Introduction
For deep learning applications, larger datasets and bigger models lead to significant improvements in accuracy
[Amodei et al.2015], but at the cost of longer training times. Moreover, many applications such as computational finance, autonomous driving, oil and gas exploration, and medical imaging, will almost certainly require training datasets with billions of training elements and terabytes of data. This highly motivates the problem of accelerating the training time of Deep Neural Nets (DNN). For example, finishing 90epoch ImageNet1k training with ResNet50 on a NVIDIA M40 GPU takes 14 days. This training requires single precision operations in total. On the other hand, the world’s current fastest supercomputer can finish single precision operations per second [Dongarra et al.2017]. Thus, if we can make full use of the computing capability of a supercomputer for DNN training, we should be able to finish the 90epoch ResNet50 training in five seconds. So far, the best results on scaling ImageNet training have used synchronous stochastic gradient descent (syncronous SGD). The synchronous SGD algorithm has many inherent advantages, but at the root of these advantages is sequential consistency. Sequential consistency implies that all valid parallel implementations of the algorithm match the behavior of the sequential version. This property is invaluable during DNN design and during the debugging of optimization algorithms. Continuing to scale the synchronous SGD model to more processors requires ensuring that there is sufficient useful work for each processor to do during each iteration. This, in turn, requires increasing the batch size used in each iteration. For example engaging 512 processors in synchronous SGD on a batch size of 1K would mean that each processor only processed a local batch of 2 images. If the batch size can batch size can be scaled to 32K then each processor processes a local batch of 64, and the computation to communication ratio can be more balanced.As a result, over the last two years we have seen a focus on increasing the batch size and number of processors used in the DNN training for ImageNet1K, with a resulting reduction in training time. In the following discussion we briefly review relevant work where all details of batch size, processors, DNN model, runtime, and training set are defined in the publications. All of the following refer to training on ImageNet.
FireCaffe [Iandola et al.2015] [Iandola et al.2016] demonstrated scaling the training of GoogleNet to 128 Nvidia K20 GPUs with a batch size of 1K for 72 epochs and a total training time of 10.5 hours. Although large batch size can lead to a significant loss in accuracy, using a warmup scheme coupled with a linear scaling rule, researchers at Facebook [Goyal et al.2017] were able to scale the training of ResNet 50 to 256 Nvidia P100’s with a batch size of 8K and a total training time of one hour. Using a more sophisticated approach to adapting the learning rate in a method they named the Layerwise Adaptive Rate Scaling (LARS) algorithm [You, Gitman, and Ginsburg2017], researchers were able to scale the batch size to very large sizes, such as 32K, although only 8 Nvidia P100 GPUs were employed. A 3.4% reduction in accuracy was attributed to the absence of data augmentation.
Given the large batch sizes that the LARS algorithm enables, it was natural to ask: how much further can we scale the training of DNNs on ImagNet? This is the investigation that led to this paper. In particular, we found that using LARS we could scale DNN training on ImageNet to 1024 CPUs and finish the 100epoch training with AlexNet in 11 minutes with 58.6% accuracy. Furthermore, we could scale to 2048 KNLs and finish the 90epoch ImageNet training with ResNet50 in 20 minutes without losing accuracy. Stateoftheart ImageNet training speed with ResNet50 is 74.9% top1 test accuracy in 15 minutes [Akiba, Suzuki, and Fukuda2017]. We got 74.9% top1 test accuracy in 64 epochs, which only needs 14 minutes.
Notes.
This paper is focused on training largescale deep neural networks on
machines/processors. We use to denote the parameters (weights of the networks), to denote the local parameters on th worker, to denote the global parameter. When there is no confusion we use to denote the stochastic gradient evaluated at the th worker. All the accuracy means top1 test accuracy. There is no data augmentation in all the results.Background and Related Work
DataParallelism SGD
In data parallelism method, the dataset is partitioned into parts stored on each machine, and each machine will have a local copy of the neural network and the weights (). In synchronized data parallelism, the communication includes two parts: sum of local gradients and broadcast of the global weight. For the first part, each worker computes the local gradient independently, and sends the update to the master node. The master then updates after it gets all the gradients from workers. For the second part, the master broadcasts to all workers. This synchronized approach is a widelyused method on largescale systems [Iandola et al.2016]. Figure 2(a) is an example of 4 worker machines and 1 master machine.
Scaling synchronous SGD to more processors has two challenges. The first is giving each processor enough useful work to do; this has already been discussed. The second challenge is the inherent problem that after processing each local batch all processors must synchronize their gradient updates via a barrier before proceeding. This problem can be partially ameliorated by overlapping communication and communication [Das et al.2016] [Goyal et al.2017], but the inherent synchronization barrier remains. A more radical approach to breaking this synchronization barrier is to pursue a purely asynchronous approach. A variety of asynchronous approaches have been proposed [Recht et al.2011] [Zhang, Choromanska, and LeCun2015a] [Jin et al.2016] [Mitliagkas et al.2016]. The communication and updating rules differ in the asynchronous approach and the synchronous approach. The simplest version of the asynchronous approach is a masterworker scheme. At each step, the master only communicates with one worker. The master gets the gradients from the th worker, updates the global weights, and sends the global weight back to the th worker. The order of workers is based on firstcomefirstserve strategy. The master machine is also called as parameter server. The idea of a parameter server was used in realworld commercial applications by the Downpour SGD approach [Dean et al.2012], which has succesfully scaled to cores. However, Downpour’s performance on 1,600 cores for a globally connected network is not significantly better than a single GPU [Seide et al.2014b].
Model Parallelism Data parallelism replicates the neural network itself on each machine while model parallelism partitions the neural network into pieces. Partitioning the neural network means parallelizing the matrix operations on the partitioned network. Thus, model parallelism can get the same solution as the singlemachine case. Figure 2(b) shows an example of using 4 machines to parallelize a 5layer DNN. Model paralleism has been studied in [Catanzaro2013, Le2013]. However, since the input size (e.g. size of an image) is relatively small, the matrix operations are not large. For example, parallelizing a 204810241024 matrix multiplication only needs one or two machines. Thus, stateoftheart methods often use dataparallelism [Amodei et al.2015, Chen et al.2016, Dean et al.2012, Seide et al.2014a].
Intel Knights Landing System
Intel Knights Landing (KNL) is the latest version of Intel’s generalpurpose accelerator. The major distinct features of KNL that can benefit deep learning applications include the following: (1) Selfhosted Platform. The traditional accelerators (e.g. FPGA, GPUs, and KNC) rely on CPU for control and I/O management. KNL does not need a CPU host. It is selfhosted by an operating system like CentOS 7. (2) Better Memory. KNL’s measured bandwidth is much higher than that of a 24core Haswell CPU (450 GBs vs 100 GBs). KNL’s 384 GB maximum memory size is large enough to handle a typical deep learning dataset. Moreover, KNL is equipped with MultiChannel DRAM (MCDRAM). MCDRAM’s measured bandwidth is 475 GB/s. MCDRAM has three modes: a) Cache Mode: KNL uses it as the last level cache; b) Flat Mode: KNL treats it as the regular DDR; c) Hybrid Mode: part of it is used as cache, the other is used as the regular DDR memory. (3) Configurable NUMA. The basic idea is that users can partition the onchip processors and cache into different groups for better memory efficiency and less communication overhead. This is very important for complicated memoryaccess applications like DNN training.
Since its release, KNL has been used in some HPC (High Performance Computing) data centers. For example, National Energy Research Scientific Computing Center (NERSC) has a supercomputer with 9,668 KNLs (Cori Phase 2). Texas Advanced Computing Center (TACC) has a supercomputer with 4,200 KNLs (Stampede 2).
In this paper, we have two chip options: (1) Intel Skylake CPU or (2) Intel KNL. Using 1024 CPUs, we finish the 100epoch AlexNet in 11 minutes and 90epoch ResNet50 in 48 minutes. Using 1600 CPUs, we finish 90epoch ResNet50 in 31 minutes. Using 512 KNLs, we finish the 100epoch AlexNet in 24 minutes and 90epoch ResNet50 in 60 minutes. Using 2048 KNLs, we finish 90epoch ResNet50 in 20 minutes.
Batch Size  Epochs  Iterations  GPUs  Iteration Time  Total Time 

512  100  250,000  1  
1024  100  125,000  2  + log(2)  + log(2) 
2048  100  62,500  4  + log(4)  + log(4) 
4096  100  31,250  8  + log(8)  + log(8)) 
8192  100  15,625  16  + log(16)  + log(16) 
…  …  …  …  
1,280,000  100  100  2500  + log(2500)  + log(2500)) 
Model  Epochs  Test Top1 Accuracy 

AlexNet  100  58% [Iandola et al.2016] 
ResNet50  90  75.3% [He et al.2016] 
LargeBatch DNN Training
Benefits of LargeBatch Training
The asynchronous methods using parameter server are not guaranteed to be stable on largescale systems [Chen et al.2016]. As discussed in [Goyal et al.2017], dataparallelism synchronized approach is more stable for very large DNN training. The idea is simple—by using a large batch size for SGD, the work for each iteration can be easily distributed to multiple processors. Consider the following ideal case. ResNet50 requires 7.72 billion singleprecision operations to process one 225x225 image. If we run 90 epochs for ImageNet dataset, the number of operations is 90 * 1.28 Million * 7.72 Billion (). Currently, the most powerful supercomputer can finish singleprecision operations per second [Dongarra et al.2017]. If there is an algorithm allowing us to make full use of the supercomputer, we can finish the ResNet50 training in 5 seconds.
To do so, we need to make the algorithm use more processors and load more data at each iteration, which corresponds to increasing the batch size in synchronous SGD. Let us use one NVIDIA M40 GPU to illustrate the case of a single machine. In a certain range, larger batch size will make the single GPU’s speed higher (Figure 3). The reason is that lowlevel matrix computation libraries will be more efficient. For ImageNet training with Alexthe Net model the, optimal batch size per GPU is 512. If we want to use many GPUs and make each GPU efficient, we need a larger batch size. For example, if we have 16 GPUs, then we should set the batch size to . Ideally, if we fix total number of data accesses and grow the batch size linearly with number of processors, the number of SGD iterations will decrease linearly and the time cost of each iteration remains constant, so the total time will also reduce linearly with number of processors (Table 2).
Model Selection
To scale up the algorithm to many machines, a major overhead is the communication among different machines [Zhang, Choromanska, and LeCun2015b]. Here we define scaling ratio, which means the ratio between computation and communication. For DNN models, the computation is proportional to the number of floating point operations required for processing an image. Since we focus on synchronous SGD approach, the communication is proportional to model size (or the number of parameters). Different DNN models have different scaling ratios. To generalize our study, we pick two representative models: AlexNet and ResNet50. The reason is that they have different scaling ratios. From Table 6, we observe that ResNet50’s scaling ratio is 12.5 larger than that of AlexNet. This means scaling ResNet50 is easier than scaling AlexNet. Generally, ResNet50 will have a much higher weak scaling efficiency than AlexNet.
In the fixedepoch situation, large batch does not change the number of floating point operations (computation volume). However, large batch can reduce the communication volume. The reason is that the singleiteration communication volume is only related to the model size and network system. Larger batch size means less number of iterations and less overall communication. Thus, large batch size can improve the algorithm’s scalability.
Difficulty of LargeBatch Training
However, synchronous SGD with larger batch size usually achieves lower accuracy than when used with smaller batch sizes, if each is run for the same number of epochs, and currently there is no algorithm allowing us to effectively use very large batch sizes. [Keskar et al.2016]. Table 3 shows the target accuracy by standard benchmarks. For example, when we set the batch size of AlexNet larger than 1024 or the batch size of ResNet50 larger than 8192, the test accuracy will be significantly decreased (Table 5 and Figure 4).
For largebatch training, we need to ensure that the larger batches achieve similar test accuracy with the smaller batches by running the same number of epochs. Here we fix the number of epochs because: Statistically, one epoch means the algorithm touches the entire dataset once; and computationally, fixing the number of epochs means fixing the number of floating point operations. Stateoftheart approaches for large batch training include two techniques:
(1) Linear Scaling [Krizhevsky2014]: If we increase the batch size from to , we should also increase the learning rate from to .
(2) Warmup Scheme [Goyal et al.2017]: If we use a large learning rate (). We should start from a small and increase it to the large in the first few epochs.
The intuition of linear scaling is related to the number of iterations. Let us use , , and to denote the batch size, the learning rate, and the number of iterations. If we increase the the batch size from to , then the number of iterations is reduced from to . This means that the frequency of weight updating reduced by times. Thus, we make the updating of each iteration more efficient by enlarging the learning rate by times. The purpose of a warmup scheme is to avoid the situation in which the algorithm diverges at the beginning because we have to use a very large learning rate based on linear scaling. With these techniques, researchers can use the relatively large batch in a certain range (Table 4). However, we observe that stateoftheart approaches can only scale batch size to 1024 for AlexNet and 8192 for ResNet50. If we increase the batch size to 4096 for AlexNet, we only achieve 53.1% in 100 epochs (Table 5). Our target is to achieve 58% accuracy even when using large batch sizes.
Team  Model  Baseline Batch  Large Batch  Baseline Accuracy  Large Batch Accuracy 

Google [Krizhevsky2014]  AlexNet  128  1024  57.7%  56.7% 
Amazon [Li2017]  ResNet152  256  5120  77.8%  77.8% 
Facebook [Goyal et al.2017]  ResNet50  256  8192  76.40%  76.26% 
Batch Size  Base LR  warmup  epochs  test accuracy 

512  0.02  N/A  100  0.583 
1024  0.02  no  100  0.582 
4096  0.01  yes  100  0.509 
4096  0.02  yes  100  0.527 
4096  0.03  yes  100  0.520 
4096  0.04  yes  100  0.530 
4096  0.05  yes  100  0.531 
4096  0.06  yes  100  0.516 
4096  0.07  yes  100  0.001 
…  …  …  …  … 
4096  0.16  yes  100  0.001 
Model  communication  computation  comp/comm 
# parameters  # flops per image  scaling ratio  
AlexNet  # 61 million  # 1.5 billion  24.6 
ResNet50  # 25 million  # 7.7 billion  308 
Batch Size  LR rule  warmup  Epochs  test accuracy 

512  regular  N/A  100  0.583 
4096  LARS  13 epochs  100  0.584 
8192  LARS  8 epochs  100  0.583 
32768  LARS  5 epochs  100  0.585 
Batch Size  epochs  Peak Top1 Accuracy  hardware  time 

256  100  58.7%  8core CPU + K20 GPU  144h 
512  100  58.8%  DGX1 station  6h 10m 
4096  100  58.4%  DGX1 station  2h 19m 
32768  100  58.5%  512 KNLs  24m 
32768  100  58.6%  1024 CPUs  11m 
Batch Size  Data Augmentation  epochs  Peak Top1 Accuracy  hardware  time 

256  NO  90  73.0%  DGX1 station  21h 
256  YES  90  75.3%  16 KNLs  45h 
8192  NO  90  72.7%  DGX1 station  21h 
8192  NO  90  72.7%  32 CPUs + 256 P100 GPUs  1h 
8192  YES  90  75.3%  32 CPUs + 256 P100 GPUs  1h 
16384  YES  90  75.3%  1024 CPUs  52m 
16000  YES  90  75.3%  1600 CPUs  31m 
32768  NO  90  72.6%  512 KNLs  1h 
32768  YES  90  75.4%  512 KNLs  1h 
32768  YES  90  75.4%  1024 CPUs  48m 
32768  YES  90  75.4%  2048 KNLs  20m 
32768  YES  64  74.9%  2048 KNLs  14m 
Batch Size  256  8K  16K  32K  64K  note 

MSRA  75.3%  75.3%  —  —  —  weak data augmentation 
IBM  —  75.0%  —  —  —  — 
SURFsara  —  75.3%  —  —  —  — 
76.3%  76.2%  75.2%  72.4%  66.0%  Heavy data augmentation  
Our version  73.0%  72.7%  72.7%  72.6%  70.0%  no data augmentation 
Our version  75.3%  75.3%  75.3%  75.4%  73.2%  weak data augmentation 
Scaling up Batch Size
In this paper, we use LARS algorithm [You, Gitman, and Ginsburg2017] together with warmup scheme [Goyal et al.2017] to scale up the batch size. Using these two approaches, synchronous SGD with a large batch size can achieve the same accuracy as the baseline (Table 7
). To scale to larger batch sizes (e.g. 32K) for AlexNet, we need to change the local response normalization (LRN) to batch normalization (BN). We add BN after each Convolutional layer. Specifically, we use the refined AlexNet model by B. Ginsburg
^{1}^{1}1https://github.com/borisgin/nvcaffe0.16/tree/caffe0.16/models/alexnet_bn. From Figure 4, we can clearly observe the effects of LARS. LARS can help ResNet50 to preserve the high test accuracy. The current approaches (linear scaling and warmup) has much lower accuracy for batch size = 16k and 32k (68% and 56%). The target accuracy is about 73%.Experimental Results
Experimental Settings.
The dataset we used in this paper is ImageNet1k [Deng et al.2009]. The dataset has 1.28 million images for training and 50,000 images for testing. Without data augmentation, the top1 testing accuracy of our ResNet50 baseline is 73% in 90 epochs. For versions without data augmentation, we achieve stateoftheart accuracy (73% in 90 epochs). With data augmentation, our accuracy is 75.3%. For the KNL implementation, we have two versions:
(1) We wrote our KNL code based on Caffe
[Jia et al.2014] for singlemachine processing and use MPI [Gropp et al.1996] for the communication among different machines on KNL cluster.(2) We use Intel Caffe, which supports multinode training by Intel MLSL (Machine Learning Scaling Library).
We use the TACC Stampede 2 supercomputer as our hardware platform^{2}^{2}2portal.tacc.utexas.edu/userguides/stampede2. All GPUrelated data are measured based on B. Ginsburg’s nvcaffe^{3}^{3}3https://github.com/borisgin/nvcaffe0.16. The LARS algorithm is opened source by NVIDIA Caffe 0.16. We implemented the LARS algorithm based on NVIDIA Caffe 0.16.
ImageNet training with AlexNet
Previously, NVIDIA reported that using one DGX1 station they were able to finish 90epoch ImageNet training with AlexNet in 2 hours. However, they used halfprecision or FP16, whose cost is half of the standard singleprecision operation. We run 100epoch ImageNet training with AlexNet with standard singleprecision. It takes 6 hours 9 minutes for batch size = 512 on one NVIDIA DGX1 station. Because of LARS algorithm [You, Gitman, and Ginsburg2017], we are able to have the similar accuracy using large batch sizes (Table 7). If we increase the batch size to 4096, it only needs 2 hour 10 minutes on one NVIDIA DGX1 station. Thus, using large batch can significantly speedup DNN training.
For the AlexNet with batch size = 32K, we scale the algorithm to 512 KNL sockets with a total of about 32K processors cores. The batch size allocated per individual KNL socket is 64, so the overall batch size is . We finish the 100epoch training in 24 minutes. When we use 1024 CPUs, with a batch size per CPU of 32, we finish 100epoch AlexNet training in 11 minutes. To the best of our knowledge, this is currently the fastest 100epoch ImageNet training with AlexNet. The overall comparison is in Table 8.
ImageNet training with ResNet50
Facebook [Goyal et al.2017] finishes the 90epoch ImageNet training with ResNet50 in one hour on 32 CPUs and 256 NVIDIA P100 GPUs. P100 is the processor used in the NVIDIA DGX1. After scaling the batch size to 32K, we are able to more KNLs. We use 512 KNL chips and the batch size per KNL is 64. We finish the 90epoch training in 32 minutes on 1600 CPUs using a batch size of 32K. We finish the 90epoch training in 31 minutes on 1600 CPUs using a batch size of 16,000. We finish the 90epoch training in 20 minutes on 2048 CPUs using a batch size of 32K. The version of our CPU chip is Intel Xeon Platinum 8160 (Skylake). The version of our KNL chip is Intel Xeon Phi Processor 7250. Note that we are not affiliated to Intel or NVIDIA, and we do not have any a priori preference for GPUs or KNL. The overall comparison is in Table 9.
Codreanu et al. reported their experience on using Intel KNL clusters to speed up ImageNet training by a blogpost^{4}^{4}4https://blog.surf.nl/en/imagenet1ktrainingonintelxeonphiinlessthan40minutes/. They reported that they achieved 73.78% accuracy (with data augmentation) in less than 40 minutes on 512 KNLs. Their batch size is 8k. However, Codreanu et al. only finished 37 epochs. If they conduct 90epoch training, the time is 80 minutes with 75.25% accuracy. In terms of absolute speed (images per second or flops per second), Facebook and our version are much faster than Codreanu et al. Since both Facebook and Codreanu used data augmentation, Facebook’s 90epoch accuracy is higher than that of Codreanu.
ResNet50 with Data Augmentation
Based on the original ResNet50 model [He et al.2016], we added data augmentation to our baseline. Our baseline achieves 75.3% top1 val accuracy in 90 epochs. Because we do not have Facebook’s model file, we failed to reproduce full match their results of 76.24% top1 accuracy. The model we used is available upon request. Codreanu et al. reported they achieved 75.81% top1 accuracy in 90 epochs; however, they changed the model parameters (not only hyperparameters). The overall comparison is in Table 10. We observe that our scaling efficiency is much higher than Facebook’s version. Even though our baseline’s accuracy is lower than Facebook’s, we achieve a correspondingly higher accuracy when we increase the batch size above 10K. Akiba et al. [Akiba, Suzuki, and Fukuda2017] reported finishing the 90epoch ResNet50 training within 15 minutes on 1,024 Nvidia P100 GPUs. However, the baseline accuracy is missing in the report, so it is difficult to tell how much their 74.9% accuracy using the 32k batch size diverges from the baseline. Secondly, both Akiba et al. and Facebook’s work [Goyal et al.2017] are ResNet50 specific, while we also show the generality of our approach with AlexNet. It is worth noting that our online preprint is two months earlier than Akiba et al.
NVIDIA P100 GPU and Intel KNL
Because stateoftheart models like ResNet50 are computational intensive, our comparison is focused on the computational power rather than memory efficiency. Since deep learning applications mainly use singleprecision operations, we do not consider doubleprecision here. The peak performance of P100 GPU is 10.6 Tflops^{5}^{5}5http://www.nvidia.com/object/teslap100.html. The peak performance of Intel KNL is 6 Tflops^{6}^{6}6https://www.alcf.anl.gov/files/HC27.25.710KnightsLandingSodaniIntel.pdf. Based on our experience, the power of one P100 GPU is roughly equal to two KNLs. For example, we used 512 KNLs to match Facebook’s 256 P100 GPUs. However, using more KNLs still requires the larger batch size.
Scaling Efficiency of Large Batches
To scale up deep learning, we need to a communicationefficient approach. Communication means moving data. On a shared memory system, communication means moving data between different level of memories (e.g. from DRAM to cache). On a distributed system, communication means moving the data over the network (e.g. master machine broadcast its data to all the worker machines). Communication often is the major overhead when we scale the algorithm on many processors. Communication is much slower than computation (Table 11). Also, communication costs much more energy than computation (Table 12).
Let us use the example ImageNet training with AlexNetBN on 8 P100 GPUs to illustrate the idea. The baseline’s batch size is 512. The larger batch size is 4096. In this example, we focus the the communication among different GPUs. Firstly, our target is to make training with the larger batch size achieve the same accuracy as the small batch in the same fixed number of epochs (Figure 5). Fixing the number of epochs implies fixing the number of floating point operations (Figure 6). If the system is not overloaded, the larger batch implementation is much faster than small batch for using the same hardware (Figure 7). For finishing the same number of epochs, the communication overhead is lower in the large batch version than in the smaller batch version. Specifically, the larger batch version sends fewer messages (latency overhead) and moves less data (bandwidth overhead) than the small batch version. For Sync SGD, the algorithm needs to conduct an allreduce operations (sum of gradients on all machines). The number of messages sent is linear with the number of iterations. Also, because the gradients has the same size with the weights (). Let us use , , and to denote the number of epochs, the total number of pictures in the training dataset, and the batch size, respectively. Then we can get the number of iterations is . Thus, when we increase the batch size, we need much less number of iterations (Table 2 and Figure 8). The number of iterations is equal to the number of messages the algorithm sent (i.e. latency overhead). Let us denote as the neural network model size. Then we can get the communication volume is . Thus, the larger batch version needs to move much less data than smaller batch version when they finish the number of floating point operations (Figures 9 and 10). In summary, the larger batch size does not change the number of floating point operations when we fix the number of epochs. The larger batch size can increase the computationcommunication ratio because it reduces the communication overhead (reduce latency and move less data). Finally, the larger batch size makes the algorithm more scalable on distributed systems.
Network  (latency)  (1/bandwidth) 

Mellanox 56Gb/s FDR IB  s  s 
Intel 40Gb/s QDR IB  s  s 
Intel 10GbE NetEffect NE020  s  s 
Operation  Type  Energy (pJ) 

32 bit int add  Computation  0.1 
32 bit float add  Computation  0.9 
32 bit register access  Communication  1.0 
32 bit int multiply  Computation  3.1 
32 bit float multiply  Computation  3.7 
32 bit SRAM access  Communication  5.0 
32 bit DRAM access  Communication  640 
Conclusion
In recent years the ImageNet 1K benchmark set has played a significant role as a benchmark for assessing different approaches to DNN training. The most successful results on accelerating DNN training on ImageNet have used a synchronous SGD approach. To scale this synchronous SGD approach to more processors requires increasing the batch size. Using a warmup scheme coupled with a linear scaling rule, researchers at Facebook [Goyal et al.2017] were able to scale the training of ResNet 50 to 256 Nvidia P100’s with a batch size of 8K and a total training time of one hour. Using a more sophisticated approach to adapting the learning rate in a method they named the Layerwise Adaptive Rate Scaling (LARS) algorithm [You, Gitman, and Ginsburg2017], researchers were able to scale the batch size to 32K; however, the potential for scaling to larger number of processors was not demonstrated in that work, and only 8 Nvidia P100 GPUs were employed. Also, data augmentation was not used in that work, and accuracy was impacted. In this paper we confirmed that the increased batch sizes afforded by the LARS algorithm could lead to increased scaling. In particular, we scaled synchronous SGD batch size to 32K and using 1024 Intel Skylake CPUs we were able to finish the 100epoch ImageNet training with AlexNet in 11 minutes. Furthermore, with a batch size of 32K and 2048 KNLs we were able to finish 90epoch ImageNet training with ResNet50 in 20 minutes. Stateoftheart ImageNet training speed with ResNet50 is 74.9% top1 test accuracy in 15 minutes [Akiba, Suzuki, and Fukuda2017]. We got 74.9% top1 test accuracy in 64 epochs, which only needs 14 minutes. We also explored the impact of data augmentation in our work.
Acknowledgements
The large batch training algorithm was developed jointly with I.Gitman and B.Ginsburg done during Yang You’s internship at NVIDIA in the summer 2017. The work presented in this paper was supported by the National Science Foundation, through the Stampede 2 (OAC1540931) award. JD and YY are supported by the U.S. DOE Office of Science, Office of Advanced Scientific Computing Research, Applied Mathematics program under Award Number DESC0010200; by the U.S. DOE Office of Science, Office of Advanced Scientific Computing Research under Award Numbers DESC0008700; by DARPA Award Number HR001112 20016, ASPIRE Lab industrial sponsors and affiliates Intel, Google, HP, Huawei, LGE, Nokia, NVIDIA, Oracle and Samsung. Other industrial sponsors include Mathworks and Cray. In addition to ASPIRE sponsors, KK is supported by an auxiliary Deep Learning ISRA from Intel. CJH also thank XSEDE and Nvidia for independent support.
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