Hi Sebastian,
yes, that was my point, I agree completely with you.
Fixing my test was not the issue, my question was whether we want to
define explicitly the semantics of this scenario.
Personally, I believe the combiner should be ready to receive 0
messages, as it's the case of BasicVertex::initialize(), putMessages()
and compute(), and act accordingly.
In the particular example, I believe the SimpleSumCombiner is bugged.
It's true that the sum of no values is 0, but it's also true that the
null return semantics of combine() is more suitable for this exact
situation.
On Mon, Jan 9, 2012 at 2:21 PM, Sebastian Schelter <s...@apache.org> wrote:
> I think we currently implicitly assume that there is at least one
> element in the Iterable passed to the combiner. The messaging code only
> invokes the combiner only if at least one message for the target vertex
> has been sent.
>
> However, we should not rely on implicit implementation details but
> explicitly specify the semantics of combiners.
>
> --sebastian
>
> On 09.01.2012 13:29, Claudio Martella wrote:
>> Hello list,
>>
>> for GIRAPH-45 I'm touching the incoming messages and hit an
>> interesting problem with the combiner semantics.
>> currently, my code fails testBspCombiner for the following reason:
>>
>> SimpleSumCombiner::compute() returns a value even if there are no
>> messages in the iterator (in this case it returns 0) and for this
>> reason the vertices get activated at each superstep.
>>
>> At each superstep, under-the-hood, I pass the combiner for each vertex
>> an Iterable, which can be empty:
>>
>> public Iterable<M> getMessages(I vertexId) {
>> Iterable<M> messages = inMessages.getMessages(vertexId);
>> if (combiner != null) {
>> M combinedMsg;
>> try {
>> combinedMsg = combiner.combine(vertexId, messages);
>> } catch (IOException e) {
>> throw new RuntimeException("could not combine", e);
>> }
>> if (combinedMsg != null) {
>> List<M> tmp = new ArrayList<M>(1);
>> tmp.add(combinedMsg);
>> messages = tmp;
>> } else {
>> messages = new ArrayList<M>(0);
>> }
>> }
>> return messages;
>> }
>>
>> the Iterable returned by this methods is passed to
>> basicVertex.putMessages() right before the compute().
>> Now, the question is: who's wrong? The combiner code that returns a
>> sum of 0 over no values, or the framework that calls the combiner with
>> 0 messages?
>>
>>
>>
>
--
Claudio Martella
claudio.marte...@gmail.com
