Concurrent requests to same Diffusion Pipeline causes index out of bounds error ( Diffusion Pipeline 同時多筆請求會 index out of bounds )

[JingRui0607]

1. Qwen-Image pipeline concurrent requests causing an index out of bounds error

Issue

When two image generation requests run concurrently using the same preloaded pipeline instance for inference (num_inference_steps=50), both requests fail around step 26 with the following error:

index 51 is out of bounds for dimension 0 with size 51

---

Suspected root cause

Both requests are sharing the scheduler’s internal timestep index.
When each request runs ~26 steps concurrently (26 × 2 = 52), the combined progression exceeds the scheduler’s timesteps size (50), resulting in an out-of-bounds access.

---

Code

import threading
from diffusers import DiffusionPipeline
import torch

self.device = "cuda"
self.torch_dtype = torch.bfloat16

self.pipe = DiffusionPipeline.from_pretrained(
    "Qwen/Qwen-Image",
    torch_dtype=self.torch_dtype,
    local_files_only=True,
    cache_dir=self.cache_dir
).to(self.device)

images = self.pipe(
    prompt=prompt,
    negative_prompt=negative_prompt,
    width=width,
    height=height,
    num_inference_steps=50,
    true_cfg_scale=true_cfg_scale,
    num_images_per_prompt=num_images_per_prompt,
    generator=generator
).images

Known workarounds

Add a lock to serialize requests
→ Safe, but eliminates concurrency

Batching requests
→ Requires request aggregation, increases latency

Multiple pipeline instances
→ VRAM usage doubles

Questions

Is the DiffusionPipeline designed to be non–thread-safe?

Are there any parameters or configurations that allow a single pipeline instance to safely handle concurrent requests?

Are there any recommended best practices for handling this type of scenario?

Log

0%| | 0/50 [00:00<?, ?it/s]
2%|▏ | 1/50 [00:00<00:18, 2.59it/s]
0%| | 0/50 [00:00<?, ?it/s]
4%|▍ | 2/50 [00:00<00:22, 2.18it/s]
2%|▏ | 1/50 [00:00<00:38, 1.28it/s]
... (omitted) ...
52%|█████▏ | 26/50 [00:20<00:19, 1.26it/s]
52%|█████▏ | 26/50 [00:20<00:19, 1.25it/s]
CRITICAL | Unexpected error: index 51 is out of bounds for dimension 0 with size 51

[hsliuustc0106]

you can check vllm-omni for online serving of qwen-image series
https://github.com/vllm-project/vllm-omni/tree/main/examples/online_serving/image_to_image