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SubjectRe: [patch 1/13] Qsort
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.


- Would you tell me, please, which way I ought to go from here?
- That depends a good deal on where you want to get to.
-- Lewis Carroll "Alice's Adventures in Wonderland"
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