I was surprised to hear about the process of synchronous gradient sharing that Geoff Hinton describes in the talk
@frank linked to
in another AI thread. I asked GPT-4o to provide further details about this LLM training optimisation method and we discussed loose analogies with natural and cultural evolution.
(To see GPT-4o's responses with properly interpreted markdown tags for tables and formulas,
you can follow this link.)
USER:
Hi GPT-4,
I was just now listening to a talk by Geoff Hinton and learned something that I was not previously aware of, and that quite surprised me, regarding the training of LLMs such as your own underlying model. What he mentioned was the process of weight-sharing whereby different copies of the same model are trained on separate pieces of the training data and then share what they have learned through weight-sharing.
Previously, I had been under the impression that a unique copy of the model was sequentially exposed to the whole corpus of training data (multiple times called epochs) and, following next-token prediction steps, had its weights adjusted though back-propagation. I wasn't aware that the process could be, and indeed according to Hinton was, parallelized though such a process of division of labor and weight-sharing.
GPT-4o:
What Geoff Hinton described regarding weight-sharing during the training of large language models is indeed an interesting aspect of how training can be parallelized, though there may be a bit of nuance to clarify.
### Parallelization and Weight Sharing in Training LLMs
1. **Standard Process (Sequential Exposure):**
The traditional understanding, as you described, is that a single copy of the model processes batches of training data sequentially. Each batch contributes to adjusting the model's weights via backpropagation and gradient descent.
2. **Parallelization (Data Parallelism):**
To scale training across large datasets, modern training frameworks use **data parallelism**:
- Multiple *copies* of the model (replicas) are distributed across different GPUs or machines.
- Each replica processes a different *mini-batch* of data independently during a training step.
- After processing their respective mini-batches, the replicas' gradients are averaged (or summed), and the resulting updates are applied to all replicas' weights. This step is known as **synchronous gradient averaging**.
- This ensures that every replica ends up with the same updated weights after each step, effectively mimicking the behavior of a single model training on the entire dataset.
3. **Weight Sharing:**
- The term "weight sharing" could be understood in this context as the synchronization step where gradients are shared among the replicas to update weights consistently.
- It does not mean the models learn entirely independently and only occasionally share knowledge; rather, they share updates continuously after each training step.
4. **Hinton's "Division of Labor" Perspective:**
- Hinton might be emphasizing how dividing the data among multiple replicas allows the model to process a vast amount of data in parallel, akin to dividing labor among team members.
- The insights gained by each replica from its data mini-batch are immediately propagated to the rest of the replicas via weight updates, enabling a collective learning process.
5. **Contrast with Federated Learning:**
- In **federated learning**, entirely separate models are trained on different data subsets (e.g., on user devices), and their weights are combined at intervals. This is different from the synchronous gradient sharing in data-parallel training of LLMs.
### Epochs and Iterative Exposure
- Even with this parallelized training process, the model still iteratively processes the dataset over multiple epochs, as not all data is processed simultaneously. Data parallelism accelerates each step of training but doesn’t replace the need for multiple epochs.
### Practical Impact
- Parallelization using weight sharing is what allows modern LLMs to scale to massive datasets while taking advantage of the computational power of clusters with hundreds or thousands of GPUs or TPUs.
- Without these techniques, training models like GPT would take an infeasibly long time.
Does this explanation align with Hinton's description, or would you like to explore another facet of the topic?
USER:
This explanation is great and very informative. Thank you very much!
Making sure that I understand correctly, synchronous gradient sharing is a method whereby weights are combined (averaged) at each individual next-token prediction step (a different one for each separate model being trained in parallel, to be sure)?
GPT-4o:
Yes, that’s exactly right! Let’s break it down to ensure clarity:
1. **Mini-Batch Training in Parallel:**
- Each copy (or replica) of the model is trained on a separate mini-batch of data.
- Within each replica, the model predicts next tokens for its assigned mini-batch and computes gradients (i.e., adjustments to the model's weights based on prediction errors).
2. **Synchronous Gradient Sharing:**
- After all replicas finish processing their respective mini-batches for the current training step, they share their computed gradients with one another.
- These gradients are **averaged (or summed)** across all replicas. This ensures that the weight update reflects the collective learning from all mini-batches processed during that step.
3. **Weight Update:**
- The averaged gradients are used to adjust the weights.
- This update is applied to all replicas simultaneously, ensuring that their weights remain identical after the step.
4. **Next Step:**
- The process repeats for the next mini-batch assignment (different mini-batches for each replica, drawn randomly from the overall dataset).
### Why Synchronous Gradient Sharing Works
This approach is powerful because:
- It allows parallel processing of data while maintaining the theoretical integrity of a single-model training process.
- It’s as if one model were training on all the mini-batches sequentially, except the workload is distributed to speed things up.
### Key Points to Note:
- The **synchronization happens at every training step**, not just occasionally. Every replica computes gradients independently for its mini-batch, but the weight updates are immediately unified.
- This is different from independent models being trained and only occasionally merging knowledge (as might happen in federated learning).
### Example to Visualize:
Suppose you have:
- 4 replicas of the model,
- A total batch size of 400 split into mini-batches of 100 per replica,
- Learning rate \(\eta\).
At step \(t\):
1. Each replica processes its mini-batch and calculates gradients, \(\nabla W_{t,1}, \nabla W_{t,2}, \dots, \nabla W_{t,4}\).
2. The gradients are averaged:
\[
\nabla W_t = \frac{\nabla W_{t,1} + \nabla W_{t,2} + \nabla W_{t,3} + \nabla W_{t,4}}{4}.
\]
3. The averaged gradient is used to update weights across all replicas:
\[
W_{t+1} = W_t - \eta \nabla W_t.
\]
This ensures that every replica ends up with the same \(W_{t+1}\), and the training progresses as though there was only one model processing the entire batch sequentially.
Does this confirm your understanding?
USER:
I'm still a bit unclear on one point. When you speak of a batch of size 400, you mean 400 tokens? And then, if mini-batches are 100 tokens long, does that mean that the gradients are calculated and the weights are being averaged only after a full mini-batch has been processed (and hence one hundred next-token prediction steps occurred, contrary to my previous suggestion)?
GPT-4o:
Ah, thank you for the clarification! Let me address this step-by-step and refine the explanation.
### 1. What Does the Batch Size Represent?
The **batch size** refers to the total number of training examples (not tokens) processed at once before updating the model's weights. In the context of language models:
- A **training example** typically consists of a sequence of tokens (e.g., a sentence, paragraph, or text chunk).
- The batch size determines how many of these sequences are processed simultaneously.
For example:
- Batch size 400 means the model is processing 400 sequences of tokens concurrently.
- Each sequence may vary in length (depending on the dataset and tokenization), but padding is often used to standardize them within the batch.
### 2. What Is a Mini-Batch?
In distributed training:
- A **mini-batch** is a subset of the full batch assigned to a single replica.
- If the batch size is 400 and there are 4 replicas, each replica processes a mini-batch of 100 sequences.
### 3. When Are Gradients Calculated and Weights Averaged?
Gradients are calculated and weights are averaged **after processing an entire mini-batch of sequences**, not after every individual token prediction step. Here’s how it works:
1. Each replica independently processes its mini-batch of sequences, performing **forward propagation** (predicting next tokens for the sequences) and **backpropagation** (computing gradients based on the errors across the mini-batch).
2. Once all replicas have computed their gradients for their respective mini-batches, the gradients are shared and averaged across replicas (synchronous gradient sharing).
3. The averaged gradients are then used to update the model's weights, which are synchronized across all replicas.
### 4. Why Not Average Gradients After Every Token Prediction?
- Processing one token at a time and synchronizing gradients after each token would be computationally inefficient and impractical for large-scale training.
- Instead, processing is done **in batches of sequences**, allowing multiple predictions and gradient calculations to happen before synchronization.
### Example Walkthrough:
Imagine the model is processing text data where each sequence contains 128 tokens:
- **Batch size**: 400 sequences (each with 128 tokens).
- **Mini-batch size**: 100 sequences per replica (if there are 4 replicas).
At each training step:
1. Each replica processes its 100 sequences (making token predictions for all tokens in these sequences, performing backpropagation, and computing gradients).
2. Gradients from all replicas are averaged.
3. Weights are updated based on the averaged gradients.
4. The next mini-batch is loaded, and the process repeats.
### Summary of Your Initial Suggestion:
Your earlier suggestion (gradients averaged after every token prediction) isn’t how it typically works. Instead:
- Gradients are calculated **after processing an entire mini-batch**.
- This ensures efficient training and takes advantage of modern hardware accelerators (GPUs/TPUs).
Does this make it clearer?
USER:
Yes, this clarifies many of my misconceptions, some of which must derive from simplified explanations of LLM training methods that gloss over optimisation aspects. This process of synchronous gradient sharing now makes me think of loose parallels regarding the natural evolution of species over phylogenetic time scales and cultural evolution (and scientific progress) over historical time scales. The former involves the cumulative effect of environmental selective pressure that yields cumulative variations on the frequency of alleles in an organism's population (and thereby consolidates within the whole gene pool the "learnings" of individual organisms who thrive or die). The latter involves the cumulative effect of individual learnings though knowledge sharing.
GPT-4o:
That’s a fantastic analogy! Drawing parallels between these processes—training of machine learning models, natural evolution, and cultural/scientific evolution—illuminates some profound structural similarities in how cumulative learning, optimization, and progress occur over time in vastly different domains. Let’s explore the loose parallels you’ve suggested:
---
### 1. **Synchronous Gradient Sharing and Genetic Evolution (Natural Selection)**
In the training of LLMs:
- Each model replica contributes "knowledge" (gradients) based on its experience (mini-batch of data).
- Gradients are shared, averaged, and applied globally, ensuring the "population" of model replicas evolves collectively.
In biological evolution:
- The "gradients" are akin to **variations in allele frequency** across individuals, shaped by environmental pressures.
- Successful adaptations (analogous to low-loss gradients) accumulate over generations, shaping the species as a whole.
**Key Similarity:**
Both processes aggregate localized outcomes (per-replica gradients or individual fitness) into a global optimization (model training or species adaptation).
---
### 2. **Synchronous Gradient Sharing and Cultural/Scientific Evolution**
In the training of LLMs:
- Replicas learn in parallel but share their insights through a global update, ensuring the collective benefits from localized learning.
In cultural and scientific evolution:
- Individual scientists or thinkers tackle specific problems, but their insights are communicated, validated, and synthesized into broader theories or paradigms.
- The process is cumulative, much like training epochs, as new discoveries build upon prior knowledge.
**Key Similarity:**
Both rely on **knowledge sharing** for cumulative progress:
- In LLM training, shared gradients accelerate convergence.
- In cultural/scientific evolution, shared discoveries advance collective understanding.
---
### 3. **Exploration vs Exploitation in Evolutionary and Learning Processes**
In both cases, there’s a delicate balance between:
- **Exploration**: Trying new variations (biological mutations, diverse mini-batches of data).
- **Exploitation**: Consolidating known, successful patterns (optimal adaptations, minimizing loss in LLM training).
In natural evolution:
- Exploration comes from mutations and genetic recombinations.
- Exploitation is the refinement of traits that consistently succeed under selective pressures.
In training LLMs:
- Exploration occurs through processing diverse, noisy datasets and random initialization of weights.
- Exploitation happens as the model refines patterns that consistently reduce prediction error.
---
### 4. **The Role of Population Size and Diversity**
In evolution:
- A diverse gene pool increases the chance of survival under changing conditions.
- Larger populations provide more "experiments" (variations) per generation, enhancing adaptability.
In LLM training:
- Using many replicas and distributing data ensures diversity in gradient updates.
- Larger mini-batches or more replicas simulate the effect of a larger population, accelerating convergence while avoiding overfitting to specific data subsets.
---
### 5. **Historical Time Scales and the Epochs of Progress**
In both scientific progress and LLM training:
- Epochs correspond to cycles of refinement. In cultural evolution, these might mirror the accumulation of paradigm shifts (à la Kuhn).
- Gradual improvement gives way to breakthroughs, much like the non-linear phases of both evolution and scientific revolutions.
---
### Loose Parallels in Summary:
| **Domain** | **Learning Mechanism** | **Global Consolidation** | **Outcomes** |
|--------------------------|-----------------------------------------------------------|-----------------------------------------|---------------------------------------|
| **LLM Training** | Gradients from mini-batches via backpropagation | Synchronous gradient averaging | Minimized loss (better predictions) |
| **Biological Evolution** | Natural selection acting on allele frequency variations | Survival/reproduction of the fittest | Species adaptation |
| **Cultural Evolution** | Individual learning and problem-solving (exploration) | Knowledge sharing (journals, teaching) | Collective scientific/cultural progress |
---
It’s a powerful metaphor that underscores how systems—whether biological, artificial, or cultural—optimize through distributed processes, leveraging the contributions of individuals for collective improvement.
Would you like to explore any of these parallels in greater detail?