1. Introduction
With the everincreasing sizes of datasets and larger deep neural networks, training often takes several days if not weeks (For example, training ResNet50 (He et al., 2016) takes 29 hours using 8 Tesla P100 GPUs). Extremely long training time impedes the research and development progress. Due to the single machine’s limited computing resources, it is natural to distribute the workload to clusters and use supercomputing power to increase the throughput of data flow. A commonly adopted solution is distributed synchronous Stochastic Gradient Descent (SGD) which parallelizes the tasks across machines. To make full use of the hardware, minibatch size per machine should be properly set and cannot be too small. In addition, it is common to use large batch to achieve weak scaling (Goyal et al., 2017; Cho et al., 2017; You et al., 2017b; Akiba et al., 2017; Codreanu et al., 2017; Smith et al., 2017). In this way, the speedup is obtained by utilizing overall throughput of the system and fewer updates of the model.
However, there are two challenges when using large batch across large clusters:

Challenge 1: Larger minibatch size often leads to lower test accuracy, as there exists a generalization gap (Keskar et al., 2016).

Challenge 2: When using large clusters, it is harder to achieve nearlinear scalability as the number of machines increases, especially for models with the high communicationtocomputation ratio.
Challenge 1
. Larger minibatch size reduces the variance of gradients by taking the average of the gradients in minibatch, and it provides a more accurate estimate of the gradients
(Goodfellow et al., 2016). Thus it allows the model to take bigger step size and in turn makes the optimization algorithm progress faster. However, as reported in (You et al., 2017b), when increasing the minibatch size to 64K, the test accuracy of ResNet50 drops from 75.4% to 73.2%. Codreanu et al. (2017) also train ResNet50 with batch 64K and achieves the accuracy of 73.9%, which does not meet the baseline accuracy.Challenge 2. A distributed training system with data parallelism strategy typically divides batches across each GPU, and requires a gradient aggregation step between each training step. This communication step usually becomes the bottleneck of the system when the number of GPUs becomes large. To achieve high performance for such distributed training system, we need to improve both the single GPU performance and the overall system performance. Given that the training throughput with one GPU is , if we use GPUs with the scaling efficiency , then the system throughout should be . When the number of GPUs is fixed, we need to increase both and to improve the overall throughput of the training system . To improve the throughput, we need faster computation and more efficient bandwidth utilization, to improve the scaling efficiency, we need more efficient collective communication primitives that can handle a system with thousands of GPUs.
In this paper, we have addressed the above two challenges. Our contributions are as follows:

We successfully scale the minibatch size to 64K for AlexNet and ResNet50 training without loss of accuracy. To achieve this, we have adopted and proposed several strategies (i.e., mixedprecision training with LARS, eliminated weight decay on bias and parameters for batch normalization, and adding proper batch normalization layers).

We build a highthroughput distributed deep learning training system which contains two main features to improve the single GPU performance and the system scaling efficiency . 1) To improve , our system supports halfprecision training, which theoretically, could achieve two times throughput improvement compared to its singleprecision counterpart. 2) To improve , our system uses a hybrid strategy which combines our optimized adaptive allreduce collective with the ringbased allreduce in NCCL.
The rest of this paper is structured as follows. We show the related work in Section 2, then describe the design and implementation of our system and optimizations in Section 3 and Section 4. Finally, we discuss our experimental results in Section 5 and conclude with experiences we have learned through building such a system in Section 6.
2. Related Work
This section describes the research landscape of distributed deep learning training system in three fields: 1) largebatch training; 2) lowprecision training; 3) distributed training on heterogeneous clusters.
2.1. Largebatch Training
Goyal et al. (2017) first trains the ResNet50 ImageNet model with a large minibatch size of 8K over 256 Tesla GPUs and finishes the training process within one hour. They adopt the linear scaling rule to adjust the learning rate as a function of minibatch size. They also develop a warmup scheme, i.e., starting from small learning rate and slowly increasing the learning rate through a few epochs, in order to overcome the optimization challenges in the first few epochs. Cho et al. (2017) use the same training configuration and finish ResNet50 training in 50 minutes with 256 GPUs. You et al. (2017b) further increase the minibatch size of ResNet50 from 8K to 32K. They use LARS to enable large minibatch size and can finish the ResNet50 training in 20 minutes with 2048 KNL chips. Besides ResNet50, they also experiment on AlexNet and finish the training of minibatch size 32K on ImageNet in 11 minutes with 1024 Skylake CPUs. Akiba et al. (2017)
demonstrate the training of ResNet50 in 15 minutes with a minibatch size of 32K over 1024 Tesla P100 GPUs. They adopt techniques such as RMSprop warmup, batch normalization without moving average and a slowstart learning rate schedule. However, their reported test accuracy is 74.9%, which is lower than the baseline
75.3%. Codreanu et al. (2017) use a combination of techniques such as aggressive learning rate scheduling and improved weight decay. They reach 75.3% test accuracy using 32K minibatch size in 42 minutes and 74.6% test accuracy using 49K minibatch size in 28 minutes. In addition to adjusting the learning rate, Smith et al. (2017) propose to increase the minibatch size instead of decaying the learning rate. Their work is the first to train ImageNet with less time (30 minutes) without losing accuracy after Goyal et al. (2017). All the above research towards largebatch training either fails to scale to more nodes and more GPUs with larger minibatch size, or trade accuracy loss for better performance.Devarakonda et al. (2017) use dynamic minibatch size and decay the learning rate at the same time. However, adapting the minibatch size is only tested in piecewise constant learning rate schedule, but cannot be easily applied in polynomial decay, whose curve is more smooth.
2.2. Lowprecision Training
Lowprecision computation is often used to lower the time and energy cost of machine learning. Unfortunately, the benefits of lowprecision (LP) arithmetic come with a cost. The roundoff or quantization error that results from converting numbers into a lowprecision representation introduces noise that can affect the convergence rate and accuracy of SGD. Conventional wisdom says that, for training, lowprecision introduces a tradeoff of the numberofbits used versus the statistical accuracy: the fewer bits used, the worse the solution will become. Theoretical upper bounds on the performance of lowprecision SGD [9] and empirical observations of implemented lowprecision algorithms
(Courbariaux et al., 2015) further confirm that current algorithms are limited by this precisionaccuracy tradeoff. De Sa et al. (2018) describe a simple lowprecision stochastic gradient descent variant called HALP, which converges at the same theoretical rate as fullprecision algorithms despite the noise introduced by using low precision throughout execution. The key idea is to use Stochastic Variance Reduced Gradient (SVRG) to reduce gradient variance, and to combine this with a novel technique called bit centering to reduce quantization error. Micikevicius et al. (2017)propose three techniques for preventing the loss of critical information. Firstly, they recommend maintaining a singleprecision copy of weights that accumulates the gradients after each optimizer step (this copy is rounded to halfprecision for the forward and backpropagation). Secondly, they propose lossscaling to preserve gradient values with small magnitudes. Although the lossscaling technique was not a requirement for successful mixedprecision training when minibatch size is not large enough. Thirdly, they use halfprecision arithmetic that accumulates into singleprecision outputs, which are converted to halfprecision before storing to memory. While all tensors in the forward and backward passes were in FP16 format, a master copy of weights was updated in FP32 format. However, they have not applied this with largebatch training strategy such as LARS to achieve better performance.
2.3. Distributed Training on Heterogeneous Clusters
Most distributed machine learning frameworks such as TensorFlow
(Martin Abadi, 2015) adopt centralized deployment mode. One bottleneck of the centralized algorithm is the high communication cost on the central nodes. Baidu (Gibiansky, 2017) first introduced the ringbased allreduce algorithm (Barnett et al., 1994) to deep learning. This is a very important contribution to the field of distributed training. The ring allreduce algorithm greatly reduces the communication load when the number of nodes increases. However, the original version is low in bandwidth utilization because of splitting up the tensors data into too small slices when tensor sizes are small compared to the number of nodes in the cluster. The IBM’s PowerAI Distributed Deep Learning (DDL) system (Cho et al., 2017) has mentioned a new allreduce algorithm. However, since the implementation is closed source, it is difficult to be applied to other works. Goyal et al. (2017) use an implementation of allreduce consists of three phases for intranode and internode communication: 1) intranode reduce, 2) internode allreduce, and 3) intranode broadcast, which reduces the communication load across nodes and improves scalability. Horovod (Sergeev and Balso, 2018) introduced gradient fusion strategy to the allreduce algorithm, which reduces tensor fragmentation and improves bandwidth utilization. However, this indiscriminate fusion results in unnecessary memory copy and no significant gains have been made in our test scenario. The DAG model proposed by Shi et al. (2018) for scheduling the computation and communication tasks in synchronized SGD guides us to design our optimized allreduce algorithm.Our system adopts several useful parts from the above work. Together with other optimizations, they help us yield high scalability on ImageNet training with both AlexNet and ResNet50.
3. System Overview
Figure 1 is an overview of our distributed deep learning training system. At a high level, our system contains the following three modules: 1) input pipeline module; 2) training module; and 3) communication module.

The input pipeline module delivers data for the next step before the current step has finished. It uses pipelining in order to minimize both CPU and GPU idle time.

The training module includes model construction and variable management. In this module, we have incorporated optimizations such as forward/backward computation with mixedprecision and model update with LARS.

The communication module uses tensor fusion and hybrid allreduce to optimize the scaling efficiency according to tensor size and cluster size.
4. System Implementation and Optimizations
4.1. MixedPrecision Training with LARS
As Micikevicius et al. (2017) have mentioned, the motivation of using halfprecision (FP16) in the training phase is to lower memory bandwidth pressure as well as increase arithmetic throughput. The former can be achieved by using fewer bits to store the same number of values, the latter is achieved on processors that offer higher throughput for reduced precision math. Orthogonal to halfprecision training, You et al. (2017a) first proposed LARS to enable larger minibatch size for distributed training. The algorithm introduces a local learning rate for each layer (as shown in Equation 1), which is the ratio of the L2norm of weights and gradients weighted by a LARS coefficient . Gradients are multiplied with its adaptive local learning rate. A natural choice is to combine halfprecision training with LARS to achieve larger minibatch size with scalability. However, a naïve implementation would introduce several problems because using LARS directly on halfprecision training will cause the computed learning rate to be out of the dynamic range of IEEE halfprecision format (FP16), and thus cause the gradients to vanish and stall the training process.
(1) 
To cope with this situation, we have proposed a training strategy which uses mixedprecision training with LARS as shown in Figure 2. In our strategy, the operations in forward and backward propagation are performed in FP16, while the weights and gradients are cast to singleprecision (FP32) format before applying LARS and cast back to FP16 afterward.
Mixedprecision training with LARS is one of the critical reasons that our system could keep good scalability while increasing the minibatch size to 64K. Table 1 shows that on ResNet50 with the minibatch size of 64K, using LARS with mixedprecision training could maintain the top1 accuracy as 76.2%.
MiniBatch Size  Number of Epochs  LARS  Top1 Accuracy 

64K  90  NO  73.2% 
64K  90  YES  76.2% 
4.2. Improvements on Model Architecture
Improvements in model architecture could often lead to better performance. In our system, we have improved the model architecture from the following two aspects: 1) eliminated weight decay on the bias and batch normalization; and 2) adding proper batch normalization layers for AlexNet.
Weight decay is a commonlyused strategy to get better generalization for the model by adding a regularization term to the loss function
(Krogh and Hertz, 1992).(2) 
If gradient descent is used for learning, the last term of the loss function leads to a new term in the gradients update:
(3) 
In neural network training, it is a typical practice to penalize only the weights of the affine transformation at each layer and leaves the biases unregularized (Goodfellow et al., 2016). What we have observed in our training for AlexNet is that if we also leave two parameter sets and in batch normalization unregularized, our model could achieve better convergence and usually with less time to train for the same number of epochs. and are two trainable parameters in batch normalization as shown in below formulas, where is the mean of minibatch and is the variance of minibatch. and contrals the scale and shift of the normalized result.
As Goodfellow et al. (2016) has noted, the reason that our model achieves better convergence could be that , and usually have much less parameters compared to weights (for AlexNet model, , and parameters only amount to 0.02% of all parameters), which means that leaving them unregularized would not give too much variance and regularizing could instead, introduce a significant amount of underfitting. As shown in Table 2, for AlexNet, we get around 1.3% improvement in accuracy with the same number of epochs. The slight improvements of training run time come from the reduced computations in L2 regularization.
Batch  Epochs  Regularize , and  Top1 

64K  95  Yes  55.8% 
64K  95  No  57.1% 
As mentioned in LARS (You et al., 2017a), replacing Local Response Normalization layers with Batch Normalization(BN) could improve the accuracy of AlexNet (Ioffe and Szegedy, 2015). However, as shown in Table 2, such AlexNetBN model cannot reach baseline top1 accuracy when the minibatch size increases to 64K. By analyzing the parameters and feature map distributions, we find that the feature map distribution after Pool5 has a larger variance and maximum values as training go on (as shown in Figure 4(a)). The significant change of feature scaling makes the training difficult. This motivates us to insert another BN layer after Pool5 to rescale the feature map as shown in Figure 3. The refinedAlexNetBN model could reach 58.8% top1 accuracy with 64K minibatch size for 95 Epoch training.
4.3. Improvements on Communication Strategies
For large batch training with distributed synchronized SGD, efficient gradients aggregation across all GPUs after each iteration is crucial to the training performance (Watcharapichat et al., 2016)(Shi and Chu, 2017). Goyal et al. (2017) have pointed out that for models with a larger set of parameters and GPUs with more computing power, it becomes harder to hide the cost of aggregation in the backprop phase. In our case, for the minibatch size of 64K and 1024 GPUs, gradients aggregation using collective communication primitives such as allreduce has become the bottleneck of the system. NCCL 2.0 is optimized for dense multiGPU systems such as NVIDIA’s DGX1. In our case, communication happens over a hundred of nodes on a cluster, the traditional ringbased allreduce implementation does not scale due to the following reason: In a cluster with GPUs, Ring allreduce will split the data on each GPU into chunks and do the reduce in iterations (Thakur et al., 2005). When gets larger, the messages passing between nodes will become smaller and fail to utilize the full bandwidth of the network. To cope with this problem, we have developed two strategies:

Tensor Fusion. An efficient communication strategy in a distributed training system should maximize the throughput as well as reduce the latency. The main challenge of training deep neural networks with multiple layers is that the sizes of gradient tensors to aggregate vary a lot for different types of layers. Usually, gradient tensor sizes for convolution layers are much smaller than fullyconnected layers. Sending too many small tensors in the network will not only cause the bandwidth to be underutilized but also increase the latency. To cope with this problem, we adopt the technique of tensor fusion. The core idea of tensor fusion is to pack multiple small size tensors together before allreduce to better utilize the bandwidth of the network. We set a parameter . In the backward phase, as tensors from each layer come in, we fuse them into a buffer pool if the total size is less than , and only send the fused tensor out for allreduce when the total size is larger than . This strategy could be easily generalized to distributed training for other neural networks. Figure 6 and Figure 7 shows the fusion strategy for AlexNet and ResNet50 respectively.

Hierarchical Allreduce. In our experiments for ResNet50, when using tensor fusion to combine all the tensors into a single tensor, the endtoend performance will increase by 8x. However, the high throughput also increases the latency, since fusing into a single tensor will prevent the parallelization of gradient aggregation of last few layers and backward propagation of earlier layers. To reduce latency, we need to restrict the condition for tensor fusion, i.e. allow smaller, and multiple tensors in tensor fusion phase. However, ring allreduce perform worse on small tensors. Hierarchical allreduce could solve this problem for small tensor communication. Instead of using ring allreduce where each GPU sends and receives bytes of data in steps. We can group GPUs together, then use a threephase algorithm to do the allreduce across all GPUs (Figure 5: first we do a reduce within GPUs of the same group, store the partial results to a master GPU in each group, then we launch Ring allreduce across groups, after each master GPU gets the final result, we do a broadcast within each group to propagate the final result to every GPU. The threephase algorithm reduces the running steps from to since the intragroup reduce and broadcast each costs steps. The decrease of computation steps makes the threephase algorithm perform better in latencysensitive case (i.e. for small tensor size and the large number of GPUs). We set as a tunable parameter and observe the highest performance is achieved when is set to 16 in our 1024 GPU cluster.

Hybrid Allreduce. Hierarchical allreduce can bring performance gain for convolution layers which usually have a smaller number of weights. However, for fullyconnected layers which usually have a much larger number of weights, ringbased allreduce still outperforms our hierarchical allreduce. To enjoy the best of both worlds, we use a hybrid strategy in our system. We set a parameter to represent the size of the tensor to aggregate in bytes. By tuning this parameter, we can switch between the traditional ringbased allreduce and our customized allreduce. Combined with tensor fusion, hybrid allreduce could help us achieve better performance.
5. Experimental Results
5.1. Experimental Settings
Model. We choose AlexNet (Krizhevsky et al., 2012) and ResNet50 (He et al., 2016) for our experiments because they represent two typical types of CNN: As shown in Table 3, the parameter size of AlexNet is around 2.5 times as ResNet50, while the computation of ResNet50 is around 5.6 times as AlexNet. Thus, the bottleneck of AlexNet lies in communication, while the bottleneck of ResNet50 lies in computation. The baseline top1 accuracy of AlexNet is 58.8% (You et al., 2017b) and the baseline top1 accuracy of ResNet50 is 75.3% (He et al., 2016).
Model  Input Size  Parameter Size  FLOPs  Baseline Top1 

AlexNet  227x227  62M  727 M  58.8% 
ResNet50  224x224  25M  4 G  75.3% 
Dataset. We use ImageNet (Deng et al., 2009) dataset in the following experiments. Both models are trained on 1.28 million training images and evaluated on 50,000 validation images by top1 test accuracy for its 1000 classes task. The training images are partitioned into 1024 chunks and the validation images are partitioned into 128 chunks. Images are stored in the format of TFRecord. In all our experiments, we use data augmentation offered in TensorFlow.
Software. We use TensorFlow (Abadi et al., 2016) as a training framework for its flexible design, various use cases, and a large user/developer community. We build our distributed gradient aggregation algorithm with NVIDIA Collective Communication Library (NCCL), and OpenMPI.
Hardware. Our GPU cluster includes 256 nodes, and each node contains 8 NVIDIA Tesla P40 GPUs that are interconnected with PCIe. For local storage, each server has two 2T NVMe SSDs. For network connectivity, each server has a Mellanox ConnectX4 100Gbit Ethernet network card. We use RoCEv2 (RDMA over Converged Ethernet) for communications among nodes in cluster, which is a common Remote Direct Memory Access (RDMA) implementations ^{1}^{1}1RDMA is a technology which supports zerocopy networking by enabling the network adapter to transfer data directly to or from application memory, eliminating the need to copy data between application memory and the data buffers in the operating system.. We also use GPUDirect RDMA (GDR) to enable direct data exchange between GPUs on different nodes. All of these technologies can reduce the latency and increase the scaling efficiency in our cluster.
5.2. Overall experimental results
Team  Batch  Hardware  Software  Top1 Accuracy  Time 

You et al. (2017b)  512  DGX1 station  NVCaffe  58.8%  6h 10m 
You et al. (2017b)  32K  CPU 1024  Intel Caffe 
58.6%  11min 
This work  64K  Tesla P40 512  TensorFlow  58.8%  5m 
This work  64K  Tesla P40 1024  TensorFlow  58.7%  4m 
Team  Batch  Hardware  Software  Top1 Accuracy  Time 

He et al. (2016)  256  Tesla P100 8  Caffe  75.3%  29h 
Goyal et al. (2017)  8K  Tesla P100 256  Caffe2  76.3%  1h 
Cho et al. (2017)  8K  Tesla P100 256  Torch  75.0%  50min 
Codreanu et al. (2017)  32K  KNL 1024  Intel Caffe  75.3%  42min 
You et al. (2017b)  32K  KNL 2048  Intel Caffe  75.4%  20min 
Akiba et al. (2017)  32K  Tesla P100 1024  Chainer  74.9%  15min 
This work  64K  Tesla P40 1024  TensorFlow  76.2%  8.7m 
This work  64K  Tesla P40 2048  TensorFlow  75.8%  6.6m 
For ResNet50, as shown in Table 5, our system finishes the training in only 8.7 minutes with 76.2% top1 accuracy over 1024 Tesla P40 GPUs and 6.6 minutes with 75.8% top1 accuracy over 2048 Tesla P40 GPUs, which to our best knowledge is the stateoftheart for ImageNet training. Compared to Akiba et al. (2017), our work saves around 40% cost with similar hardware but much shorter training time. Compared to He et al. (2016)’s work which uses 8 GPUs, we achieve more than 248x speedup. Based on the same 1024 GPUs, our work is 1.61 times faster than Akiba et al. (2017). Note that for ResNet50 training, we adopt halfprecision communication during the allreduce gradients aggregation phase due to its reduced memory usage.
5.3. Convergence Analysis
In this subsection, we show that with our optimizations, we can maintain the same convergence as previous works on ImageNet training with a larger minibatch size. The overall training curve of top1 accuracy for ResNet50 and AlexNet are shown in Figure 8 and Figure 9 separately.
Compare the convergence of mixedprecision training and singleprecision training. As explained in Section 4.1, we adopt a mixedprecision training strategy to avoid the precision loss in halfprecision computation. A master copy of weights was updated in FP32 to avoid loss of accuracy, and all tensors in the forward and backward passes were in FP16. The updated gradients (weight gradients multiplied by the learning rate) become too small to be represented in FP16 as any value whose magnitude is smaller than becomes zero in FP16. In fact, when the minibatch size is less than 16K, a master copy of FP32 is enough to get the same top1 test accuracy of baseline. With the minibatch size of 16K, lossscaling is required to maintain the same accuracy as the baseline, or else gradients vanishing will start to appear. When the minibatch size increases to 32K, LARS technique was required for successful mixed precision training. To make LARS perform properly, we have to set its coefficient to a small number: 0.001. This will cause the local learning rate to become zeroes in FP16. Because of this, we need to assign an FP32 copy to LARS. Also, in order to avoid overfitting, the weight decay should be increased from 0.0001 to 0.0005 when the minibatch size grows to 64K. To validate the effectiveness of our mixedprecision training strategy, we compare it with plain singleprecision training. The experiment result in Figure 10 shows that our mixedprecision training strategy has similar top1 accuracy for ResNet50 at 90 epoch as singleprecision training (76.3% for singleprecision training and 76.2% for mixedprecision training).
Effect of LARS. We compare the test accuracy of ResNet50 with and without applying LARS (You et al., 2017a). As shown in Table 6, LARS could improve the top1 accuracy from 60.6% to 71.9%. Also, Figure 8 shows that with LARS, the training curve is more smooth than the curve without LARS. However, even with both mixedprecision and LARS, we still cannot reach the baseline accuracy yet.
Batch  LARS  Top1 Accuracy 

64K  60.6%  
64K  ✓  71.9% 
Effect of model improvements. Eliminating weight decay on bias and batch normalization generates positive effects on convergence. Table 7 shows that eliminated weight decay on batch normalization(BN) for ResNet50, combined with mixedprecision and LARS, could improve top1 accuracy from 71.9% to 76.2%, which meets the baseline test accuracy. Note that for ResNet50 training, we ignore the bias tensor for weight decay as its influence is negligible.
Batch  No Decay BN  Top1 

64K  71.9%  
64K  ✓  76.2% 
Batch  No Decay Bias  No Decay BN  pool5 BN  Top1 Accuracy 
64K  55.8%  
64K  ✓  56.3%  
64K  ✓  56.4%  
64K  ✓  ✓  57.1%  
64K  ✓  ✓  ✓  58.8% 
For AlexNet, we test the effect of optimization strategies including not regularizing bias, not regularizing batch norm parameters and inserting batch normalization after Pool5 layer. As shown in Table 8, when applying all strategies, the top1 accuracy of minibatch size 64K reaches its peak value of 58.8%, which meets the baseline accuracy. Figure 9 also shows that after applying a series of optimization strategies, the convergence speed gets improved and the final test accuracy is higher than using the LARS algorithm only.
5.4. Training Speed and Scalability
In this subsection, we show the training speed and scalability of our distributed training system.
Compare the speed of mixedprecision training and singleprecision training. As shown in Table 9, using mixedprecision training can speedup singlenode performance of ResNet50 from 172 images/second to 218 images/second. This improvement comes from the FP16 computation speedup and the reduced communication parameter size.
Batch/GPU  Data Type  Images/Sec 

64  FP32  172 
64  mixed  218 
Scalability. Figure 11 shows that our customized allreduce has high scaling efficiency. When per GPU minibatch size is fixed to 64, the scaling efficiency of 1024 GPUs (8 GPUs 128 nodes) compared to singlenode (8 GPUs) could reach 99.2%, which is close to the optimal scalability. When comparing the scaling efficiency before and after optimization, we can see the improvements is significant. For 1024 GPUs, we improved the scaling efficiency from 9.0% to 99.2%.
When per GPU minibatch size is fixed to 32, it is harder to scale out. Because the smaller minibatch size often leads to faster computation, which causes the communication to become the bottleneck. As reported in (Akiba et al., 2017), the scaling efficiency of 1024 GPUs with 32 batch/GPU is 80.0%. As shown in Figure 12, our system can reach 87.9% for the same batch settings as (Akiba et al., 2017). Due to our efficient communication strategies, we have achieved higher scaling efficiency than the stateoftheart with the same minibatch size.
Figure 13 shows the scalability of AlexNet training with a minibatch size of 128 per GPU. The baseline is FP32 allreduce (with RDMA). When comparing the scaling efficiency between using 8 GPUs and 512 GPUs, introducing tensor fusion could achieve an improvement from 70% to 81%, and using FP16 allreduce gives 82.7% scalability. When combining FP16 and tensor fusion strategies with hybrid allreduce, we get 91.4% scaling efficiency.
6. Conclusion
Largescale deep neural networks require a large amount of computation to converge to a good testing accuracy. Synchronized gradient descent methods with data parallelism are widely used to train the models in distributed environments. However, data communication between machines in the cluster easily becomes the bottleneck of the system throughput. Though using a large minibatch size can improve the scalability of the system, it becomes more difficult to keep the good generalization ability of the models. In this study, we build a highly scalable deep learning training system to address the problem. We first use the mixedprecision techniques to improve the throughput of a single GPU without losing accuracy. Then we propose optimization approaches (e.g., eliminated weigh decay in batch normalization layers) to successfully train AlexNet and ResNet50 using a minibatch size of 64K without losing accuracy. To further increase the scalability of the system, we propose highly optimized allreduce algorithms which achieve much better performance than the NCCLbased counterpart. As a result, on the training of the ImageNet dataset, we achieve 58.7% top1 test accuracy with AlexNet (95 epochs) in only 4 minutes using 1024 Tesla P40 GPUs, and achieve 75.8% top1 test accuracy with ResNet50 (90 epochs) in only 6.6 minutes using 2048 Tesla P40 GPUs, which outperforms the existing systems.
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