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OMEGA GNN Cost Modeling Framework

OMEGA (Observing Mapping Efficiency over GNN Accelerator) framework is the cost model for inter-phase Graph Neural Network (GNN) dataflows. OMEGA can be used to model other SpMM and GEMM multiphase dataflows as well.

GNN Dataflow Analysis Using OMEGA Framework

<img src="computations.png">

GNNs are becoming increasingly popular because of their ability to accurately learn representations from graph structured data. GNN inference runtime is dominated by two phases: (1) Aggregation which is an SpMM computation with irregular, workload dependent data accesses, and (2) Combination computations that can be cast as GEMMs, similar to dense DNNs as shown in the figure above. Prior works on DNN dataflow studies have described the data orchestration and data movement in DNN accelerators. However, these works only model dense computations and model one GEMM or convolution operation at a time. GNNs offer an additional knob of pipelining between the two phases which also leads to interdependence of the two dataflows.

Taxonomy for GNN Dataflows

We aim to provide analysis of the design-space of GNN dataflows over flexible accelerator (for example - MAERI) which captures both individual phase dataflows (Intra-phase dataflows) and dataflows between the two phases (Inter-phase dataflows). To enable this, we propose a taxonomy that expresses: (1) Aggregation intra-phase dataflow (2) Combination intra-phase} dataflow (3) Inter-phase strategy, and (4) phase ordering targetting a flexible accelerator like MAERI which can support execution of all possible dataflows. OMEGA is a cost model that models performance and energy for the GNN dataflows.

OMEGA Framework

OMEGA is built around STONNE simulator, STONNE codebase

It instantiates SpMM and GEMM on STONNE's flexible accelerator model MAERI and feeds the statistics to an inter-phase cost model that returns the metrics of a pipelined inter-phase dataflow as shown in Figure below.

<img src="omega.png">

Resources

Publication

For more details, please refer to our IPDPS paper. ArXiv Link - here

Update: The paper is published in IPDPS 2022 and was nominated for the best paper award (Top 5 from 474 submissions).

Bibtex

If you use OMEGA and/or our GNN dataflow taxonomy in your reseach, please cite-

@INPROCEEDINGS{garg2022understanding,  
author={Garg, Raveesh and Qin, Eric and Muñoz-Matrínez, Francisco and Guirado, Robert and Jain, Akshay and Abadal, Sergi and Abellán, José L. and Acacio, Manuel E. and Alarcón, Eduard and Rajamanickam, Sivasankaran and Krishna, Tushar},  
booktitle={2022 IEEE International Parallel and Distributed Processing Symposium (IPDPS)},   
title={Understanding the Design-Space of Sparse/Dense Multiphase GNN dataflows on Spatial Accelerators},   
year={2022},  
volume={},  
number={},  
pages={571-582},  
doi={10.1109/IPDPS53621.2022.00062}}

Tutorials

Docker Image

We have created a docker image for OMEGA for the purpose of ASPLOS tutorial! This is the most stable version. Everything is installed in the image so using the simulator is much easier. Just type the next docker command to download and run the image:

docker run -it stonnesimulator/stonne-simulators
cd OMEGA/omega-code

ASPLOS 2023

OMEGA was presented at ASPLOS 2023 STONNE+OMEGA Tutorial. It was also presented in ASPLOS 2022 tutorial in past.

For OMEGA demo and video presentation of GNN Dataflows, please refer to this video

Codebase - Master Branch

For the most stable and most convenient to install codebase, please refer to the docker image used in the tutorial.

Most stable version of this github repo will always be "master"

Dependencies

Please refer to the C++ and python requirements in the STONNE codebase

Documentation

omega-code (Directory which has the Makefile and the example scripts)
    |-->sample_graphs (Contains example input graphs in csr format)
    |-->stonne (Directory with stonne codebase and modifications for OMEGA)
            |--> src (Contains stonne cpp files and modifications for OMEGA)
                  |-->omega.cpp (Wrapper that calls the stonne instances and has the analytical model)

Please refer to the STONNE simulator for details on simulation of an individual kernel. OMEGA is a wrapper around the STONNE simulator that instanciates SpMM and GEMM simulation, takes the individual kernel statistics and applies analytical equations on these statistics to return the statistics for Inter-phase dataflows.

OMEGA takes the following inputs

<ul> <li>Dimensions <li>Tile sizes for both phases. VF matrix is shared across phases so we use 'a' and 'c' to refer to the phase for which tile size is being specified <li>Hardware Parameters <li>Path to Input files for the adjacency matrix (CSR representation). Refer to sample_graphs directory </ul>

An example command is as follows:

./omega -V=1168 -F=28 -G=2 -E=2590 -T_Va=18 -T_N=1 -T_Fa=28 -T_Vc=18 -T_G=1 -T_Fc=28 -pe_agg=512 -pe_cmb=512 -dn_bw_agg=512 -rn_bw_agg=512 -dn_bw_cmb=512 -rn_bw_cmb=512 -vertex_path="sample_graphs/vertex_mutag_batch64.txt" -edge_path="sample_graphs/edge_mutag_batch64.txt"

For running an example simulation, go to the omega_code directory and first compile.

cd omega-code

make all

source example_simulation.sh