Fun with low-rank vocab matrices (and a bonus test loss reduction?)
I was nerdsniped ! On the HF discussion page for one of the models I created for my previous post , asked if I'd considered trying out factorised embeddings -- something he's using for his model, Maba, and which was previously used in some other models, like ALBERT . It's a really nifty idea; you use a similar trick to LoRA as a way of reducing the number of parameters used for your embeddings and your output head. And for small models, those can be a disproportionate number of the total. For example, with my 163 million parameter GPT-2 small-style models, the embeddings are about 39 million of them ; the output head is the same size, so with those two taken together, that's half the model just getting stuff in and out rather than actually doing the thinking. Even if you use weight tying, like the original GPT-2 models did, you'll wind up spending 30% of your "budget" on the single matrix shared between embeddings and the output head. Similarly, while larger models have a smaller percentage spent that way, even medium-sized MoE models can wind up spending a lot of the active parameters on embeddings; I calculated that for Qwen 3.6 35B MoE, with 3B active parameters, there were about 1B of them used across both the input embeddings and the output head. So anything that can reduce that -- so long as it doesn't come at a high cost in terms of the model's capabilities -- is worth considering. You save on your parameter count, which either means smaller models and potentially quicker training, or allows you to "invest" the savings in more thinking parameters -- that is, a wider network with a higher embedding dimensionality, or a deeper one with more layers. Andrew reported that the impact of using this trick was minimal: At rank 128 on a 50k vocabulary, the penalty on validation cross-entropy is negligible (typically within +0.02 to +0.04 loss, or <0.5 perplexity delta). Now, that suggests that he's getting a loss of around 2.5 (that being the point where a loss increase of 0.04 implies a perplexity change of 0.5), which is much lower than I typically get with my GPT-2 architecture (3.5 is more like it for me), but still, perhaps I'd get good numbers too? I decided to train a few models to see what happened. The results were interesting! I found that using this trick on both the embeddings and the output head caused a somewhat larger increase in loss than Andrew described -- about 0.07 in my first test, 0.10 in the second, and 0.14 in the third. Those are relatively large, though potentially recoverable from further training or larger models. But more surprisingly, I found that using the trick on the input embeddings only seemed to reduce test loss. Before I'd started, I'd expected it to be less harmful on the input side than it was on the output side, but seeing an improvement was certainly unexpected. Getting loss down by reducing your number of parameters is not what normally happens! So, it's definitely worth digging in a bit. Let's start with the theory: what are these factorised embeddings, and how do they work? Like LoRA, they rely on low-rank factors, so firstly I'll define those. Our models are made up of a large set of matrices -- embeddings, output heads, attention weights, FFN linear layers, and so on. Making them smaller obviously reduces the number of parameters for the model, though you would expect it to come at a cost. The idea behind low-rank factors is that you can replace a larger matrix with two smaller ones that will contain most of the important information. Imagine that you've got a matrix of size m × n . Now, from matrix multiplication, we know that if you multiply an m × r matrix by an r × n one, you'll get a result that is m × n . If we do that, then we've factorised the matrix (in the same way as we might factorise 12 into 3 and 4 because 3 × 4 = 12 ), and the value r is referred to (logically enough) as the rank of the factorisation. If r is sufficiently smaller than m and n , then these two matrices -- the low-rank factors -- combined will contain fewer parameters than the full matrix. Let's make this specific, using the output head of a GPT-2 style model without weight tying. It takes in the embeddings that result from the Transformer layers (after normalisation), and converts them into logits across the vocabulary. For the GPT-2 small size, our embeddings are 768-dimensional, and the vocab size is 50,257 tokens. So when we use a linear layer to do that mapping, it has 768 × 50257 = 38597376 parameters. If we were to replace it with two matrices -- say, one of 768 × 128 and one of 128 × 50257 , then combined they would take up 768 × 128 + 50257 × 128 = 98304 + 6432896 = 6531200 parameters. That is almost six times smaller! So: the idea is that instead of creating our model with an m × n matrix, we create it with a pair, m × r and r × n , and train those instead. If all goes well, that pair will be almost as capable of learning what we want as the full one would have been, so we'll get results that are close enough. The maths and the implementation work out simply, too. For a linear layer with no bias, we can write the matrix multiplication that takes our inputs X and a weight matrix W , and produces an output Z , like this: 1 Now, if we're using a pair of low-rank factor matrices A and B instead of a full one, W , we can say the "virtual" weights we want to use for the calculation are W ′ such that: So that means that our calculation for this neural network is this: Matrix multiplication is associative, which means that you can rewrite that as: ...which hopefully you can see is the same as feeding X through a layer using A as its weights, and then feeding the result through a second one using B . In code, you've taken something like this: ...and replaced it with this: It's probably intuitively obvious, though, that this comes at a cost. If you have fewer parameters then you can store less information, so this part of your model is "dumber". If it's not clear, though, imagine that in the code above was one. You would be taking the (for GPT-2 small) 768-dimensional embedding, converting it to a single number, and then expanding that single number out to a 50,257-dimensional set of logits. Stuff is going to get lost -- the idea behind low-rank factors is that, so long as you choose an appropriate value for r (which is often referred to as the width of the low-rank bottleneck ), you won't lose the important stuff. This works surprisingly well in many cases -- in particular, LoRA, which allows you to fine-tune models that are too hard to fully train on your hardware, or to fine-tune them faster, uses it to easily train something like low rank factor "diffs" to your weight matrices 2 . But for this post, we'll try the simplest version: what if we replace both the input embeddings and the output head -- the vocabulary matrices -- in their entirety with low-rank bottlenecks? Low-rank factorisation of the vocabulary matrices is a bit of a mouthful, so based on the name "LoRA", I decided to call this trick LoRE, for low-rank embeddings. It's not strictly accurate (after all, we're doing it both to the embedding matrix at the start of the LLM and to the output head at the end), and I don't expect it to take off, but I rather like it and will use it in this post :-) The argument behind it is that the embeddings are just a simple lookup table, so are exactly the kind of place you'd expect to be able to make savings by only considering the important parts of our huge matrix; the same might apply to the output head at the end, though personally I was less convinced by this part. It's worth looking into that asymmetry a bit. My intuition was that while embeddings really are a lookup table, the output head is doing something a bit more subtle. It's projecting from the continuous embeddings that come out of our Transformer layers into logits, which we interpret (via softmax) as a probability distribution over possible next tokens. That's a significantly less simple job than mapping from "cat" to the appropriate embedding: as a (simplified) example, if you have an embedding that means something adjacent to "cat", "dog", "gerbil", "household pet" and so on, then projecting that to the appropriate values for the next token is non-trivial. 3 Still, implementation-wise, it was really simple. I decided to extend the PyTorch code that I had from Sebastian Raschka 's book " Build a Large Language Model (from Scratch) ", by adding an optional section to the model config JSON. This would allow us to switch on LoRE mode for the input embeddings and the output head independently, and would specify the rank -- that is, the r in the section above, which says how wide the low-rank bottleneck between matrices A and B would be. So, in code, my first cut changed this: ...to this: ...and this: ...became this: That in itself was a perfectly reasonable implementation of the underlying concept. But it had a problem. I wanted to compare the loss with and without LoRE, and also find out what the effects of input embedding-only and output-head-only LoRE would be. But I've found in other experiments that the initial random weights that my models start with can have a significant effect on the loss that the resulting model gets. The code above would give quite different random weights with different configurations. The fix was simple, though: With that code, so long as there is any LoRE config, we create the same weights regardless of whether it's used in the input embeddings or the output head. I would need to train a baseline which had config for LoRE but just had both and set to there -- that would actually be a normal, non-LoRE model but would have the same initial random weights in any parts that it shared with an equivalent model that did use LoRE. Additionally, Andrew had said that we need to be careful about initialisation: The main failure mode to watch for during training is early gradient spikes: scaling the bottleneck projection initialization to 1 / sqrt(r) keeps variance uniform and ensures training dynamics match the full-rank baseline from step 1. ...and later : Keep standard GPT-2 init on the embedding table, but scale the inner linear projection matrices by std = 1.0 / math.sqrt(128). (The 128 in there is because we were talking about a setup with r = 128 , which is actually what I wound up using.) Now, I was using PyTorch's default initialisation rather than the classic GPT-2 setup, but I went ahead and added a flag to handle the scaling as Andrew described: if it was set, then right at the end of the model creation I applied the initialisation to the low rank factor matrices that were innermost in my model (as in, closest to the Transformer layers): Even though that was not run for every configuration, because it happened right at the end of the model creation, it would not affect the other weights, so I figured that it would be safe. You can see the full diff from the normal GPT-2 code here . Those changes looked correct to me, and I asked GPT-6 Astra (on "extra high" thinking) to confirm that it looked right to it in the context of the discussion, which it did. That done, I double-checked with Andrew , as I really wanted this to be a clean attempt at a repro of his results. He confirmed, so it was time to train some models! I decided that I wanted five training runs to check this out thoroughly: To save time, I decided to train them in the cloud: the first three, and then the last two, in two batches of parallel runs. I used 8x A100 machines with 40 GiB VRAM per GPU on Lambda . I'll break with my normal tradition of giving detailed runthroughs of each training run, as there are (spoiler) quite a lot of them in this post! However, for reproducibility, the commands I ran for each one looked like this: ...with the run ID, , replaced with the appropriate one for the given run settings . Here are the results from the first three runs: the baseline, and the ones with LoRE at both ends, with and without smart initialisation. The model name links go to the uploaded models on Hugging Face. You can see that I had some issues with the last run; there were shortages of available instances, and I wound up starting one in Japan. Connectivity to it from my home in Portugal was really slow and unreliable; when I tried to download the model after the training run, was predicting 10 hours (which at the machine's cost of US$15.92/hour would have been insane). However, I was able to copy it to my PythonAnywhere account quickly, perhaps because that is in a US Amazon datacenter and had better connectivity to Japan, and then I could copy it from there to my machine, again quite quickly. Unfortunately while scrabbling around with that I didn't capture all of the output. Still, the important data at this stage was in the test loss column. As expected, using LoRE made the test loss worse. And smart initialisation did seem to help reduce that penalty. The loss delta, at +0.07, was worse than the +0.02 to +0.04 that Andrew described in his message, but not wildly different -- certainly small enough that it seemed plausible that you could recover it, and perhaps more, by investing the 64M parameter difference in a deeper or wider model. Promising! The cost and time savings were real, too: the run in Japan had its cost bumped up by all of the woes I had copying the model down, so the one is the best indication: US$38.55 rather than US$43.96, so a saving of more than 10%. And while going down from a bit more than 2h30m to 2h14m might not seem like much, with a larger model or a longer training run, an almost 14% reduction could be valuable in and of itself. (And that's not even considering the alternative of "reinvesting" the saved parameters in larger models.) So that was pretty good news! It was time to do the ablation runs to see whether having LoRE just on the token embeddings, or just on the output head, made things any different. I switched the setting for the remaining two models to , and kicked them off. Now, there's a bunch of interesting stuff in there, but let's focus on the test loss. My lab notes for this bit say "well, that's a bit of a shocker!" Quite amazingly -- at least to me -- we got lower test loss with the input-only LoRE, with 3.526610, than we did with the no-LoRE baseline, which got 3.599611. Things get even more interesting when you compare the different versions (excluding the non-smart-init) against each other, in terms of their difference from the no-LoRE baseline (which I'll call the delta for conciseness below). Rounding to 3dps: If you add the two single-LoRE loss deltas together -- the 0.138 and the -0.073 -- you get 0.065, which is surprisingly close to the delta of 0.072 for the dual-LoRE option, just 0.007 away. Could we be looking at two almost independent effects, one for each option of where to apply the LoRE? And was that apparent benefit of the input-only LoRE a real thing? Or was it due to chance -- perhaps we had "good luck" when initialising the LoRE weights, and "bad luck" when initialising the non-LoRE ones. If there was an overlap between the best loss you could get with LoRE and the worst loss you could get without, then this result could be within the noise. I decided I was going to do a second batch of training runs to see if I could get evidence one way or the other. But before we get into that, it's worth taking a look at some of the other numbers in those two tables. Firstly, let's take a look at the training time. The training machines looked like they were running at close to 100% during all of the runs, so tentatively let's assume that time taken is proportional to compute. Unsurprisingly, the baseline run came in with the longest time, at 9,339s. And equally unsurprisingly, the full-LoRE run that we have timing for was much faster, at 8,057s. It certainly makes sense that replacing big matrices with pairs of (much) smaller low-rank factors will mean fewer calculations on both the forward and the backward pass, meaning that the training run is faster. There was something that initially surprised me with the single-LoRE runs. Using LoRE on the output head sped things up quite a lot, but input embedding LoRE had much less effect. However, after thinking about it a bit, it became clear what was going on. An embedding module in a network is actually really quick to run, both on the forward and backward pass. While I like to think of embeddings as conceptually being a projection of a one-hot vector in vocab space into embedding space via a matrix multiplication, and I still think that's an excellent model of interpreting what's going on, in practice it's just something much more like an array lookup, with almost zero cost. Likewise on the backward pass, we only need to work out gradients for the chosen embedding, so that should be faster too. Indeed, with that all in mind, it might not have been surprising if input-only LoRE had turned out to be a bit slower than the baseline, as it, at least, needed to do forward and backward passes through actual matrices, even though they were fairly small. Doing another of those delta tables, the time difference in seconds (using the "ordinary init" run for the both-on number): You can see that if you add the delta of the single-LoRE options, you get -1,251, which is pleasingly close to the delta of the dual-LoRE one. And that is much less surprising than the loss result -- with training compute, you really would expect improvements like this to be additive. Finally, there was something interesting in the initial training loss. Because my training code prints out the loss it got on the first global step, I decided to note it down. There's something in this data that will become important later: in both of the training runs with LoRE on the output head and smart initialisation, it went up noticeably. Normally, the initial training loss on my GPT-2-style models is a bit less than 11. This fits in well with the architecture; you'd expect that a model that was predicting tokens purely randomly (well, strictly, uniformly) would have a perplexity roughly equal to the vocab size; the vocab size for the GPT-2 tokeniser is 50,257, and loss is the natural logarithm of perplexity, so that implies a loss of roughly 10.82. But both of those smart-init models with LoRE on the output head had an initial loss about one point higher, at 11.865 and 11.817, meaning that they were significantly worse than random. What might be going on here? We'll find out later :-) Anyway, for now, that was an interesting set of results. I wanted to dig in more; in particular, would that surprising improvement of test loss for the input-only LoRE model reproduce if I tried again with a different random seed? I had two working hypotheses at this point; either A solid and simple test to try to distinguish between these was just to train another set of models, with a different random seed. If I got the same kind of results, input LoRE beating non-LoRE, that would be evidence in favour of (2) rather than (1), whereas if things changed, it would work the other way. I decided not to bother with a non-smart-init model this time, so I set things up for four new models. My training code uses a random seed of 42 by default, but allows it to be overridden in the config file, so I decided to use 123 for these, and created: Once I had the config together, it was time to train the models -- but it was hard to get available instances on Lambda. I wound up having to use my script to start things up, but after a few hours I got an alert on my phone that I had instances running, and could kick things off. The second training run with input-only LoRE came in with a test loss that once again beat the baseline, so I did the full set of runs, and here are the results: Another matrix of deltas against the baseline: So, once again, input-only LoRE improved test loss (though at -0.013, it was a smaller effect than the -0.073 the previous set of models had). And once again, the sum of the input-only and output-only LoRE deltas was close to the combined-LoRE model's -- indeed it was even closer, 0.102 being just 0.001 away from 0.101. The training time results and the higher initial training loss for models with LoRE on the output head also held up, which was good to see. So at this point it was time to wrap things up -- or was it? While I was doing the second batch of training runs, I started writing up this experiment. As I normally do , I ran a draft past various AI models, and Claude Opus 5.5 on max thinking spotted something. Andrew had written : Keep standard GPT-2 init on the embedding table, but scale the inner linear projection matrices by std = 1.0 / math.sqrt(128). I'd interpreted that "inner" as meaning that the "innermost" matrices in our low rank factors -- the ones that were closest to the Transformer layers -- should be initialised as he said, with values drawn from a normal distribution with a standard deviation of 1 / r , and a mean of zero. So that implied the second matrix in the embeddings, and the first one in the output head: Claude felt that it should be the second low-rank factor in both cases -- that is, we should change It made a very good case for doing things that way, and digging into that is useful because it changes this "smart" initialisation from being a blindly-implemented "secret sauce" into something actually meaningful. Let's imagine that we're not using LoRE, and consider the output head only. It is a PyTorch , which means that: ...the values are initialized from 𝒰 ( − k , k ) , where k = 1 in_features So, we have a matrix, which we can call W , that is full of numbers drawn from that probability distribution -- a uniform (flat) distribution within the range from − k to k . For LoRE, we're replacing W with two separate matrices, which we'll call A and B : Let's say that W is shaped m × n , which means that A is m × r , and B is r × n . We want the combined effect of those two matrices W ′ = A B to start off with random numbers that look like -- in terms of their randomness -- the full matrix W . But what do the numbers actually look like in our "virtual" matrix W ′ with the code above? On the face of it, it sounds unlikely to be anything like the probability distribution for W . After all, W was initialised based on a formula that used in_features . You can see that A will have been initialised using the same distribution, as it has the same number of input features, but the number of input features for B is r , so its random numbers will have been based on that instead. Their combination into W ′ will presumably blend something from each of those two different probability distributions. We can actually work out what that will be. Maths incoming: click here to skip . Per Wikipedia , the variance of the product of two independent random scalar variables (so, not matrices, but we'll come back to that) X and Y is this: ...where σ X 2 is X 's variance, μ X is its mean, and likewise for Y . Now, let's say that the mean is zero for both X and Y (like it is for PyTorch's distribution for the initial weights, 𝒰 ( − k , k ) ). That simplifies the above: How does that apply to our matrices? Well, let's write out how we calculate W ′ by multiplying A and B , assuming that m = 3 , n = 4 , and r = 2 : By normal matrix maths, the value at position 0 , 0 in the result will be the dot product of the zeroth row in the first matrix (taken as a vector) and the zeroth column in the second: We can rewrite that for an arbitrary low rank bottleneck r like this: ...and even more generally, we can define all of the items in W ′ like this: So, every element is the sum of r products, where each product multiplies a number from whatever distribution was used to initialise A 's elements by a number from whatever was used for B 's. Using the notation from above, A 's elements have variance σ A 2 and B 's have variance σ B 2 . So that means that the variance of each component of that sum is σ A 2 · σ B 2 . There are r of them; if you add together independent variables, their variances add, so that means that the variance of the elements of our resulting W ′ is: Now, let's remember that our matrix A actually already was initialised in the way we would have liked our non-LoRE version W to have been done. The distribution of the random initial weights for an is dependent entirely on the number of input features -- and of course A has the same number of input features as W . So how can we make Var ( W ′ ) be σ A 2 ? We need to make σ B 2 = 1 / r , and then the equation above drops out like this: Now, let's look at the code (with Claude's correction in there): We're setting the weights on our B matrix to be drawn from a distribution with a standard deviation of 1 / r . SD is, of course, the square root of the variance (which is why we've been using things like σ B 2 with that squaring in there to represent variance -- σ on its own is used for standard deviation). So the variance of the matrix after we've called that on it will be 1 / r as desired, and W ′ will have random initial weights with the same variance as W would have done if we'd created it directly. That's rather satisfying :-) So that's the output head. How about the input embeddings? They're s, which are initialised somewhat differently: initialized from 𝒩 ( 0 , 1 ) That's a normal distribution, with a mean of 0 and a variance (and thus also a standard deviation) of 1. The maths above applied to any distribution with a mean of zero -- so the smart initialisation should work for those too. Our A matrix would have the correct variance, 1, and a mean of zero, and therefore so would A B , given that (for embeddings) B already had the smart initialisation. So does that mean that our W ′ "virtual" matrix created by multiplying A and B is identical in terms of randomness to the matrix W that it's replacing? Well, not quite. Remember that the output head had numbers drawn from a uniform distribution, and the embeddings from a normal distribution. We're multiplying the A matrices that are the "input" low rank factor matrices by B s that are initialised from a normal distribution. The variance of the A matrices is maintained by the smart initialisation, as we've shown, and the mean (being zero) is going to come through cleanly too. But the shape will change. I did a bit of digging around and it started to feel like a bit of a rabbit hole, but the one thing I was able to be certain of is that a variable drawn from a uniform distribution multiplied by another from a normal distribution gives a result that is neither uniform nor normal, and that normal times normal gives something called a normal product distribution (or also a product-normal distribution), which is also neither uniform nor normal. Remember that each element of our W ′ matrix is defined by this: So each one is a sum of r numbers, each of which is drawn from a product normal distribution (for embeddings) or from whatever the uniform × normal distribution is called. What shape would that be? Again: rabbit hole, but the central limit theorem states ( per Wolfram MathWorld ): the normalized sum of independent random variates with finite variances approaches a normal distribution So, for large r , we can say that the result is going to be close to a normal distribution. 4 What the smart initialisation does is make sure that our "virtual" weights, W ′ = A B , have the same mean and variance as the ones we're trying to approximate, W . But it won't keep the same shape for the distribution that they're drawn from -- certainly not for the output head, which goes from uniform to normal-ish. The embeddings are less impacted (though not completely untouched) because they were normal and are still almost so. Still, it's a neat trick! Was it what Andrew had actually meant? Having convinced myself that the smart initialisation code I'd been using was wrong, I decided to check with Andrew. My first thought was that perhaps he was using weight tying -- remember, with weight tying, the embedding matrix is just re-used in transposed form for the output head. That would mean that the embeddings would be W ′ = A B , and you'd use the smart initialisation on B , but the output head -- being that transpose -- would use W ′ T = B T A T for the same B and A , so the smart initialisation would be the other way around. But as it turned out, it was just a miscommunication . He said that he keeps his weights untied, and had been using the word "inner" in "inner linear projection matrices" to mean something different to what I'd interpreted it as. He confirmed that the real smart init code should always be applied to the second matrix, as the maths (and Claude) suggested. So that was a simple change. You can see the diff here , but in short: Now, the good news (both for my own sanity and my wallet) was that testing this fix was not going to require another four training runs. My training code is essentially deterministic -- no dropout or any other randomness is used after the model is created 5 . Because the smart initialisation code was at the end of the model creation, and smart initialisation of the output head was right at the end, that meant that changing it would not affect any other code that used randomness. So if I did two training runs -- with LoRE switched on for the output head only, and with it switched on for both embeddings and output -- then those would be "compatible" with earlier training runs that had no LoRE at all, and that only had it on the embeddings. You can see the configs here (with no explicit random seed, so they'd use my default of 42): Here are the results; I've put them into a table with their counterparts from the original seed-42 training run: Before we look at the test loss, check out that "Initial training loss" column. The weirdness that I'd noticed earlier, where that value was much worse than uniform, is gone! That actually makes some kind of sense. With the smart initialisation on the wrong part of the low-rank output head, our variance was completely wrong, and so the model was initially producing particularly bad results. 6 So that was promising! But the test loss results were even more interesting. In our original seed-42 results with the incorrect init, the output-head-LoRE-only model got a test loss of 3.737128, but here we got 3.785209. It was actually worse with the corrected smart initialisation! And that carried through to the result for models with LoRE on both input and output; the original model with the incorrect init got 3.671585, and here we got 3.741209 -- an even bigger worsening. Combining those two together, an image I like in terms of the loss landscape is that previously we were starting on a mountain that was near a deep valley -- we started with high loss, but there was a low-loss place nearby. But with the smart initialisation fixed, we were now starting on a hill with a less-deep valley nearby. So what happens if we do one of our test loss delta tables showing how each model performed against the baseline? If we add up the effect of the input-only test loss delta of -0.073 and the output-only delta of 0.186, we get 0.113 -- quite different to the delta of 0.142 that we actually got with them both. That nice "additive" property that we had, where the delta from input LoRE only plus the delta from output LoRE only summed to almost exactly the same as the delta for LoRE on both ends, appears to have gone away :-( Still, an interesting set of results! Let's bring everything together. Here are all of the training runs together in one table. I've removed the columns for the cost, the end training loss, and the training time to keep things manageable, and I've sorted it by the test loss. A few things stand out: So what does that all tell us? The numbers above suggest three things -- with important caveats below: Even more caveated, we might say: Neither the larger of the two apparent improvements from input-only LoRE nor the worsening from output-only were small -- when I was trying various interventions into my GPT-2 models (like removing dropout, scheduling the learning rate, gradient clipping and so on), many of them had a similar or smaller level of effect. Similarly, training a comparable non-LoRE model on double the number of Chinchilla-optimal tokens (ie. on 40 per parameter rather than 20) improved loss by 0.09, while investing the same compute resources to do a Chinchilla-optimal training run on a larger model improved it by roughly 0.13 over the baseline (see this post for the raw numbers). So, is input-only LoRE One Weird Trick To Improve Your Test Loss? Of course not. Are output-only LoRE and both-ends LoRE Bad Things? Not necessarily. The first thing to remember is that this is a really limited set of experiments. Although I wound up spending about US$450 on training runs (I really should be investing in building my machine up as a better training box to save on Lambda costs), I was only able to test: Even in order to say anything definitive about whether LoRE works for this specific architecture, it would be necessary to do tests over a much larger number of random seeds and different lengths (in terms of number of tokens) of training run: To get solid results on whether smart initialisation is a good thing, you'd need to try a number of seeds to see whether the poor performance of the "correct" smart initialisation against the "incorrect" one worked out as a general result. And if it did, why might that be? Beyond that: the apparent improvement in the loss from input-only LoRE is something that came up in these experiments. Maybe it would turn out to be a real thing, maybe not, but the real benefit that Andrew mentioned in his original message was about something quite different: LoRE saves a lot of parameters and only (as he said later) costs a small amount of loss. If you reinvest those saved parameters in a deeper network with more layers, you can hopefully gain back the loss that you, um, lost, and then some -- and wind up with a more capable model. So important further tests (which I think I will do, but later on) would be: It's worth noting that in my Chinchilla experiments, I scaled up the model by 2 . That's about a 41% increase, which is quite close to the 39% we save by using LoRE on both ends. So if the loss improvements carried over, perhaps it might work? On the other hand, those models were also trained on 41% more tokens, so... So there's a huge number of further experiments that would be needed to build out these results -- and that's even before we move beyond this specific architecture. Andrew reports solid results with his setup, and his model Maba does appear to be quite different. But would it extend to a Qwen-style one? Or Mistral? Next, there's what happens with scaling. LoRE is valuable in small models because embeddings and the output head are such a large proportion of the total parameter count. My guess is that it would become increasingly unhelpful as models got larger. To take an extreme case, looking at the Kimi K3 paper , it has a vocab size of 160k, and its hidden dimension (which I'll assume is what the embeddings are using) is 7,168. That means that the embedding layer uses something like 1,146,880,000 parameters, as does the output head, for a total of ~2.3B parameters (assuming they're not using weight tying, which seems likely for a monster like this). It has 104.2B active parameters per token, so if it were to use LoRE you'd save something of the order of 2% of the active parameter count. But for purposes of "reinvesting" in further layers, the real comparison point is the total number of parameters, not the active ones, and with about 2.8T total, the number used for embeddings is less than 0.1%. It doesn't sound like it would help much. Unless, of course, the benefit of the input-only LoRE loss improvement actually turned out to be a real thing, and didn't weaken as the model was scaled... But anyway, that's speculation. I think a good set of experiments for that would be to try the same training runs as before, but for larger models. Maybe just trying GPT-2 medium and large sizes would be a good start? Lots of work. If I had access to Google's resources then I think I'd be kicking off a bunch of training runs. OTOH if I had access to Google's resources I probably wouldn't be blogging about this... I think that there's one thing I'm going to take away from this. LoRE is an interesting intervention -- and I think is well worth trying out on any new model where vocab matrices are a significant fraction of the parameters, just to see if it helps. I'll be doing that in future. And if anyone else out there wants to try LoRE-style training runs, I'd love to hear from you. Any results, positive or negative, would be really interesting. Thanks for reading! And many, many thanks to Andrew for suggesting this as an idea to play around with -- it's been fun :-) This is often written Z = X W T to reflect the fact that weights are generally kept in d out × d in format, at least in PyTorch, but that's really an implementation detail . ↩ I'll write more about LoRA in the future -- I've been playing with it a bit -- but essentially, what you do is: For each matrix that you want to train (which might not be all of them -- for fine-tuning, people often only train a subset), you add on a low-rank adapter , which is a pair of matrices like our A and B above, and which sits "beside" the existing one, like this: Gradient tracking is enabled on the matrices in the low-rank adapter, and its results are added to the results of the original weights. The idea is that the adapter pair learns to create a "diff" to the results of the original weights, which allows a certain amount of training of that part of your model. And it works surprisingly well. Again: I'll write about this in more depth in the future. ↩ When I was discussing this with an LLM -- sadly I've forgotten which -- the softmax bottleneck came up here, as it did in later correspondence with Andrew. That's a reference to this paper . I'll need to work through it carefully at some later point, but the relevant point appears to be that the rank (which we can loosely read as how many independent ways the predictions can vary from one context to another) of the values coming out of softmax is limited by the rank of what is going into it. If we force our (in my GPT-2 small case) 768-dimensional embeddings through a 128-dimensional bottleneck, we're reducing their inherent rank, and it's that reduced rank that will drive softmax's, so the results will be worse than what you'd get with the original 768. Seems logical enough. Linkrot-proof reference: Yang, Z., Dai, Z., Salakhutdinov, R., & Cohen, W. W. (2018). Breaking the softmax bottleneck: A high-rank RNN language model. In International Conference on Learning Representations (ICLR) . ↩ Thanks to Claude Opus 5.5 for pointing me at the central limit theorem. I definitely need to brush up on my statistics, and I'm sure I knew about it at some point back in the 90s... ↩ Modulo CUDA jitter of various flavours -- though even then, a while back I did two training runs on 8x A100 machines with the same config, months apart, and the Safetensors files were identical bit-for-bit. So I think the effect of that in my setup is minuscule. ↩ The temptation to dig into the details of why, and exactly what the mistaken smart initialisation did to the variance of the "virtual" weights W ′ , is strong. But I will resist. ↩ When I ran a draft of this post past my editorial board of LLMs , Grok made the point that perhaps the apparent benefit of having LoRE only on the input embeddings might have been due to that model seeing significantly more tokens per parameter. It's an interesting thought! If we do the maths, the input-only and output-only LoRE models were trained on ~25 tokens per parameter and the both-ends ones were trained on about 33, vs 20 for the non-LoRE ones. However, in my Chinchilla experiments, I only got 0.09 improvement in loss by training on 40 tokens per parameter rather than 20. Now, I'm sure that gain mostly happened early in the run -- overtraining is subject to diminishing returns -- but the result of 0.073 for input-only LoRE would imply that almost all of it happened in the first extra 5 tokens per parameter, which seems dubious. Those are different training runs with different seeds, and were using my JAX code, which has a better baseline loss than my PyTorch code, so we can't compare them directly. But it feels unlikely that the effect would be drastically different. Also, if we use reductio ad absurdum on Grok's logic, it does sound a bit weak -- if we get rid of parameters but train on the same number of tokens, the higher tokens per parameter will not in general counteract the damage from the smaller model. Imagine, for example, removing 32M parameters worth of Transformer layers and training on the Chinchilla-optimal tokens for the size of the model before you did that. I'm not going to run the experiment, but I'd be... somewhat more than surprised if it improved the model. ↩ A baseline. As I said earlier, the LoRE stuff would have an effect on the randomness used for the initial weights, so I wanted to train one model with LoRE "enabled" but not actually used. Config: , . A model trained with LoRE enabled on both the input embeddings and the output head, but without the smart initialisation of the low-rank matrices. This was simply to confirm to myself that it was required. Config: , . A model with LoRE on both ends, with the smart initialisation -- essentially, what Andrew had described. Config: , . A model with LoRE on the input embeddings only, using smart initialisation if that had turned out to be necessary, and not using it if it had not. I deliberately made the config invalid, using instead of or for the flag, so that I'd be reminded to fill those in later. Config: , . A model with LoRE on the output head only, with smart initialisation the same as with (4). Config: , . What we were seeing was due to luck of the draw with the random initialisation of the weights -- good luck with LoRE, bad luck without. There really was an effect here: input-only LoRE actually improved test loss in the resulting model. A new baseline with no LoRE. Config: , . An input-only LoRE. I wanted to run this one as the first non-baseline one, because if it got worse (higher) loss than the baseline, then it would look rather like the effect I'd previously seen was due to random weight initialisation, so I might want to consider stopping there. Config: , . If I decided to continue, the next one would be LoRE on both the input and output sides. Config: , . Finally, I'd do an output head LoRE only one. Config: , . I changed the config parameter so that instead of just being or , it could be , for the original incorrect interpretation, or for the fixed one. I updated all of the existing training configurations that had set to to be . I added on yet more config for some more training runs to test this, setting to the new option. Both: , . Output-only: , . The input-only LoRE models lead the pack -- though it's a close-run thing, and the gap between and could well be within the noise at 0.013. In that post, I trained three models with different weight initialisation seeds, and got models that differed by up to ~0.017. The two models with the corrected smart initialisation are the worst -- even worse than the model with no extra initialisation, . LoRE can reduce the number of parameters in these models dramatically even if only applied to the input embeddings, and that comes with potentially an improvement in test loss (0.073 in one test, 0.013 in a second). When applied to the output head, it has the same size of reduction in the parameter count, and causes a worsening in test loss (over three tests, 0.115, 0.138 and 0.186). The results with LoRE on both ends seemed to come in somewhere around (in two of the three cases, very close to) the sum of the input-only and output-only deltas, though it doesn't seem to be a strictly additive relationship. The mathematically cleaner implementation of what I've been calling "smart initialisation" actually worked worse than the mistaken version I used originally, though this was only tested with one seed, so the evidence is much weaker. Against my own GPT-2 style model (important differences from the original: no weight tying or bias on the QKV weights, no dropout, and PyTorch default weight initialisation). With two seeds. With two options for smart initialisation, and one of them only with one seed. Over a number of tokens chosen to be Chinchilla-optimal for the baseline non-LoRE model. 7 Maybe the effect was just limited to the seeds I tested? Two is better than one, but three would be better still, four even better, and so on. Maybe the effect disappears in longer training runs? Perhaps LoRE causes some kind of loss in capacity that stops models from improving past some point. Intuitively that could make sense -- if you imagine training as being in some sense compressing information from the training data into the parameters, having fewer parameters is clearly going to limit your capacity in some sense. Isoparameter tests: Input-only LoRE saves 32M parameters, so what happens if you "spend" that on extra layers? Or alternatively make the model wider (in terms of its embedding dimension)? Or some combination of both? Output-only LoRE likewise. And, of course, both input and output. Isocompute tests. Notably, input-only LoRE doesn't save you much compute, as I mentioned earlier -- embedding layers are cheap in terms of compute, if not in terms of parameters. So here perhaps it would be more interesting to see what happens if you did output-only LoRE, but trained the model on more data so that the total training compute budget matched. Would you gain back the lost loss? This is often written Z = X W T to reflect the fact that weights are generally kept in d out × d in format, at least in PyTorch, but that's really an implementation detail . ↩ I'll write more about LoRA in the future -- I've been playing with it a bit -- but essentially, what you do is: Freeze all of the matrices that make up your parameters (eg. in PyTorch, set to ). For each matrix that you want to train (which might not be all of them -- for fine-tuning, people often only train a subset), you add on a low-rank adapter , which is a pair of matrices like our A and B above, and which sits "beside" the existing one, like this: Gradient tracking is enabled on the matrices in the low-rank adapter, and its results are added to the results of the original weights. When I was discussing this with an LLM -- sadly I've forgotten which -- the softmax bottleneck came up here, as it did in later correspondence with Andrew. That's a reference to this paper . I'll need to work through it carefully at some later point, but the relevant point appears to be that the rank (which we can loosely read as how many independent ways the predictions can vary from one context to another) of the values coming out of softmax is limited by the rank of what is going into it. If we force our (in my GPT-2 small case) 768-dimensional embeddings through a 128-dimensional bottleneck, we're reducing their inherent rank, and it's that reduced rank that will drive softmax's, so the results will be worse than what you'd get with the original 768. Seems logical enough. Linkrot-proof reference: Yang, Z., Dai, Z., Salakhutdinov, R., & Cohen, W. W. (2018). Breaking the softmax bottleneck: A high-rank RNN language model. In International Conference on Learning Representations (ICLR) . ↩ Thanks to Claude Opus 5.5 for pointing me at the central limit theorem. I definitely need to brush up on my statistics, and I'm sure I knew about it at some point back in the 90s... ↩ Modulo CUDA jitter of various flavours -- though even then, a while back I did two training runs on 8x A100 machines with the same config, months apart, and the Safetensors files were identical bit-for-bit. So I think the effect of that in my setup is minuscule. ↩ The temptation to dig into the details of why, and exactly what the mistaken smart initialisation did to the variance of the "virtual" weights W ′ , is strong. But I will resist. ↩ When I ran a draft of this post past my editorial board of LLMs , Grok made the point that perhaps the apparent benefit of having LoRE only on the input embeddings might have been due to that model seeing significantly more tokens per parameter. It's an interesting thought! If we do the maths, the input-only and output-only LoRE models were trained on ~25 tokens per parameter and the both-ends ones were trained on about 33, vs 20 for the non-LoRE ones. However, in my Chinchilla experiments, I only got 0.09 improvement in loss by training on 40 tokens per parameter rather than 20. Now, I'm sure that gain mostly happened early in the run -- overtraining is subject to diminishing returns -- but the result of 0.073 for input-only LoRE would imply that almost all of it happened in the first extra 5 tokens per parameter, which seems dubious. Those are different training runs with different seeds, and were using my JAX code, which has a better baseline loss than my PyTorch code, so we can't compare them directly. But it feels unlikely that the effect would be drastically different. Also, if we use reductio ad absurdum on Grok's logic, it does sound a bit weak -- if we get rid of parameters but train on the same number of tokens, the higher tokens per parameter will not in general counteract the damage from the smaller model. Imagine, for example, removing 32M parameters worth of Transformer layers and training on the Chinchilla-optimal tokens for the size of the model before you did that. I'm not going to run the experiment, but I'd be... somewhat more than surprised if it improved the model. ↩
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