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Fix illegal acces mean/stdev, sum add Kahan Summation #2223
Fix illegal acces mean/stdev, sum add Kahan Summation #2223
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As discussed offline, we are still loosing accuracy here, because we cannot do atomic compensated summation. In a follow up PR, we should strive to improve this. A few notes:
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I believe you would need to make use of extra smem here
smu[ColsPerBlk * RowsPerBlkPerIter ]
then store the each outputs something likesmu[ thisColId * RowsPerBlkPerIter + thisRowId ] = thread_sum
, followed by per-thread working on summing upRowsPerBlkPerIter
from a single warp0 with kahan algo ifRowsPerBlkPerIter
is small and for largerRowsPerBlkPerIter
like 32 you can use shfl based reduction with kahan algo applied on each of its 5 iteration.There was a problem hiding this comment.
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Yes, within the block we can use a second shared memory atomicAdd to store the compensation. With the current blockdim we only have 4 threads adding their intermediate values. I tried that but decided to skip for now until addition across blocks is not compensated afterwards.
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This is not compensated right?
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The BlockReduce is not, which is why the compensation is added to the value beforehand.