Curriculum Learning: A Regularization Method for Efficient and Stable Billion-Scale GPT Model Pre-Training

Note: This tutorial is updated on 10/29/2021. Changes include: 1) A more detailed tuning strategy. 2) Pipeline parallelism support. 3) Token-based learning rate decay. 4) A new GPT-2 example at github.com/microsoft/Megatron-DeepSpeed. See details below.

In this tutorial, we introduce DeepSpeed’s curriculum learning-based data pipeline, which presents easier or simpler examples earlier during training. By enabling stable training with 8x/4x larger batch size/learning rate (whereas the baseline approach struggles with training divergence), we observe that curriculum learning (based on sequence length) provides stable and 3.3x faster GPT-2 pre-training (tested on 117M and 1.5B parameters), together with better token-wise convergence speed and zero-shot WikiText-103/LAMBADA evaluation results. In addition, since curriculum learning only affect the data pipeline, its benefit is complementary to many DeepSpeed features and other system optimization techniques. For example, curriculum learning is compatible with DeepSpeed’s ZeRO Redundancy Optimizer, ZeRO-Offload, and 3D Parallelism.

To illustrate the benefits and usage of curriculum learning, we use the Megatron-LM GPT-2 pre-training task as example. For more details on this task, please refer to the tutorial. In addition, we also have a paper which provides the technical details including implementation and evaluations.

1. Configurations and tuning strategy

Curriculum learning can be used by setting the DeepSpeed configuration as the following example json config file:

{
  "train_batch_size": 4096,
  "gradient_accumulation_steps": 1,
  "steps_per_print": 1,
  "optimizer": {
    "type": "Adam",
    "params": {
      "lr": 0.00015,
      "max_grad_norm": 1.0,
      "betas": [0.9, 0.95]
    }
  },
  "gradient_clipping": 1.0,
  "fp16": {
    "enabled": true,
    "loss_scale": 0,
    "loss_scale_window": 1000,
    "hysteresis": 2,
    "min_loss_scale": 1
  },
  "curriculum_learning": {
    "enabled": true,
    "curriculum_type": "seqlen",
    "min_difficulty": 8,
    "max_difficulty": 1024,
    "schedule_type": "fixed_linear",
    "schedule_config": {
      "total_curriculum_step": 15000,
      "difficulty_step": 8
    }
  }
}

To support curriculum learning, we add the following new parameters:

curriculum_type is the type of curriculum difficulty metric. Currently we support the seqlen metric which presents shorter sequences earlier in training. We implement this type of curriculum learning by performing training data sequence truncation before the actual forward pass. We will describe how to implement this in the Megatron-LM GPT-2 pre-training example below.

min_difficulty is the starting difficulty level. For seqlen metric it means we start with sequence length as min_difficulty. We observe that lower min_difficulty usually provides better stability/convergence speed benefit but with two caveats: First, sometimes (especially for large models) starting with too small difficulty level may lead to severe overfitting (e.g., training loss divergence or validation perplexity fluctuations) thus hurt the convergence. Second, for seqlen metric it is recommended to set min_difficulty as multiple of 8 (for FP16 data) or 16 (for INT8 data) in order to enable NVIDIA GPU’s Tensor Core acceleration. To tune this hyperparameter for seqlen metric, we recommend to start with min_difficulty at 8 (million-scale models) or 64 (billion-scale models), and then increase it if you observe divergence or validation perplexity fluctuations at the very beginning.

max_difficulty is the ending difficulty level. For seqlen metric it should be set as the full sequence length (e.g., 1024 for Megatron-LM GPT-2 pre-training).

schedule_type is the scheduling policy for curriculum learning (i.e., which difficulty level to use at certain step). Currently we support three schedules: fixed_linear, fixed_root, and fixed_discrete. We recommend to first try the fixed_linear schedule, which is easier to tune and provides great training stability/efficiency gain in our tests. Each schedule has its own configurations:

1.1 fixed_linear schedule

For fixed_linear schedule there are two configurations:

"schedule_type": "fixed_linear",
"schedule_config": {
  "total_curriculum_step": 15000,
  "difficulty_step": 8
}

The total_curriculum_step is the total number of steps for the curriculum learning. For fixed_linear schedule the difficulty level will linearly increase from min_difficulty to max_difficulty during the total_curriculum_step duration. This configuration needs to be tuned for each training task. We observe that too small and too large total_curriculum_step are both suboptimal: with too small total_curriculum_step curriculum learning might not be able to provide enough training stability benefit so the training might still diverge; with too large total_curriculum_step the model may overfit too much during curriculum learning on the easier/simpler training data thus hurt the overall convergence. To tune this hyperparameter, we recommend performing a binary search to find the largest total_curriculum_step that does not have significant validation perplexity fluctuation during the first few multiples of LR warmup steps. The underlying rationale can be found in our paper Appendix A.1.

The difficulty_step configuration ensures that at anytime the difficulty level must be multiple of difficulty_step. A smaller value is preferable since it gives more smooth curriculum and better stability. We usually set it as 8 (for FP16 data) or 16 (for INT8 data) to enable NVIDIA GPU’s Tensor Core acceleration. If this is unrelated to your hardware, you can set it as 1.

1.2 fixed_root schedule

For fixed_root schedule there are three configurations:

"schedule_type": "fixed_root",
"schedule_config": {
  "total_curriculum_step": 15000,
  "difficulty_step": 8,
  "root_degree": 2
}

The total_curriculum_step and difficulty_step have the same meaning as in the fixed_linear schedule case. The root_degree determines the root degree of the root function of the schedule. The difficulty level at certain step is determined as ((current step/total_curriculum_step)**(1/root_degree)) * (max_difficulty - min_difficulty) + min_difficulty. Thus fixed_linear is basically a special case of fixed_root with root_degree as 1. In our (limited) study, we find the fixed_root schedule does not provide any clear advantage over fixed_linear schedule, while requiring one additional parameter.

1.3 fixed_discrete schedule

For fixed_discrete schedule there are two configurations:

"schedule_type": "fixed_discrete",
"schedule_config": {
  "difficulty": [1,2,3],
  "max_step": [5,10]
}

The difficulty is a list of difficulty levels to be used during schedule. The max_step is a list of step timestamp to determine when to switch to next difficulty level. For example, the json config above means that at step 1-5 difficulty 1 is used, at step 6-10 difficulty 2 is used, from step 11 difficulty 3 is used. This fixed_discrete schedule provides the most flexible curriculum learning scheduling. However, we find that one risk of this kind of schedule is that if the model stays at certain difficulty level for too long, training divergence may happen when switching to next difficulty due to severe overfitting.

2. Curriculum learning for Megatron-LM GPT-2 pre-training

Watch out! After the update on 10/29/2021, now there are two curriculum learning examples for Megatron-LM GPT-2 pre-training. Both of them have some unique features and limitations. See details below.

We provide two curriculum learning examples for Megatron-LM GPT-2 pre-training:

The first one is at Megatron-DeepSpeed/tree/main/examples/curriculum_learning. This integration is based on a newer Megatron-LM fork, and only this curriculum learning example supports pipeline parallelism. However, currently (10/29/2021) we haven’t verified ZeRO-2 and ZeRO-3 on this fork. Overall, we highly recommend you to use this example if your model does not require ZeRO-2/3.

The second one is at DeepSpeedExamples/Megatron-LM-v1.1.5-ZeRO3/curriculum_learning/. This integration is based on an older Megatron-LM hard copy that we will eventually deprecate and this curriculum learning example does not support pipeline parallelism. We recommend you to ONLY use this example if your model requires ZeRO-2/3.

Besides the additional DeepSpeed curriculum learning json configurations described above, there are some other necessary changes on the user side to integrate curriculum learning:

2.1 Training data truncation

To enable seqlen-based curriculum learning, we need to add the functionality of training data truncation based on the given curriculum sequence length. For the case without pipeline parallelism, it is necessary to add a curriculum_seqlen argument in the model’s forward pass and use it to perform training data sequence length truncation. For Megatron-LM GPT-2 pre-training, we implement this in forward() in megatron/model/gpt2_model.py and in forward_step() in pretrain_gpt2.py.

For the case with pipeline parallelism, due to DeepSpeed engine limitations we cannot inject the curriculum_seqlen argument in the forward pass. Instead, we create a duplicate of deepspeed.runtime.data_pipeline.curriculum_scheduler on the user side, and use it to retrieve the curriculum_seqlen. This implementation can be found in megatron/training.py.

2.2 Disable batch size warmup (–rampup-batch-size)

In our paper section 5.4 we demonstrate that curriculum learning (seqlen-based) provides much better training stability than the batch size warmup technique introduced by Open AI GPT-3. So when using CL you need to remove the --rampup-batch-size config in your training script. It’s not recommended to use both CL and batch size warmup, because both of them will reduce the number of tokens in a batch. Another related change you might want is to increase your micro batch size, since without batch size warmup your batch size will be fixed now.

2.3 Token-based training termination

Because curriculum learning changes length of each sequence/sample during training, it is very hard/impossible to use number of steps/samples to terminate the training exactly at the desired number of tokens. Thus we add a --train-tokens config as an alternative accurate token-based termination. We recommend increase your original --train-samples or --train-iters to a large enough number (e.g., 3X of what you used for baseline), and set --train-tokens at the exact desired number of training tokens.

2.4 Token-based LR decay

Again because curriculum learning changes the number of tokens per batch, in our paper Appendix A.2 we show that it is also necessary to change the LR decay to token-based (to avoid decaying LR too fast). Thus we add a --lr-decay-tokens which will be the number of LR decay tokens. If previously you were using --lr-decay-samples, you can calculate your --lr-decay-tokens simply by multiplying the former by full seqlen (e.g. 1K for GPT-2 and 2K for GPT-3). If previously you were using --lr-decay-iters, you can calculate your --lr-decay-tokens by multiplying the former by full seqlen and the global batch size. Then you need to replace --lr-decay-samples or --lr-decay-iters with --lr-decay-tokens in your script.

2.5 LR warmup adjustment

For LR warmup we don’t change it to token-based, because doing so for curriculum learning means slowing down the LR warmup, which is both unnecessary and harmful. However, to avoid too fast warmup you may need to adjust your --lr-warmup-samples or --lr-warmup-iters from non-CL cases for various reasons (e.g., if you used --rampup-batch-size in non-CL case, for CL we don’t use it so the number of samples per batch will be different at beginning). Assuming you want to use X tokens to warmup the LR (for OpenAI GPT-3 this was 375M tokens), then for curriculum learning case you shall set --lr-warmup-samples as X divided by the min_difficulty, or set --lr-warmup-iters as X divided by min_difficulty * --global-batch-size. This is a rough estimation based on that curriculum learning starts from seqlen min_difficulty and it won’t increase too much during LR warmup.