Re: [patch 1/13] Qsort

[Rafael J. Wysocki]

On Sunday, 23 of January 2005 06:05, Jesper Juhl wrote:
> On Sun, 23 Jan 2005, Andi Kleen wrote:
> 
> > > How about a shell sort?  if the data is mostly sorted shell sort beats 
> > > qsort lots of times, and since the data sets are often small in-kernel, 
> > > shell sorts O(n^2) behaviour won't harm it too much, shell sort is also 
> > > faster if the data is already completely sorted. Shell sort is certainly 
> > > not the simplest algorithm around, but I think (without having done any 
> > > tests) that it would probably do pretty well for in-kernel use... Then 
> > > again, I've known to be wrong :)
> > 
> > I like shell sort for small data sets too. And I agree it would be 
> > appropiate for the kernel.
> > 
> Even with large data sets that are mostly unsorted shell sorts performance 
> is close to qsort, and there's an optimization that gives it O(n^(3/2)) 
> runtime (IIRC),

Yes, there is.

> and another nice property is that it's iterative so it  
> doesn't eat up stack space (as oposed to qsort which is recursive and eats 
> stack like ****)...

To be precise, one needs ~(log N) of stack space for qsort, and frankly, one
should use something like the shell (or should I say Shell?) sort for sorting
small sets of elements in qsort as well.

> Yeah, I think shell sort would be good for the kernel.

I agree.

Greets,
RJW


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