you hit L2 and/or L3 cache you start doing worse then merge sort or quick sort, but difference is tolerable (typically less then 2x). The real disaster happens once you hit main memory: to read one number from random address in memory you need about 250-300 ticks (and this is what happens with heapsort because prefetch can not cope with it's memory access patterns), while sustained sequential access gives you next 32bit number in 2-3 ticks. Since some part of heap will be in cache even for large arrays heapsort does not suddenly become 50-100 times slower then mergesort when you pass L3 border, to get to this level of difference you need hundreds of gigabytes of data.