pipeline is really slow - consulting [D]
Our take
The challenges faced by practitioners in the realm of imitation learning for robotics, as highlighted in the recent article, underscore the intricate nature of AI training pipelines. The author, navigating a frustrating bottleneck in their model's training speed, raises important questions that resonate with many in the field. The combination of a shared ResNet18 encoder, a Diffusion Transformer policy backbone, and the careful structuring of their dataset reflects a thoughtful approach to tackling complex tasks. However, the unexpectedly slow training times—where GPU utilization hovers between 20-30% despite high CPU usage—point to a deeper issue that many data scientists and engineers may encounter as they push the boundaries of what current technologies can achieve.
This scenario serves as a compelling case study for understanding the nuanced dynamics of hardware and software interactions in machine learning workflows. As the author notes, even attempts to optimize the setup through batch size adjustments or synthesizing data yield only marginal improvements. This reality emphasizes the need for innovative solutions and a reevaluation of existing practices. The situation mirrors broader discussions in the AI community, such as those presented in our article on Pandas vs Polars vs DuckDB: Which Library Should You Choose?, where the choice of tools can significantly impact performance and user experience.
In analyzing the author's predicament, it becomes evident that the intricacies of model architecture, data management, and hardware capabilities are intricately linked. The use of an indexed reference-based storage system like Zarr may contribute to efficient data access, but the lack of shuffling and contiguous train/val splits could inhibit the model's ability to generalize effectively. Furthermore, the choice to freeze the encoder after initial training raises questions about the balance between leveraging pre-trained models and allowing for adaptive learning. This scenario illustrates a pivotal moment in AI development, where practitioners must navigate these complexities to optimize their systems effectively.
The implications of such challenges extend beyond individual projects; they reflect a broader trend in the AI landscape. As organizations increasingly adopt AI-driven solutions, understanding these bottlenecks—and addressing them—will be crucial for driving productivity and innovation. The need for accessible, human-centered tools that streamline the development process has never been more pressing. This aligns with the ongoing conversation in our article, From Prototype to Profit: Solving the Agentic Token-Burn Problem, highlighting the importance of creating efficient workflows that adapt to the evolving needs of users.
As we consider the future of AI and robotics, the lessons gleaned from such experiences will undoubtedly shape the next generation of tools and methodologies. The community's collective knowledge will be vital in transforming these challenges into opportunities for growth and innovation. The question that remains is how we, as an industry, will respond to these bottlenecks. Will we continue to push the envelope of what's possible, or will we reconsider our strategies to foster a more efficient, user-friendly environment for AI development? The answers will likely define the trajectory of AI technology in the years to come.
Hi, after a long debugging process and many discussions, I wanted to ask for advice from people who may have encountered similar training bottlenecks.
My goal is imitation learning for robotics.
Model / Pipeline
- Observation space:
- 4 RGB robot cameras
- image resolution: 128x128x3
- small vector of robot joint velocities (14 dims)
- Pipeline:
- Shared ResNet18 encoder processes each image
- Each image embedding dimension is 128
- Final input to policy:
- 4 * 128 image embedding
- concatenated with 14-dim state vector
- Policy backbone:
- DiT (Diffusion Transformer)
- ~8 layers
- hidden dim: 512
- 8 attention heads
- total params: ~50M
- Diffusion setup:
- predict action chunks of length ~50
- diffusion timesteps: 4
Dataset / Storage
- Dataset stored in Zarr
- Data access is indexed/reference-based (not loading huge chunks into RAM)
- train/val split is contiguous
- no shuffling
Current encoder setup
- Initially trained end-to-end
- During debugging I switched to ImageNet pretrained ResNet18
- Encoder is currently frozen
Hardware / Software
- GPU: NVIDIA A4500
- RAM: 48GB
- Storage: SSD
- CUDA: 12.8
- PyTorch: 2.9
- Precision: bf16 mixed precision (also tested fp32)
Dataloader
- batch size: 2
- 8 persistent workers
- pinned memory enabled
Preprocessing
- preprocessing is minimal
- normalization + float conversion only
- preprocessing happens inside the multimodal encoder on GPU
Profiler results (PyTorch profiler)
Current workload split:
- train_dataloader_next:
- 4.41s / 41.84s = 10.5%
- batch_to_device:
- 0.32s / 41.84s = 0.77%
- training_step:
- 12.78s = 30.5%
- backward:
- 10.83s = 25.9%
- optimizer_step (wrapper total):
- 26.09s = 62.4%
Problem
The training is much slower than I expected.
Current behavior:
- CPU utilization: ~100%
- GPU utilization: ~20–30%
- GPU utilization can even become LOWER with synthetic data
- VRAM usage is relatively low
- Throughput is around 10 iterations/sec
- Epoch of ~50k samples takes around 30 minutes
Additional observations
- Increasing batch size does NOT reduce epoch wall-clock time
- Sometimes larger batches make things slower
- Freezing the encoder did not improve throughput much
- Replacing dataset samples with synthetic/random tensors improved throughput by only ~50%
- Synthetic dataset was initialized directly in memory
I do not believe this setup should be this slow. At this rate, training takes multiple days.
For comparison, I saw papers with somewhat similar architectures mentioning ~10 hour training times on RTX 4090. With my setup 10 hours is completely not enough.
Does anyone see something obviously wrong or have suggestions for where I should investigate next?
Please help, can't know what to do!
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