I think it would be pretty straightforward to add an option to not
re-activate a halted vertex on new messages. Would that satisfy your
issue Nick?
Avery
On 8/4/12 2:42 AM, Sebastian Schelter wrote:
Hi Nick,
I think you are running into the issue that vertex
activation/scheduling and data transfer are not separated in the
Pregel/Giraph model. I ran into the same problem when trying to
implement Adaptive PageRank on Giraph.
I think the problem comes from the vast generality of Pregel's BSP
model: vertices can send any kind of message to any other vertex.
However, in a lot of algorithms, vertices only send their state to
their neighbors. Other platforms such as GraphLab separate
activation/scheduling from messaging, in this system a vertex can only
schedule its neighbors and once one of the neighbors is invoked, it
can read the state of its direct neighbors.
I think that the Pregel model is simply not suited for such algorithms
(unfortunately), let me know if you find a clever workaround.
Best,
Sebastian
2012/8/3 Nick West <[email protected]>
Thanks for the reply.
Is there an easy modification that I can make to remove condition 2? Can you
point me to the code that addresses this?
The problem I am facing is the following: At every iteration a non-halted
vertex needs messages from all of its neighbors. When deciding to send
messages, a given vertex doesn't know if its neighbors will vote-to-halt in the
current superstep, thus it must send a message to each of its neighbors. In
the case that all vertices have voted to stop, the sending of a messages by any
vertex will cause the algorithm to continue, yet in this situation it is
desirable to terminate.
I have worked out a few solutions that involve either increasing the amount of
data a vertex saves each iteration or augmenting the messages sent with
additional information, but I think it would be beneficial, and more general,
to allow this type of termination instead.
Do you have any thoughts on this?
Thanks,
Nick
On Aug 3, 2012, at 10:14 AM, Alessandro Presta wrote:
Hi Nick,
You are pretty much correct, except that not all vertices need to vote to halt
at the same time: some vertices might have voted to halt at a previous
superstep and never received any messages after then, in which case they are
never reactivated.
In other words, I think you can rephrase that as:
All vertices are halted after a given superstep
No messages were sent in that superstep
Hope it helps.
Alessandro
From: Nick West <[email protected]>
Reply-To: "[email protected]" <[email protected]>
Date: Friday, August 3, 2012 2:48 PM
To: "[email protected]" <[email protected]>
Subject: Termination Conditions
Excuse me if this is stated somewhere obvious, but I haven't been unable to
find it. What are the exact termination criteria for the global algorithm?
Reading the documentation on voteToHalt, looking at the Shortest Path Example
code, and looking at the results of my own application, these two conditions
must both hold for the global BSP algorithm to terminate:
1) All vertices vote to halt in a given superstep
2) No messages are sent in that supersetp
Is that correct?
Thanks,
Nick West
Benchmark Solutions
101 Park Avenue - 7th Floor
New York, NY 10178
Tel +1.212.220.4739 | Mobile +1.646.267.4324
www.benchmarksolutions.com
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Nick West
Benchmark Solutions
101 Park Avenue - 7th Floor
New York, NY 10178
Tel +1.212.220.4739 | Mobile +1.646.267.4324
www.benchmarksolutions.com
