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# Tensor parallelism

Tensor parallelism (TP) splits weight matrices column-wise or row-wise across GPUs. Each GPU holds a shard, computes a partial result, and synchronizes with an all-reduce to produce the full output.

TP relies on frequent cross-GPU communication. It works best on hardware with fast intra-node links such as NVLink.

```text
    ┌─────────────────────────────┐
    │       X  (replicated)       │
    └────┬──────────┬─────────┬───┘
         │          │         │
    ┌────▼───┐ ┌────▼───┐ ┌───▼────┐
    │ ▓▓▓ W₀ │ │ ░░░ W₁ │ │ ███ W₂ │
    │  X@W₀  │ │  X@W₁  │ │  X@W₂  │
    └────┬───┘ └────┬───┘ └───┬────┘
         └──────────┼─────────┘
               Y₀+Y₁+Y₂
    ┌────────────────────────────┐
    │          Y (full)          │
    └────────────────────────────┘
```

Transformers supports TP for architectures whose config defines `base_model_tp_plan`. Check that field first to see whether a model supports native TP.

```py
from transformers import AutoConfig

config = AutoConfig.from_pretrained("Qwen/Qwen3-0.6B")
print(config.base_model_tp_plan is not None)
print(config.base_model_tp_plan)
```

If a model supports TP, set `tp_plan="auto"` in [`~PreTrainedModel.from_pretrained`]. Transformers initializes the device mesh and shards the supported layers for you.

> [!WARNING]
> Don't use `device_map` with `tp_plan`. The two conflict at the weight-loading level. `device_map` places whole modules on specific GPUs, while `tp_plan` shards those same parameters across all GPUs.

```py
import torch

from transformers import AutoModelForCausalLM

model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen3-0.6B",
    dtype=torch.bfloat16,
    tp_plan="auto",
)
```

[`Trainer`] detects `tp_plan`, reads `tp_size` from the model, and creates a [`~accelerate.parallelism_config.ParallelismConfig`] automatically.

Launch training on one node with 4 GPUs.

```shell
torchrun --nproc-per-node 4 train_tp.py
```

## ParallelismConfig

Pass [`~accelerate.parallelism_config.ParallelismConfig`] explicitly when combining TP with other parallelism techniques like [FSDP](./fsdp).

```py
import torch

from accelerate import ParallelismConfig
from transformers import AutoModelForCausalLM, TrainingArguments

model = AutoModelForCausalLM.from_pretrained(
    "Qwen/Qwen3-0.6B",
    dtype=torch.bfloat16,
    tp_plan="auto",
)

parallelism_config = ParallelismConfig(tp_size=4)

args = TrainingArguments(
    ...,
    parallelism_config=parallelism_config,
)
```

## Next steps

- Read the [Tensor Parallelism](https://huggingface.co/spaces/nanotron/ultrascale-playbook?section=tensor_parallelism) chapter from The Ultra-Scale Playbook for more details about how it works.
- Read the [tensor parallelism inference guide](./perf_infer_gpu_multi) to learn more about partitioning strategies, manual TP plans, and implementation details.
