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From Fabian Hueske <fhue...@gmail.com>
Subject Re: [gelly] Spargel model rework
Date Tue, 27 Oct 2015 17:39:11 GMT
I'll try to have a look at the proposal from a performance point of view in
the next days.
Please ping me, if I don't follow up this thread.

Cheers, Fabian

2015-10-27 18:28 GMT+01:00 Martin Junghanns <m.junghanns@mailbox.org>:

> Hi,
> At our group, we also moved several algorithms from Giraph to Gelly and
> ran into some confusing issues (first in understanding, second during
> implementation) caused by the conceptional differences you described.
> If there are no concrete advantages (performance mainly) in the Spargel
> implementation, we would be very happy to see the Gelly API be aligned to
> Pregel-like systems.
> Your SSSP example speaks for itself. Straightforward, if the reader is
> familiar with Pregel/Giraph/...
> Best,
> Martin
> On 27.10.2015 17:40, Vasiliki Kalavri wrote:
>> Hello squirrels,
>> I want to discuss with you a few concerns I have about our current
>> vertex-centric model implementation, Spargel, now fully subsumed by Gelly.
>> Spargel is our implementation of Pregel [1], but it violates some
>> fundamental properties of the model, as described in the paper and as
>> implemented in e.g. Giraph, GPS, Hama. I often find myself confused both
>> when trying to explain it to current Giraph users and when porting my
>> Giraph algorithms to it.
>> More specifically:
>> - in the Pregel model, messages produced in superstep n, are received in
>> superstep n+1. In Spargel, they are produced and consumed in the same
>> iteration.
>> - in Pregel, vertices are active during a superstep, if they have received
>> a message in the previous superstep. In Spargel, a vertex is active during
>> a superstep if it has changed its value.
>> These two differences require a lot of rethinking when porting
>> applications
>> and can easily cause bugs.
>> The most important problem however is that we require the user to split
>> the
>> computation in 2 phases (2 UDFs):
>> - messaging: has access to the vertex state and can produce messages
>> - update: has access to incoming messages and can update the vertex value
>> Pregel/Giraph only expose one UDF to the user:
>> - compute: has access to both the vertex state and the incoming messages,
>> can produce messages and update the vertex value.
>> This might not seem like a big deal, but except from forcing the user to
>> split their program logic into 2 phases, Spargel also makes some common
>> computation patterns non-intuitive or impossible to write. A very simple
>> example is propagating a message based on its value or sender ID. To do
>> this with Spargel, one has to store all the incoming messages in the
>> vertex
>> value (might be of different type btw) during the messaging phase, so that
>> they can be accessed during the update phase.
>> So, my first question is, when implementing Spargel, were other
>> alternatives considered and maybe rejected in favor of performance or
>> because of some other reason? If someone knows, I would love to hear about
>> them!
>> Second, I wrote a prototype implementation [2] that only exposes one UDF,
>> compute(), by keeping the vertex state in the solution set and the
>> messages
>> in the workset. This way all previously mentioned limitations go away and
>> the API (see "SSSPComputeFunction" in the example [3]) looks a lot more
>> like Giraph (see [4]).
>> I have not run any experiments yet and the prototype has some ugly hacks,
>> but if you think any of this makes sense, then I'd be willing to follow up
>> and try to optimize it. If we see that it performs well, we can consider
>> either replacing Spargel or adding it as an alternative.
>> Thanks for reading this long e-mail and looking forward to your input!
>> Cheers,
>> -Vasia.
>> [1]: https://kowshik.github.io/JPregel/pregel_paper.pdf
>> [2]:
>> https://github.com/vasia/flink/tree/spargel-2/flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/spargelnew
>> [3]:
>> https://github.com/vasia/flink/blob/spargel-2/flink-libraries/flink-gelly/src/main/java/org/apache/flink/graph/spargelnew/example/SSSPCompute.java
>> [4]:
>> https://github.com/grafos-ml/okapi/blob/master/src/main/java/ml/grafos/okapi/graphs/SingleSourceShortestPaths.java

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