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# Ulysses sequence parallelism

Ulysses sequence parallelism (SP) trains on very long sequences by splitting them across multiple GPUs. To compute attention correctly, an all-to-all collective swaps the sharding dimension from sequence to attention heads. Each GPU then has the full sequence and computes attention locally over a subset of heads. A second all-to-all returns to the sequence-sharded layout so the rest of the forward pass continues locally on each chunk.

```text
                        GPU 0                       GPU 1
                   ┌───────────────┐          ┌───────────────┐
  forward          │ tokens 0..N/2 │          │ tokens N/2..N │  ← each GPU holds half the sequence
  (seq-sharded)    │  all H heads  │          │  all H heads  │
                   └───────┬───────┘          └───────┬───────┘
                           └───────── all-to-all ──────┘
                   ┌───────────────┐          ┌───────────────┐
  attention        │ all N tokens  │          │ all N tokens  │  ← now each GPU has the full sequence
  (head-sharded)   │ heads 0..H/2  │          │ heads H/2..H  │  ← but only half the heads
                   └───────┬───────┘          └───────┬───────┘
                           └───────── all-to-all ──────┘
                   ┌───────────────┐          ┌───────────────┐
  forward          │ tokens 0..N/2 │          │ tokens N/2..N │  ← back to seq-sharded
  (seq-sharded)    │  all H heads  │          │  all H heads  │
                   └───────────────┘          └───────────────┘
```

> [!NOTE]
> This guide covers the Ulysses sequence parallelism component of [ALST](https://www.deepspeed.ai/tutorials/ulysses-alst-sequence-parallelism/) (Arctic Long Sequence Training). The full ALST system also includes TiledMLP and activation checkpoint offloading, which aren't available in Transformers. See the [DeepSpeed ALST tutorial](https://www.deepspeed.ai/tutorials/ulysses-alst-sequence-parallelism/) for the complete system.

## Configure

Sequence parallelism requires Accelerate v1.12.0 and at least 2 GPUs. Configure sequence parallelism in Accelerate's [`~accelerate.ParallelismConfig`] and pass it to [`~TrainingArguments#parallelism_config`] or an [Accelerate config file](./accelerate#accelerate-config-file).

<hfoptions id="launch">
<hfoption id="parallelism_config">

```py
from accelerate.utils import ParallelismConfig, DeepSpeedSequenceParallelConfig

parallelism_config = ParallelismConfig(
    sp_backend="deepspeed",
    sp_size=4,
    dp_replicate_size=1,
    sp_handler=DeepSpeedSequenceParallelConfig(
        sp_seq_length_is_variable=True,
        sp_attn_implementation="flash_attention_2",
    ),
)

training_args = TrainingArguments(
    ...,
    deepspeed="path/to/deepspeed_config.json",
    parallelism_config=parallelism_config,
)
```

Run [accelerate launch](https://huggingface.co/docs/accelerate/en/package_reference/cli#accelerate-launch) with a [`Trainer`]-based script.

```shell
accelerate launch --num_processes 4 train.py \
--output_dir output_dir \
--per_device_train_batch_size 1 \
--gradient_accumulation_steps 1
```

</hfoption>
<hfoption id="Accelerate config file">

Run the [accelerate config](https://huggingface.co/docs/accelerate/en/package_reference/cli#accelerate-config) command and answer questions about your hardware and training setup to create a `default_config.yaml` file in your cache.

```yaml
distributed_type: DEEPSPEED
deepspeed_config:
  deepspeed_config_file: path/to/ds_config.json
machine_rank: 0
num_machines: 1
num_processes: 4
parallelism_config:
  parallelism_config_sp_size: 4
  parallelism_config_dp_replicate_size: 1
  parallelism_config_sp_backend: deepspeed
  parallelism_config_sp_seq_length_is_variable: true
  parallelism_config_sp_attn_implementation: flash_attention_2
```

Run [accelerate launch](https://huggingface.co/docs/accelerate/en/package_reference/cli#accelerate-launch) with a [`Trainer`]-based script.

```shell
accelerate launch --config_file alst_config.yaml train.py \
--output_dir output_dir \
--per_device_train_batch_size 1 \
--gradient_accumulation_steps 1
```

</hfoption>
</hfoptions>

The following fields are important for configuring sequence parallelism.

> [!TIP]
> The [`Trainer`] automatically handles DataLoader sharding, `position_ids` generation, label shifting, and loss aggregation across SP ranks. If you're writing a custom training loop, see the Accelerate [Sequence Parallelism](https://huggingface.co/docs/accelerate/concept_guides/sequence_parallelism) guide instead.

- `sp_backend` must be set to `"deepspeed"` to use Ulysses sequence parallelism.

- `sp_size` is the number of GPUs that process a single sequence in parallel. Each SP rank receives a unique data stream from the DataLoader, unlike tensor parallelism where all ranks receive identical data. The effective `dp_world_size = world_size / sp_size`, so with 4 GPUs and `sp_size=4`, `dp_world_size=1` for batch size calculations. Sequences must also be padded to a multiple of `sp_size`. Set `pad_to_multiple_of` in your data collator accordingly.

    > [!WARNING]
    > The number of attention heads must be divisible by `sp_size`. A model with 32 heads supports `sp_size` of 1, 2, 4, 8, 16, or 32.

    ```py
    from transformers import DataCollatorForLanguageModeling

    data_collator = DataCollatorForLanguageModeling(
        tokenizer=tokenizer,
        mlm=False,
        pad_to_multiple_of=sp_size,
    )
    ```

- `sp_seq_length_is_variable` controls variable sequence length handling. Set it to `True` (recommended) for varying lengths between batches. Set it to `False` when all sequences pad to a fixed length specified by `sp_seq_length`.

- `sp_attn_implementation` sets the attention backend. Supported values are `"sdpa"`, `"flash_attention_2"`, or `"flash_attention_3"`. FlashAttention is recommended, especially when packing multiple samples in a batch. SDPA can attend incorrectly across sample boundaries when samples are packed. Eager attention isn't supported because its 4D `attention_mask` is discarded for memory and scaling reasons.

## Combining with data parallelism

Sequence parallelism and data parallelism use the same GPUs, and SP doesn't require additional hardware. To run both, set `dp_replicate_size` or `dp_shard_size` so that `dp_replicate_size × dp_shard_size × sp_size` equals your total GPU count.

For example, with 8 GPUs and `sp_size=4`, set `dp_replicate_size=2` (2 × 1 × 4 = 8).

```py
parallelism_config = ParallelismConfig(
    sp_backend="deepspeed",
    sp_size=4,
    dp_replicate_size=2,
    sp_handler=DeepSpeedSequenceParallelConfig(
        sp_seq_length_is_variable=True,
        sp_attn_implementation="flash_attention_2",
    ),
)
```

## Next steps

- The Accelerate [Sequence Parallelism](https://huggingface.co/docs/accelerate/concept_guides/sequence_parallelism) guide covers the Ulysses implementation in more depth and shows how to write a custom training loop.
- The [DeepSpeed ALST tutorial](https://www.deepspeed.ai/tutorials/ulysses-alst-sequence-parallelism/) covers the full ALST system, including TiledMLP and activation checkpoint offloading.
- The [parallelism methods](./perf_train_gpu_many) guide shows how to combine sequence parallelism with other strategies like ZeRO.
- The [Ulysses Sequence Parallelism: Training with Million-Token Contexts](https://huggingface.co/blog/ulysses-sp) blog post explains how Ulysses works and how it's integrated in Accelerate, Trainer, and SFTTrainer.
