[GIRAPH-837] GiraphStats in MasterCompute has wrong values

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 1.1.0
• Component/s: bsp
• Assignee: Unassigned
• Reporter: Maxymilian Śmiech
• Created: 02/Feb/14 15:46
• Updated: 02/Feb/14 15:46

Description

I am trying to implement new feature (small class assignment) – voting to change computation "phase" – using MasterCompute.

Basically when every Vertex "votes to advance to next phase" I need to check conditions like in:

BspServiceMaster.coordinateSuperstep()

if (masterCompute.isHalted() ||
        (globalStats.getFinishedVertexCount() ==
        globalStats.getVertexCount() &&
        globalStats.getMessageCount() == 0)) {
      globalStats.setHaltComputation(true);
    }

In my solution I check if all vertices have "voted" and there are no messages to deliver. If that is true I broadcast next phase number through an aggregator.

Now I have problems obtaining GlobalStats.getMessageCount(). I see that coordinateSuperstep() is performing those actions (this is example with superstep 15 and 16):

1. Call AbstractComputation.compute() on every Vertex with superstep 15
2. Call MasterCompute.compute() with superstep 16
3. Aggregate GlobalStats from superstep 15
4. Check halting conditions after superstep 15
5. Copy GlobalStats to GiraphStats
6. Go to step 1

I can access GiraphStats.getInstance().getSentMessages() in MasterCompute.compute()

I wrote small test with two nodes and two edges:

• superstep 0:
  • MasterCompute does nothing
  • each vertex sends one message to all its neighbours
• superstep 1:
  • MasterCompute reads GiraphStats.getInstance().getSentMessages().getValue() and broadcasts it through an IntOverwriteAggregator
  • each vertex sets its value to that aggregator's current value and halts

After that, every vertex has value of 0. I believe this is wrong. If I perform actions from superstep 1 in superstep 2, I got correct vertex value of 2.

Explanation is simple: MasterCompute.compute() called in superstep has GiraphStats from superstep-2. That is because it is called too early – before GlobalStats from superstep-1 are gathered and put in GiraphStats.

My proposed solution is:

BspServiceMaster.coordinateSuperstep()

GlobalStats globalStats = aggregateWorkerStats(getSuperstep());
/* ... */
updateCounters(globalStats);
/* ... */
doMasterCompute();
/* ... */
if (masterCompute.isHalted() ||
        (globalStats.getFinishedVertexCount() ==
        globalStats.getVertexCount() &&
        globalStats.getMessageCount() == 0)) {
      globalStats.setHaltComputation(true);
    }
/* ... */

I am new to Giraph. Maybe this beahvior is intended. Maybe it is not trivial to change. But for me it is not working as expected.