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From Benjamin Heitmann <benjamin.heitm...@deri.org>
Subject Resources or advice on minimising memory usage in Giraph/Hadoop code ?
Date Wed, 06 Jun 2012 02:38:57 GMT
Hello, 

can somebody recommend a web page, article or book on minimising the memory usage of Giraph/Hadoop
code ? 
I am looking for non-obvious advice on what *not* to do, and for best practices on what to
do inside of Hadoop...

E.g. is it preferable to use Java Strings or Hadoop Text Writables ? Should all strings be
externalised ? 

Currently, I am running a Giraph job with 10 workers. Each worker has a maximum heap of Xmx7G.

The concurrent garbage collection is enabled. The machine has 24 cores, and 96 GB of memory.

The job currently uses a max of around 50 GB, so there is free memory available outside of
java.

The graph itself has ~2 million vertices and ~4 million edges, which is not really "big data".

However, before starting superstep 1, I get heap space errors. Previous versions of my algorithm
where simpler, 
but they also ran into heap space errors when the data was around one order of magnitude bigger.


My suspicion is that the amount of state which my vertices have, and the amount of messages
which I am generating 
exceeds the standard use case of a pagerank rank algorithm by far. 

To list a few of the reasons why I need a lot of state: 

* I need to execute multiple runs of the same algorithm in parallel. Loading this specific
graph takes about 3 minutes, 
running the algorithm once takes about 10 seconds or so, but I have around 600 users in that
graph. And this is just a small graph, 
the whole algorithm is intended to be run for thousands of users. (... "big data"...) 

* The identities of the edges and vertices are not based on numbers but on strings. 
All edges and all vertices have a URI associated with them. 
The graph represents RDF data from different sources, such as DBpedia. 
In addition, most of the vertices have one or multiple types associated with them, and 
each type is again represented by a URI. 
These types are essential to the logic of the algorithm. 
I guess it would be possible to externalise all of those strings, but it adds a layer of complexity
which I had previously hoped to avoid. 

* As Giraph does not currently provide a central coordination point for the processing of
the graph, 
I need to send a lot of messages between vertices in order to coordinate the algorithm.

* Giraph does not allow multiple Java classes to be used for different vertices in the same
graph. 
However, different vertices have different roles in my algorithm, and each role has a different
set of states in which it can be, 
due to the missing global coordination point. 

* Taken together, the lack of a central coordination point and the inabiltity to have different
java classes as part of the same graph, 
make the whole algorithm more similar to a network protocol and not to a graph algorithm.
Thus I need a lot of messages
and a lot of state. 


If anybody has some good suggestion on how I should proceed, I would be very interested in
hearing them. 

If somebody wants to take a look at my code, then I can currently provide you with that code
in a non-public way.

sincerely, Benjamin Heitmann. 


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