Broadcasting into a PetscVec runs on the local arrays - #300
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broadcastable now returns the Vec itself, and dest .= f.(args...) checks out the local arrays of dest and of every Vec among the arguments, broadcasts over them and hands them back. At 10 000 entries a MatShell doing y .= 2 .* x goes from 139 000 allocations to 9, and from 1.56 ms to 0.002 ms.
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Base.broadcastablefell back tocollect(oneVecGetValuesper entry) andcopyto!wrote the result back with oneVecSetValuesper entry. At 10 000 entries aMatShellwith that body took 139 000 allocations and 1.56 ms; now 9 and 0.002 ms.broadcastablereturns the Vec itself, anddest .= f.(args...)checks out the local arrays ofdestand of everyPetscVecargument, broadcasts over them, and hands them back.x .= x .+ yreuses the array already checked out. On several ranks the broadcast covers the entries each rank owns, so a Julia array in the expression must have the local length.Tested serially (results, aliasing, a Julia array argument,
DimensionMismatch, aMatShellbody, and a fixed allocation count at two sizes) and on 4 ranks. Out-of-placew = 2 .* xis unchanged; it needs its own broadcast style and left for later.