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Subject: Re: [boost] [sort] Re: [review] Formal review period for Sort library begins today, November 10, and ends Wednesday, November 1
From: Steven Ross (spreadsort_at_[hidden])
Date: 2014-11-18 06:49:39

> > The input data distribution was evenly distributed 32-bit random data.
> The
> > absolute worst-case distribution I've been able to design for spreadsort
> is
> > binaryalrbreaker with an ALR_THRESHOLD of 3.
> > With 32-bit integers, it has 512 elements, and takes 15 microseconds for
> > integer_sort, and 14 microseconds for std::sort.
> > With 64-bit integers, it has 131072 elements, and takes 2.8ms for
> > integer_sort, and 1.6ms for std::sort.
> > If you add or subtract any elements, it won't be the worst-case
> > distribution! If I simply double the number of elements, 64-bit
> > integer_sort takes 5.35ms, and std::sort takes 4.56ms. I'm not sure what
> > kind of graph would make sense with this type of situation. I'd also
> like
> > to note that this distribution is specifically designed to make
> > integer_sort slow, and cannot be scaled up without improving the relative
> > speed of integer_sort.
> You've lost me here. Surely, for any N there is input that results in
> worst-case performance, and it should be possible to generate such
> input?
> The worst-case input that I've been able to devise for spreadsort
consists of data structured so that:
1) It uses the full width of the key, forcing radix sorting to use as many
iterations as possible, maximizing overhead and the number of swaps.
2) Each radix iteration only cuts the problem in half, and involves
swapping all the elements. Swaps are the slowest part of this algorithm,
so maximizing the number of swaps is important.

Using restrictions 1 and 2, I get a minimum data size of 512 for 32-bit
data and 131072 for 64-bit data (my binaryalrbreaker example does this if
you set ALR_THRESHOLD to 3). The only way to increase size above that is
to have more elements at the very bottom of the distribution, which
amortizes the radix costs over more elements and rapidly makes the
algorithm relatively more efficient compared to std::sort. Below that it's
just not the worst-case input.

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