官網鏈接：http://mxnet.readthedocs.io/en/latest/tutorials/imagenet_full.html

Training Deep Net on 14 Million Images by Using A Single Machine

This note describes how to train a neural network on Full ImageNet Dataset [1] with 14,197,087 images in 21,841 classes.We achieved a state-of-art model by using 4 GeForce GTX 980 cards on a single machine in 8.5 days.

There are several technical challenges in this problem.

How to pack and store the massive data.

How to minimize the memory consumption of the network, so we can use net with more capacity than those used for ImageNet 1K

How to train the model fast.

We also released our pre-trained model for this full ImageNet dataset.

此方法描述了如何是使用一臺機器 -4張GTX980互聯在一臺機器上，訓練8.5天訓練完整個ImageNet 數據集，此數據集有14,197,087張圖片。

???? ?? 1.存儲海量數據的方法。

??????? 2.最小化內存的計算代價。

??????? 3.如何快速訓練模型。

Data Preprocessing

The raw full ImageNet dataset is more than 1TB. Before training the network, we need to shuffle these images then load batch of images to feed the neural network. Before we describe how we solve it, let’s do some calculation first:

原始的ImageNet數據超過1TB，在訓練之前，必須分批的形式載入神經網絡。在描述之前，先進行一些計算。

假設的硬件速度：

?????????????????????????????????????????????? 4K 隨機讀取????????????????????????????????????? 順序存取

????????? 西數 黑盤：??? 0.43MB /s （110 IOPS）??????????????????????????? 170MB/s

????????? 三星 850PRO ssd?????? 40MB/s???????????????????????????????????????????? 550MB/s

Assume we have two good storage device [2]:

| Device | 4K Random Seek | Sequential Seek | | ------------------------- | --------------------- | --------------- | | WD Black (HDD) | 0.43 MB /s (110 IOPS) | 170 MB/s | | Samsung 850 PRO (SSD) | 40 MB/s (10,000 IOPS) | 550 MB/s |

A very naive approach is loading from a list by random seeking. If use this approach, we will spend 677 hours with HDD or 6.7 hours with SSD respectively. This is only about read. Although SSD looks not bad, but 1TB SSD is not affordable for everyone.

But we notice sequential seek is much faster than random seek. Also, loading batch by batch is a sequential action. Can we make a change? The answer is we can’t do sequential seek directly. We need random shuffle the training data first, then pack them into a sequential binary package.

This is the normal solution used by most deep learning packages. However, unlike ImageNet 1K dataset, where wecannot? store the images in raw pixels format. Because otherwise we will need more than 1TB space. Instead, we need to pack the images in compressed format.

一個普通的方法是 從一個列中隨機存取，若使用這種途徑，我們使用HDD 將使用667小時，或者SSD6.7小時。SSD缺點就是有點貴。

序列化存取比隨機存取快，但有時隨機存取是必須的。

另外，我們需要一種壓縮的格式進行存取數據。

The key ingredients are

• Store the images in jpeg format, and pack them into binary record.
• Split the list, and pack several record files, instead of one file.
  • This allows us to pack the images in distributed fashion, because we will be eventually bounded by the IO cost during packing.
  • We need to make the package being able to read from several record files, which is not too hard.This will allow us to store the entire imagenet dataset in around 250G space.

After packing, together with threaded buffer iterator, we can simply achieve an IO speed of around 3,000 images/sec on a normal HDD.

主要過程是：

?????? 使用jpeg格式存儲數據，并進行二次壓縮；

?????? 將列表拆分；減輕 I/O 代價；

?????? 打包存儲可以 存儲整個 ImageNet的數據集到250GB的空間里。

打包之后，我們可以多線程存取達到 3000張/秒 圖像。

Training the model.訓練模型

Now we have data. We need to consider which network structure to use. We use Inception-BN [3] style model, compared to other models such as VGG, it has fewer parameters, less parameters simplified sync problem. Considering our problem is much more challenging than 1k classes problem, we add suitable capacity into original Inception-BN structure, by increasing the size of filter by factor of 1.5 in bottom layers of original Inception-BN network.This however, creates a challenge for GPU memory. As GTX980 only have 4G of GPU RAM. We really need to minimize the memory consumption to fit larger batch-size into the training. To solve this problem we use the techniques such as node memory reuse, and inplace optimization, which reduces the memory consumption by half, more details can be found in memory optimization note

收集數據完畢，我們需要確定使用哪種網絡結構。 在此，我們使用? Inception-BN [3] 形式的模型。相對于其他模型，它有更少的參數，因此有更少的參數同步問題。.............考慮到GTX980只有4G顯存，需要分批訓練。 我們使用? node memory reuse節點內存救援 的形式，減少一半的內存耗費，詳情請見memory optimization note。

Finally, we cannot train the model using a single GPU because this is a really large net, and a lot of data. We use data parallelism on four GPUs to train this model, which involves smart synchronization of parameters between different GPUs, and overlap the communication and computation. A runtime denpdency engine is used to simplify this task, allowing us to run the training at around 170 images/sec.

我們使用多個GPU進行訓練，緩解參數過多和數據過多的問題，使用數據并行的方式訓練模型，設計到少量的 參數同步。 一個? runtime denpdency engine 用于使這種工作簡單化， 可以達到 170張 圖片/每秒 的訓練速度。

Here is a learning curve of the training process:

訓練-學習曲線

Evaluate the Performance

Train Top-1 Accuracy over 21,841 classes: 37.19%

訓練表現：

???? 在21841個類上達到 37.19% 的Top1 分辨率

There is no official validation set over 21,841 classes, so we are using ILVRC2012 validation set to check the performance. Here is the result:

| Accuracy | Over 1,000 classes | Over 21,841 classes | | -------- | ------------------ | ------------------- | | Top-1 | 68.3% | 41.9% | | Top-5 | 89.0% | 69.6% | | Top=20 | 96.0% | 83.6% |

As we can see we get quite reasonable result after 9 iterations. Notably much less number of iterations is needed to achieve a stable performance, mainly due to we are facing a larger dataset.

We should note that this result is by no means optimal, as we did not carefully pick the parameters and the experiment cycle is longer than the 1k dataset. We think there is definite space for improvement, and you are welcomed to try it out by yourself!

The Code and Model.代碼和模型

The code and step guide is publically available at https://github.com/dmlc/mxnet/tree/master/example/image-classification

We also release a pretrained model under https://github.com/dmlc/mxnet-model-gallery/tree/master/imagenet-21k-inception

How to Use The Model.使用模型

We should point out it 21k classes is much more challenging than 1k. Directly use the raw prediction is not a reasonable way.

Look at this picture which I took in Mount Rainier this summer:

We can figure out there is a mountain, valley, tree and bridge. And the prediction probability is :

對于 山巒、峽谷、樹木、橋梁的預測準確度:

We notice there are several peaks. Let’s print out the label text in among 21k classes and ImageNet 1k classes:

| Rank | Over 1,000 classes | Over 21,841 classes | | ----- | --------------------------- | -------------------------- | | Top-1 | n09468604 valley | n11620673 Fir | | Top-2 | n09332890 lakeside | n11624531 Spruce | | Top-3 | n04366367 suspension bridge | n11621281 Amabilis fir | | Top-4 | n09193705 alp | n11628456 Douglas fir | | Top-5 | n09428293 seashore | n11627908 Mountain hemlock |

There is no doubt that directly use probability over 21k classes loss diversity of prediction. If you carefully choose a subset by using WordNet hierarchy relation, I am sure you will find more interesting results.

Note

[1] Deng, Jia, et al. “Imagenet: A large-scale hierarchical image database.” Computer Vision and Pattern Recognition, 2009. CVPR 2009. IEEE Conference on. IEEE, 2009.

[2] HDD/SSD data is from public website may not be accurate.

[3] Ioffe, Sergey, and Christian Szegedy. “Batch normalization: Accelerating deep network training by reducing internal covariate shift.”arXiv preprint arXiv:1502.03167 (2015).

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