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Subject: Re: [boost] [BGL] Trying to get a correclty working Parallel BFS code
From: Sandeep Gupta (gupta.sandeep_at_[hidden])
Date: 2009-11-10 18:53:21

On Tue, Nov 10, 2009 at 2:51 PM, Nick Edmonds <ngedmond_at_[hidden]>wrote:

> --snip-- (sorry, first reply was too big)
>>> Hi Nick,
>>> Thanks for the explanation of the distributed BFS. This is indeed quite
>> helpful. Especially at later stages. I haven't gotten to a point to
>> comment
>> on the data model but the current architecture is swell from what I can
>> judge.
>> I hesitate to ask explanation at every minor detail but I now confused
>> about passing the same vertex descriptor. Specifically I didn't quite
>> grasp
>> the statement "source vertex just has to be
>> graph_traits<Graph>::vertex_descriptor, which you can construct using
>> vertex() call if you want to generate it from a global vertex index".
>> The source vertex has to graph_traits<Graph>::vertex_descriptor which I
>> takes two arguments the index and the graph --these two parameters can
>> only
>> identify local vertices. For global vertices we need global descriptors
>> which also require the owner.
>> Appreciate your help in clarifying this.
>> Thanks
>> Sandeep
> The vertex() call takes a global index and constructs a vertex_descriptor,
> which is basically a pair containing the local index and the owning process.
> vertex(i, g) will construct the i'th vertex descriptor in g on any process,
> regardless of where the vertex is actually stored. The confusion is that
> there are two indices here, global indices, and local indices. The graph
> distribution object maps between the two. vertex() takes a global index,
> specifically for this use case.
> Hope that helps,

> Hi Nick,
 Sorry, I didn't mean to imply that your first reply was too big. Your
explanation of how the PBFS is actually working is indeed quite helpful.
Just that rIght now I just too focussed on get the code working. Ofcourse,
when I dig deeper I would certainly have to refer to your exposition.

Right now in the code I am passing vertex(0,g) to all processes.
This means, based upon the explanation, that we are performing BFS on all
processes from the node with global identifier as 0. But still the code is
not outputting right result. There is something more to it or I again
misunderstood your explanation.

Thanks for your patience.


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