Comment by vessenes
14 days ago
This was an idea that sounded somewhat silly until it was shown it worked. The idea is that you encourage through training a bunch of “experts” to diversify and “get good” at different things. These experts are say 1/10 to 1/100 of your model size if it were a dense model. So you pack them all up into one model, and you add a layer or a few layers that have the job of picking which small expert model is best for your given token input, route it to that small expert, and voila — you’ve turned a full run through the dense parameters into a quick run through a router and then a 1/10 as long run through a little model. How do you get a “picker” that’s good? Well, it’s differentiable, and all we have in ML is a hammer — so, just do gradient descent on the decider while training the experts!
This generally works well, although there are lots and lots of caveats. But it is (mostly) a free lunch, or at least a discounted lunch. I haven’t seen a ton of analysis on what different experts end up doing, but I believe it’s widely agreed that they tend to specialize. Those specializations (especially if you have a small number of experts) may be pretty esoteric / dense in their own right.
Anthropic’s interpretability team would be the ones to give a really high quality look, but I don’t think any of Anthropic’s current models are MoE.
Anecdotally, I feel MoE models sometimes exhibit slightly less “deep” thinking, but I might just be biased towards more weights. And they are undeniably faster and better per second of clock time, GPU time, memory or bandwidth usage — on all of these - than dense models with similar training regimes.
The only thing about this which may be unintuitive from the name is an "Expert" is not something like a sub-llm that's good at math and gets called when you ask a math question. Models like this have layers of networks they run tokens through and each layer is composed of 256 sub-networks, any of which can be selected (or multiple selected and merged in some way) for each layer independently.
So the net result is the same: sets of parameters in the model are specialized and selected for certain inputs. It's just a done a bit deeper in the model than one may assume.
the most unintuitive part is that from my understanding, individual tokens are routed to different experts. this is hard to comprehend with "experts" as that means two you can have different experts for two sequential tokens right?
I think where MoE is misleading is that the experts aren't what we would call "experts" in the normal world but rather they are experts for a specific token. that concept feels difficult to grasp.
It's not even per token. The routing happens once per layer, with the same token bouncing between layers.
It's more of a performance optimization than anything else, improving memory liquidity. Except it's not an optimization for running the model locally (where you only run a single query at a time, and it would be nice to keep the weights on the disk until they are relevant).
It's a performance optimization for large deployments with thousands of GPUs answering tens of thousands of queries per second. They put thousands of queries into a single batch and run them in parallel. After each layer, the queries are re-routed to the GPU holding the correct subset of weights. Individual queries will bounce across dozens of GPUs per token, distributing load.
Even though the name "expert" implies they should experts in a given topic, it's really not true. During training, they optimize for making the load distribute evenly, nothing else.
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Also note that MoE is a decades old term, predating deep learning. It's not supposed to be interpreted literally.
> individual tokens are routed to different experts
that was AFAIK (not an expert! lol) the traditional approach
but judging by the chart on LLaMa4 blog post, now they're interleaving MoE models and dense Attention layers; so I guess this means that even a single token could be routed through different experts at every single MoE layer!
ML folks tend to invent fanciful metaphorical terms for things. Another example is “attention”. I’m expecting to see a paper “consciousness is all you need” where “consciousness” turns out to just be a Laplace transform or something.
So really it's just utilizing sparse subnetworks - more like the human brain.
The idea has also been around for at least 15 years; "ensemble learning" was a topic in my "Data Mining" textbook from around then.
Meta calls these individually smaller/weaker models "experts" but I've also heard them referred to as "bozos", because each is not particularly good at anything and it's only together that they are useful. Also bozos has better alliteration with boosting and bagging, two terms that are commonly used in ensemble learning.
MOE as an idea specific to neural networks has been around since 1991[1] . OP is probably aware, but adding for others following along, while MoE has roots in ensembling, there are some important differences: Traditional ensembles run all models in parallel and combine their outputs, whereas MoE uses a gating mechanism to activate only a subset of experts per input. This enables efficient scaling via conditional computation and expert specialization, rather than redundancy.
[1]:https://ieeexplore.ieee.org/document/6797059
If I have 5000 documents about A, and 5000 documents about B, do we know whether it's better to train one large model on all 10,000 documents, or to train 2 different specialist models and then combine them as you describe?
well you don't. but the power of gradient descent if properly managed will split them up for you. But you might get more mileage out of like 200 specialist models.
It probably depends on how much A and B overlap. If it's say English sci-fi and Chinese poetry two different models may be better.
> Anecdotally, I feel MoE models sometimes exhibit slightly less “deep” thinking
Makes sense to compare apples with apples. Same compute amount, right? Or you are giving less time to MoE model and then feel like it underperforms. Shouldn't be surprising...
> These experts are say 1/10 to 1/100 of your model size if it were a dense model
Just to be correct, each layer (attention + fully connected) has it's own router and experts. There are usually 30++ layers. It can't be 1/10 per expert as there are literally hundreds of them.
Cool. Those that mean I could just run the query through the router and then load only the required expert? That is could I feasibly run this on my Macbook?
I've been calling for this approach for a while. It's kinda similar to how the human brain has areas that are good at specific tasks
It's already used a lot — the paper I believe is from 1991, and GPT4 among many others is MoE
yes, and it's on a per-layer basis, I think!
So if the model has 16 transformer layers to go through on a forward pass, and each layer, it gets to pick between 16 different choices, that's like 16^16 possible expert combinations!
So this is kind of an ensemble sort of thing in ML like random forest and GBT?