Performance Tuning Guide

This tutorial provides some tips for performance tuning and debugging.

Choosing Parallel Methods

Alpa relies on analyses of primitives tensor operators to perform auto-parallelization. These analyses can be tricky for complicated computational graphs, especially those with many indexing/slicing/concatenating operators. To make sure Alpa can perform auto-parallelization correctly, we can start with simple parallel methods and gradually move to more advanced ones.

  1. Start with the basic DataParallel

    Try a small configuration of your model and run it with alpa.parallelize(func, method=alpa.DataParallel()). This is used to make sure Alpa’s basic analyses work correctly. If you see warnings like “Detect unexpected behaviors in the auto-sharding pass.”, this means some analyses fail on the model. You can submit an issue with a reproducible script to report the error.

  2. Try Zero2Parallel

    Next, try the Zero2Parallel method. You are expected to see the allocation memory size is lower if you use optimizers with element-wise states, such as Adam. Note that nvidia-smi does not correctly report the memory usage, you can use executable.get_total_allocation_size() as we did in the quick start.

  3. Try ShardParallel

    Next, try the more general ShardParallel method with different logical mesh shapes.

  4. Enable gradient accumulation.

    Next, enable gradient accumulation by

    1. replace jax.grad and jax.value_and_grad with alpa.grad and alpa.value_and_grad, respectively.

    2. set a larger global batch size and increase num_micro_batches accordingly.

  5. Try PipeshardParallel

    Try to combine pipeline parallelism and shard parallelism.

    1. Layer construction. You can use the automatic layer construction by using layer_option=AutoLayerOption(layer_num=...). You can try a few choices of the layer_num argument and see the performance. The best choice of this value depends on the number of nodes in your cluster and the number of repetitive blocks in your model. You can also do layer construction manually by using layer_option="manual" and mark_pipeline_boundary

    2. Number of micro batches. The num_micro_batches also affects the performance a lot. You can fix a large global batch size and try a few choices of num_micro_batches.

Reducing Runtime Overhead

Alpa uses a single-controller architecture. In this architecture, the user script runs on a CPU driver and sends commands to GPU workers. Users can just think of the device cluster as a single big device.

This architecture is easier to use and understand but can potentially lead to significant runtime overhead. The runtime overhead includes:

  • Send commands to launch the computation on workers

  • Send data to workers

  • Fetch data from workers

To reduce the overhead, we should avoid frequent synchronization, so we can overlap the computation with runtime scheduling. Printing or accessing the value of a DistributedArray is a case of synchronization because we have to fetch the data from workers’ GPUs to the driver’s CPU. However, accessing metadata such as shape and dtype does not need synchronization because the metadata is stored on the driver.