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From Sergey Edunov <edu...@gmail.com>
Subject Re: Questions after writing some giraph code
Date Mon, 06 Jul 2015 19:40:58 GMT
Hi Jan,

It's a bit hard to advise without seeing actual code, so my reply might
seem too generic. Feel free to send specific questions with code samples to
get more detailed advice.

"Is there a way to do this without duplicating a lot of code of the
“standalone” MasterCompute class of algorithm" - this sort of thing is
usually done by abstracting your algorithm out of giraph-related classes.
You can have a class with static methods that implement your algorithm and
then all you need to do is to pass data from vertices or master compute
into this class. This approach has other benefits such as easy testability.
E.g. you can write unit tests for algorithm.

"I need to register n aggregators (n = number of vertices)" - this is
generally a bad sign. How much data you want to store in aggregators?
Remeber, they will be send other the wire between workers and master. You
can, of course, get around by registering single Map as aggregator. You'll
need to wrap it into another class that implements Writable and then
implement readFields and write functions.

"My biggest problem right now is debugging" - unit tests are usually very
good approach. Again, depends a lot on your configuration. We also use
downsampled graphs a lot to quickly test on cluster.

" After reading through another giraph algorithm, I noticed that they do
the same" - can you point to the example? I suspect that's because they use
mutable data types or some helper functions to change value.

Regards,
Sergey Edunov


On Mon, Jul 6, 2015 at 8:18 AM, Jan Ebbing <Jan.Ebbing@gmx.de> wrote:

> Hello everybody,
>
>
>
> first of all thank you for all your work on giraph.
>
> I’m a student writing his bachelor thesis using giraph.
>
> I have already implemented an algorithm that isn’t completely trivial,
> with my new task, however, I’m running into problems.
>
>
>
> I implemented my first algorithm as a subclass of MasterCompute where I
> register aggregators etc and then do a switch on the superstep and call
> setComputation() with the appropriate AbstractComputation subclass I wrote.
> (I hope this is how you’re supposed to do it)
>
> Now I want to implement a new algorithm that calls my new algorithm as a
> subroutine: While the halting condition is not fulfilled, I first partition
> the graph using algorithm 1, then continue to work on it with algorithm 1.
> Is there a way to do this without duplicating a lot of code of the
> “standalone” MasterCompute class of algorithm 1? An additional problem that
> arises with this is that the Vertex- and EdgeValue classes would have to be
> exactly the same. I tried to work around this by defining a writable
> interface that defines all the methods needed for my algorithms which would
> make the actual classes exchangeable. However, giraph uses reflection to
> create a new instance of exactly the type the computation uses (an
> interface), which leads to an error. Is there any good way to do this or
> are giraph jobs not that flexible?
>
>
>
> In a variation of the first algorithm, I need to register n aggregators (n
> = number of vertices). However, the documentation reads like I have to
> register aggregators in MasterCompute.initialize() (nowhere else) and at
> that point, getTotalNumVertices() does not work yet. I am aware that this
> is a very costly operation (I do not use all of those aggregators but I do
> not know beforehand which of them I will use and which I will not use), but
> currently the only workaround I can think of is using
> Configuration.getInt(). (and writing the total number of vertices in such a
> config file beforehand)
>
> Also a question on the config files: I understood that there are several
> of them that overwrite each other and that e.g. I should not change the
> core-default.xml or core-site.xml since they will change my complete Hadoop
> installation. I also read somewhere that it’s possible to write a config
> file just for one job (which would be what I need) but I never found out
> how that file should be called/where I have to place it.
>
>
>
> My biggest problem right now is debugging, though: Is there an easy way to
> test giraph code on small sample graphs? Right now, to test my
> implementations I have to package my code with maven, copy the long command
> into the terminal to run the giraph job (changing the output folder since
> they have to be different each time), wait a few minutes for the job to
> complete, open the web GUI, click through a few pages there until I see my
> debug statements/if the job completed I have to run through a text file via
> the console. Compared to what I was used to (1 click in eclipse and almost
> instantly seeing the output on the console) this is very annoying,
> especially since I do dumb small mistakes like switching the if and else
> blocks more often than I’d like to and have to go through the whole process
> each time that happens.
>
> I also searched for that, I found GRAFT which seemed to be a useful
> debugging tool, but more for suitable for testing on real input and not to
> quickly test if the code runs at all on a small input graph.
>
> After searching through this mailing list archive, there were a few
> references to running a giraph job with one click in eclipse aswell (see
> also [1]), but most descriptions were very vague and I could not reproduce
> them.
>
>
>
> Lastly, one small question: In my first algorithm I had a small bug where
> I would use getVertexValue, then change the java object but not call
> setVertexValue which resulted in my changes not being saved leading to
> undesired behavior. After reading through another giraph algorithm, I
> noticed that they do the same (maybe it was with an EdgeValue, I’m not 100%
> sure on that) and don’t call setXValue, but apparently their code works.
> Can anybody shed some light on that? (I understand why it’s useful to have
> an explicit setVertexValue method for writing/reading vertices to/from
> disk, I just don’t understand why it is not necessary for them?)
>
>
>
> Thanks,
>
> Jan
>
>
>
>
>
> [1]
> http://ben-tech.blogspot.in/2011/08/how-to-debug-hadoop-mapreduce-jobs-in.html
>

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