Hi Avery, Many thanks for the point-by-point replies. It clarifies a lot of questions I had.
The Pregel papers did throw more light on the approach and architecture. Hi Eli, Your feedback about very large scale applications on Giraph sounds very encouraging. Thanks very much. After reading both of your replies, I have some (final!) questions regarding memory usage: . For applications with a large number of edges per vextex: Are there any built-in vertex or helper classes or at least sample code which feature spilling of edges to disk, or some kind of disk-backed map of edges, to support such vertices? Or do we have to sort of roll our own? . For graphs with a large number of vertices relative to available workers, at least in development phase, one may not always have access to a large number of workers, yet one might wish to process a very large graph. In these cases, it may happen that the workers may not be able to hold all their assigned vertices in memory. So again in this case, are there any built-in classes to allow spilling of vertices to disk, or a similar kind of disk-backed map? . Assuming some kind of disk backing is implemented to handle large number of vertices/edges (under a situation of insufficient # of workers or memory per worker), is it likely that just the volume of IO (message/IPC) could cause OOMEs? Or merely slowdowns? In general, I feel that one of the reasons for wide and rapid adoption of Hadoop is the "download, install and run" feature, where even for large data sets, the stock code will still run to completion on a single laptop (or a single Linux server, etc), except that it will take more time. But this may be perfectly acceptable for people who are evaluating and experimenting, since there is no incurred cost for hardware. A lot of developers might be OK with giving the thing a run overnight on their laptops or fire up just one spot instance on EC2 etc and let it chug along for a couple of days. I know this was the case for me when I was starting out with Hadoop. So more nodes are needed only to speed things up, but not for functionality. It might be great to include such features into Giraph also.. which will require that disk backed workers be supported in the code as standard feature. Would love to hear your thoughts on these. Thanks, Jeyendran From: Eli Reisman [mailto:apache.mail...@gmail.com] Sent: Tuesday, September 11, 2012 12:11 PM To: user@giraph.apache.org Subject: Re: Can Giraph handle graphs with very large number of edges per vertex? Hi Jeyendran, I was just sayiing the same thing about the documentation on another thread, couldn't agree more. There will be progress on this soon, I promise. I'd like us to reach a model of "if you add a new feature or change a core feature, the patch gets committed contingent on a new wiki page of docs going up on the website." There's still nothing about our new Vertex API, master compute, etc. on the wiki. I would say 8 gigs to play with is a great amount where you will most definitely be able to get very large interesting graphs to run in-memory, depending on how many workers (with 8G each) you have to work with. having 3-4 workers per machine is not a bad thing if you are provisioned to do this. And lots of machines. This is a distributed batch processing framework, so more is better ;) as far as vertices with a million edges, sure but it depends on how many of them and your compute resources. Again, can't go into much detail but Giraph has been extensively tested using real-world, large, interesting, useful graph data. This includes large social graphs that have supernodes. So if you're supplying that, and you have the gear to run your data, you've picked the right tool. You can spill to disk, run in memory, or spread the load and scale to many, many workers (Mapper tasks) hosted on many nodes and Giraph will behave well if you have the compute resource to scale to fit your volume of data. On Tue, Sep 11, 2012 at 12:27 AM, Avery Ching <ach...@apache.org> wrote: Hi Jeyendran, nice to meet you. Answers inline. On 9/10/12 11:23 PM, Jeyendran Balakrishnan wrote: I am trying to understand what kind of data Giraph holds in memory per worker. My questions in descending order of importance: 1. Does Giraph hold in memory exactly one vertex of data at a time, or does it need to hold all the vertexes assigned to that worker? All vertices assigned to that worker. 2. Can Giraph handle vertexes with, a million edges per vertex? Depends on how much memory you have. Would recommend making a custom vertex implementation that has a very efficient store for better scalability (i.e. see IntIntNullIntVertex). If not, at what order of magnitude does it break down? - 1000 edges, 10K edges, 100K edges?... (Of course, I understand that this depends upon the -Xmx value, so let's say we fix a value of -Xmx8g). 3. Are there any limitations on the kind of objects that can be used as vertices? Specifically, does Giraph assume that vertices are lightweight (eg, integer vertex ID + simple Java primitive vertex values + collection of out-edges), or can Giraph support heavyweight vertices (hold complex nested Java objects in a vertex)? Limitations are that the vertex implementation must be Writable, the vertex index must be WritableComparable, edge type Writable, message type Writable. 4. More generally, what data is stored in memory, and what, if any, is offloaded/spilled to disk? Messages and vertices can be spilled to disk, but you must enable this. Would appreciate any light the experts can throw on this. On this note, I would like to mention that the presentations posted on the Wiki explain what Giraph can do, and how to use it from a coding perspective, but there are no explanations of the design approach used, the rationale behind the choices, and the software architecture. I feel that new users can really benefit from a design and architecture document, along the lines of Hadoop and Lucene. For folks who are considering whether or not to use Giraph, this can be a big help. The only alternative today is to read the source code, the burden of which might in itself be reason for folks not to consider using Giraph. My 2c :-) Agreed that documentation is lacking =). That being said, the presentations explain most of the design approach and reasons. I would refer to the Pregel paper for a more detailed look or ask if you have any specific questions. Thanks a lot, No problem! Jeyendran
