Fix in global_scan and global_scan_inplace. #24
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Updating the elements on the first processor after exscan may lead to erroneous results.
asrivast28
commented
Mar 2, 2020
| T sum = T(); | ||
| if (n > 0) | ||
| sum = *(out+(n-1)); | ||
| T sum = *(out+(n-1)); |
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n is always greater than 0 because of the check on line 739.
asrivast28
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Mar 2, 2020
| if (n > 0) | ||
| sum = *(out+(n-1)); | ||
| T sum = *(out+(n-1)); | ||
| T presum = exscan(sum, func, nonzero_comm, commutative); |
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exscan returns T() on process 0.
asrivast28
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Mar 2, 2020
| T presum = exscan(sum, func, nonzero_comm, commutative); | ||
| // accumulate previous sum on all local elements | ||
| for (size_t i = 0; i < n; ++i) { | ||
| *o = func(presum, *o); |
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If func is min, and T is a numeric type, T() will replace all the correct results on process 0 if all the elements of the array are greater than 0.
patflick
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Mar 3, 2020
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👍 great catch. How did I miss this?? 😮
Thanks for the fix.
patflick
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Mar 3, 2020
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optimally, can you add a failing test case in test/test_reductions.cpp (ie, failing before the fix)?
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Problem: Currently, in the
mxx::global_scan*functions, the local scan results are updated on every process with the prefix sum from the previous process. However, the prefix sum on process 0 is just the zero-initialized value corresponding to the type. This may lead to erroneous results on process 0 in some cases, e.g. while scanning usingminon an array if the minimum element in the array is greater than zero.Solution: The result of
local_scanon process 0 should not be updated with thepresum.