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Both data and model parallelism suffer from system under-utilization. Wavelet exploits the under-utilized memory & compute by scaling up the number of training tasks and launching the additional tasks with a delay to fully utilize the on-chip memory and improve the compute utilization, speeding up individual jobs.
That was an unnecessarily long sentence... GRE took its toll on me!
Introduction
Background and Motivation
Distributed DNN Training Schemes
Jobs Characteristics of Distributed DNN Training
Zoom-in analysis on data parallel training
Sub-iteration analysis on model parallel training
Wavelet Design
System Overview
Wavelet in Data Parallelism
Memory overlapping
Computation overlapping
Model synchronization between waves
Wavelet in Model Parallelism
Launching multiple tock-wave tasks
Model partition switching
Inter-batch synchronization
Evaluation
Data parallelism
Single machine multi-GPU
Multi-machine multi-GPU
Model parallelism
Single machine multi-GPU
Multi-machine multi-GPU
Overhead analysis
Related Work
Resource allocation for distributed DNN training
GPU sharing
Conclusion
Bigger models & datasets call for large-scale distributed machine learning training. The current scheduling policy, gang scheduling, where all training tasks on all workers need to be launched at the same time, contributes to the under-utilization of system resources (compute & memory).
In Fig. 1 (see above), computation is memory-bounded during the forward propagation. Between time 0.4 and 0.6, memory is underutilized in the backward propagation. Moreover, ~60% of on-chip compute cores are underutilized.
In vanilla allreduce, there are only Main & Tick waves. With this Tock wave added, Wavelet doubles the number of model synchronizations. This is the same as synchronizing over 2*N data parallel tasks on 2*N GPUs, thus guaranteeing convergence.
In gang scheduling, the memory of all GPUs is underutilized during backprop. Tick-tock scheduling injects tock-wave tasks right after the tick-wave tasks finish the forward pass. To concurrently run 2 tasks (tick & tock), 2 model replicas are maintained on the GPU since the two waves train on different data. In the memory, the size of the model is way smaller than the size of the intermediate results, so no need to worry about the extra memory.
CUDA computation kernels are launched in separate CUDA streams to ensure ordered execution within a stream and non-blocking across different streams. The empty bubbles between kernels is due to the latency of CPUs sending instructions to GPUs.
In the vanilla pipelined process (white blocks), only 1 batch is active in the system and at most 1 GPU is active at a time. Each GPU also holds the same model partition during the whole training process.
In Wavelet, we inject 3 (N-1 w/ N GPUs) tock waves on 1 tick wave. The model partition is swapped on each GPU using a round-robin fashion. There exists an extra model synchronization for each model partition, and the context switching also brings overhead.
Single machine: Up to 1.88x speedup (avg: 1.4x, theoretically 2x) over DP baseline
Multiple machine: Up to 1.18x speedup over baseline. The worse throughput than baseline is the overhead kicking in: The cross-machine low-bandwidth network becomes the bottleneck during the extra allreduce
Only ~2.5x speedup in 4x/8x parallelism
Gpipe/PipeDream breaks a mini-batch into smaller micro-batches -> High-frequency but small-size data chunks
Number of CUDA kernel calls ↑
Intermediate result transfer between GPUs that hold different model partitions ↑
Linear scalability in theory
Context switch: Switching model partition, ~4% of total training time
Communication: Transferring intermediate results across GPUs (~15% of total training time)
All reduce: Model synchronization during backprop (~4% of total training time)
There are existing job multiplexing schemes that boost system utilization. lets 2 low-utilization jobs space-share a GPU. provides fine-grained memory sharing via the GPU Lane abstraction. However, neither scheme contributes to the training progress of a single job. In this work, Wavelet relaxes the gang scheduling scheme and accelerates a single job while improving the system utilization.
