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Prolog library for building self-optimizing predicates

branch: master
README.md

Usage

This library is a Prolog macro generator for building self-optimizing predicates.

Let's say you want to sort a list. Should you use merge sort, quick sort, counting sort or maybe a hand-coded sort that's optimized for tiny lists? The best choice depends on CPU cache architectures, compiler optimizations, runtime data characteristics, etc. It can be very difficult to know, a priori, which algorithm is best. Even if you choose correctly today, the best choice is likely to change as your software evolves over time.

Self-optimizing predicates solve this problem by choosing the best available algorithm at runtime based on live performance measurements. The miser library randomly chooses among available implementations while measuring their runtime characteristics. Once miser is confident it's found the best algorithm, it permanently swaps it into place so there's no ongoing overhead. It does this separately for each predicate call site.

Here's a sketch of a list sorting predicate with miser:

:- module(best_sort, []).
:- use_module(miser).

:- miserly(best_sort/2, [merge_sort,quick_sort,tiny_sort,counting_sort]).

merge_sort(List, Sorted) :-
    % a merge sort implementation goes here

quick_sort(List, Sorted) :-
    % a quick sort implementation goes here


% Implementations can specialize by restricting themselves to certain
% inputs.  Only applicable implementations will be considered.

tiny_sort(List, Sorted) :-
    length(List, Len),
    Len < 4,
    % hand-coded sort for very short lists

counting_sort(List, Sorted) :-
    list_of_integers(List),
    % a counting sort implementation goes here

A program uses the new predicate like this:

:- use_module(best_sort).

short_list(Sorted) :-
    best_sort([3,1,2], Sorted).

long_list(Sorted) :-
    best_sort([45,93,35,23,20,52,12,77,56,25,12], Sorted).

short_list/1 and long_list/1 will use different sort algorithms, depending on runtime performance characteristics.

Documentation

miserly(+NameArity, +Implementations)

A directive to create a macro with NameArity name and arity which chooses the best implementation from among Implementations at run time.

Installation

Using SWI-Prolog 6.3 or later:

$ swipl
1 ?- pack_install(miser).

Future Work

  • Use wall clock time instead of inference count to measure performance
  • Use a statistical model to choose the best runtime implementation
  • Resume search for the best algorithm if a chosen implementation fails after we've already chosen what we thought was the best one
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