Many merge operations can be broken up to work incrementally. For example,
if the merge operation is to sum *n* rank updates, then you can set mergeMsg
= (a, b) = a + b and this function will be applied to all *n* updates in
arbitrary order to yield a final sum. Addition, multiplication, min, max,
and mean are operations that work in this manner (they are associative and
commutative).
If you absolutely must operate on all *n* messages at once, for example to
find the median, then a workaround is to emit Array(m) instead of m in the
sendMsg function, and then to set mergeMsg = (a, b) = a ++ b. This will
accumulate all inbound messages into an array which you can access in vprog.
However, it will be much slower for graphs with high-degree vertices,
because the accumulated arrays can grow very large.
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