Major Description
This is the first task related to TEZ-3997
| Note: There is no API change in this proposed change. The majority of this change will be lifting some existing constraints against CONCURRENT edge type, and addition of a VertexMangerPlugin implementation. |
This includes enabling the CONCURRENT SchedulingType as a valid edge property, by removing all the sanity check against CONCURRENT during DAG construction/execution. A new VertexManagerPlugin (namely VertexManagerWithConcurrentInput) will be implemented for vertex with incoming concurrent edge(s).
In addition, we will assume in this change that
- A vertex cannot have both SEQUENTIAL and CONCURRENT incoming edges
- No shuffle or data movement is handled by Tez framework when two vertices are connected through a CONCURRENT edge. Instead, runtime should be responsible for handling all the data-plane communications (as proposed in [1]).
Note that the above assumptions are common for scenarios such as whole-DAG or sub-graph gang scheduling, but they may be relaxed in later implementation, which may allow mixture of SEQUENTIAL and CONCURRENT edges on the same vertex.
Most of the (meaningful) scheduling decisions today in Tez are made based on the notion of (or an extended version of) source task completion. This will no longer be true in presence of CONCURRENT edge. Instead, events such as source vertex configured, or source task running will become more relevant when making scheduling decision for two vertices connected via a CONCURRENT edge. We therefore introduce a new enum ConcurrentSchedulingType to describe the “scheduling timing” for the downstream vertex in such scenarios.
| public enum ConcurrentSchedulingType { /** * trigger downstream vertex tasks scheduling by "configured" event of upstream vertices */ SOURCE_VERTEX_CONFIGURED, /** * trigger downstream vertex tasks scheduling by "running" event of upstream tasks */ SOURCE_TASK_STARTED } |
Note that in this change, we will only use SOURCE_VERTEX_CONFIGURED as the scheduling type, which suffice for scenarios of whole-DAG or sub-graph gang-scheduling, where we want (all the tasks in) the downstream vertex to be scheduled together with (all the tasks) in the upstream vertex. In this case, we can leverage the existing onVertexStateUpdated() interface of VextexMangerPlugin to collect relevant information to assist the scheduling decision, and there is no additional API change necessary. However, in more subtle case such as the parameter-server example described in Fig. 1, other scheduling type would be more relevant, therefore the placeholder for ConcurrentSchedulingType will be introduced in this change as part of the infrastructure work.
Finally, since we assume that all communications between two vertices connected via CONCURRENT edge are handled by application runtime, a CONCURRENT edge will be assigned a DummyEdgeManager that basically mute all DME/VME handling.
