It's interesting that it is possible. I find the quest for bit-reproducibility to be somewhat misguided; it's probably useful for debugging parallel codes but I think that if it is something that is being required by scientific users then they are probably thinking about their problem the wrong way. To me it is similar to worrying about dynamic typing... --cjc ======================== Hi Colin I just thought you might be interested - it appears to show that we should be able to get bit-accurate summations in parallel, at low cost. So, interpreting optimistically (possibly prematurely), it means that it might be possible to make PyOP2 fully deterministic by default. In contrast, at present in PyOP2 the precise association of floating-point adds may vary due to thread to thread races, or when the mesh is recoloured or repartitioned. I think the paper's claim applies to global reductions; I'm not sure it scales nicely to addtos, though there might be other solutions for that. Paul