[GitHub] [incubator-mxnet] samskalicky opened a new issue #16108: Subgraph API creates duplicate inputs and outputs

[GitBox <gi...@apache.org>]

samskalicky opened a new issue #16108: Subgraph API creates duplicate inputs and outputs
URL: https://github.com/apache/incubator-mxnet/issues/16108
 
 
   ## Description
   Subgraph API creates duplication inputs for each consumer of an input node to a subgraph, and duplicate outputs for each consumer of a subgraph output. This results in tensor duplication, and causes OOM due to excessive memory usage. 
   
   ## Minimum reproducible example
   Consider the duplicate input problem first. Lets say we have the following graph:
   ```
   A --> B
     \
       --> C
   ```
   In this graph node A has a single output, and nodes B and C consume A's output. If nodes B and C are added to a subgraph, the graph is changed to the following:
   ```
   A --> S
   
   S [ A0 --> B
        A1 --> C]
   ```
   Heres, nodes B and C are moved into the subgraph, and their dependency on node A (which is not in the subgraph) is broken, and an input node is created for each: A0, A1. The subgraph will have 2 inputs (A0, A1) and will both have dependencies to node A. 
   
   Now lets consider the the duplicate output problem. Consider the same graph as above:
   ```
   A --> B
     \
       --> C
   ```
   But this time lets only node A is added to the subgraph, and nodes B and C are left out. Then  the graph is changed to the following:
   ```
   S --> B
     \
       --> C
   
   S [ A]
   ```
   Here, B and C have dependencies on the output from A inside the subgraph. In this case, the subgraph will have 2 outputs, one for each consumer: B and C. But both outputs will be defined as coming from the same output of node A inside the subgraph. 
   

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