On Thu, Jul 30, 2015 at 12:17 PM Mahesh Barve <mahesh.barve@tcs.com> wrote:
Hi Marcin,

 Thanks for the reply.  

1.  Which are the files in the source tree where I can get the details of implementation of queue,graph and the property map for serial as well as distributed (MPI ) cases.

BFS implementation is here:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/breadth_first_search.hpp

You can call it with your own queue:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/breadth_first_search.hpp#L101

or without:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/breadth_first_search.hpp#L116

When you call it without the queue, one is created for you:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/breadth_first_search.hpp#L140

Here is the queue header file:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/queue.hpp

and implementation:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/detail/queue.ipp

Any other implementation you want to see can be found by browsing the linked repository.
 
Also, I had the following query.

With reference to document
http://www.boost.org/doc/libs/1_58_0/libs/graph_parallel/doc/html/overview.html
For parallel version of BFS, the steps that I inferred are as below. Could you please correct me in this:

1. The root node process reads the input file which is in the metix format.
2. The root node process creates the adjacency list representation of the graph.

http://www.boost.org/doc/libs/1_58_0/libs/graph_parallel/doc/html/distributed_adjacency_list.html

Use the documentation above. If you will create the adjacency_list using the edge list constructor, you should call the constructor on every node with an equivalent list of edges. So probably you should read in your graph on every node, or distribute it from the root node. While the documentation says that every node needs an equivalent list of edges, if you look at the implementation you can get away with just the list of edges for the particular node, but you have to be careful about it.
 

3. The root node process creates the property map and "ghost cells" of the graph. The various entries are marked as white corresponding to each of the node.
4. The root node process will distribute the adjacency list and property map to each of the processes.

Everything must happen on all nodes. Here is an example of how to run BFS:

https://github.com/boostorg/graph_parallel/blob/master/example/breadth_first_search.cpp

The same program is run in every MPI process (SPMD). The example will probably answer most of your questions.
 

5. Each of the non-root nodes now has its own copy of adjacency list and property map along with details of the source node where the BFS is to start
6. Now the process having the source node starts the traversal of the graph.
7. The source node is put in a queue.
8. The source node will dequeue the source node and mark all its neighbors as gray and put them in queue. Also it will mark itself as black.
9. The property maps on each of the nodes will now be updated
10. Each of the node processes will be monitoring its own property map
       If any of the processes finds a node marked as gray, it will enqueue that particular node and start its own traversal locally.
11. Thus on every node the property map will be marked as gray and then eventually black after visit is complete. The property maps will be synchronised.
12. The steps from 8 to 11 are repeated till all the entries in every property map are marked as black.

Is my interpretation of the implementation for parallel correct?

Maybe I misunderstood your question. Are you implementing your own BFS or using the Boost one?
 
How often are the property maps on various nodes synchronized and under what condition?

Is there any synchronisation with respect to the queues ?

Yes, check the linked implementation.
 
please help,
regards,
-Mahesh




From:        Marcin Zalewski <marcin.zalewski@gmail.com>
To:        boost-users@lists.boost.org
Date:        07/30/2015 10:03 AM
Subject:        Re: [Boost-users] Details of boost parallel graph implementation
Sent by:        "Boost-users" <boost-users-bounces@lists.boost.org>





On Wed, Jul 29, 2015 at 4:26 AM Mayank Bakshi <mayank.bakshi@tcs.com> wrote:
Hello,

I am working on mpi implementation of boost for breadth first search.
What is the algorithmic detail for BFS in boost.mpi? How the graph is processed by each rank which we provide in mpirun (load distribution among different processes)?

The sequential BGL library is overloaded on distributed graphs:

https://github.com/boostorg/graph/blob/master/include/boost/graph/breadth_first_search.hpp
Look for the mpl::true_ and mpl::bool_ to see where distributed vs. not-distributed selection is made. graph_parallel provides some additional overloads of the helper, but it does not define a brand new BFS:

https://github.com/boostorg/graph_parallel/blob/master/include/boost/graph/distributed/breadth_first_search.hpp
The whole magic happens in the queue, graph, and property maps data structures. The queue distributes work, and it also implements a global "empty" check, and the property maps allow accessing remote vertices. So, if you need MPI BFS, one is already there.  _______________________________________________





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