Improve goal reorder heuristics. #3208
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This change primarily does two things: 1. For `--reorder-goals`, we use a dedicated datatype `Degree` rather than an `Int` to compute the approximate branching degree. We map 0 and 1 to the same value. We then use a lazy ordering and a shortcutting minimum function to look for the "best" goal. The motivation here is that we do not want to spend unnecessary work. Following any goal that has 0 or 1 as degree cannot really be "wrong", so we should not look at any others and waste time. This will still not always make the use of `--reorder-goals` better than not using it, but it will reduce the overhead introduced by it. 2. We use partitioning rather than sorting for most of the other goal reordering heuristics that are active in all situations. I think this is slightly more straightforward and also slightly more efficient, whether `--reorder-goals` is used or not. I have run some preliminary performance comparisons and they seem to confirm that in both cases separately (with or without `--reorder-goals`), these changes are a relative improvement over the status quo. I will run additional tests before merging this into master.
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/cc @hvr |
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I've ran a few random tests for cases which stress the solver a bit (solving a full stackage-set, trying to find (when there isn't any) install plans for packages for GHC8, etc..), and they all show a significant improvement of |
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@23Skidoo After running some tests, it still looks like this is a clear improvement, and I haven't found any result deviations either. Are you ok if I merge this? I have no idea what your policies for 1.24 are, but I wouldn't mind having it go in that release as well. |
kosmikus
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Mar 4, 2016
Improve goal reorder heuristics.
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Cherry-picked into the 1.24 branch. |
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This change primarily does two things:
For
--reorder-goals, we use a dedicated datatypeDegreerather than an
Intto compute the approximate branchingdegree. We map 0 and 1 to the same value. We then use a
lazy ordering and a shortcutting minimum function to look
for the "best" goal.
The motivation here is that we do not want to spend
unnecessary work. Following any goal that has 0 or 1 as degree
cannot really be "wrong", so we should not look at any others
and waste time.
This will still not always make the use of
--reorder-goalsbetter than not using it, but it will reduce the overhead
introduced by it.
We use partitioning rather than sorting for most of the other
goal reordering heuristics that are active in all situations.
I think this is slightly more straightforward and also slightly
more efficient, whether
--reorder-goalsis used or not.I have run some preliminary performance comparisons and they
seem to confirm that in both cases separately (with or without
--reorder-goals), these changes are a relative improvementover the status quo. I will run additional tests before
merging this into master.