As early as the 90s it was observed that CPU speed (FLOPs) was improving faster than memory bandwidth. In 1995 William Wulf and Sally Mckee predicted this divergence would lead to a “memory wall”, where most computations would be bottlenecked by data access rather than arithmetic operations.

Over the past 20 years peak server hardware FLOPS has been scaling at 3x every 2 years, outpacing the growth of DRAM and interconnect bandwidth, which have only scaled at 1.6 and 1.4 times every 2 years, respectively.

Thus for training and inference of LLMs, the performance bottleneck is increasingly shifting toward memory bandwidth. Particularly for autoregressive Transformer decoder models, it can be the dominant bottleneck.

This is driving the need for new tech like Compute-in-memory (CIM), also known as processing-in-memory (PIM). Hardware in which operations are performed directly on the data in memory, rather than transferring data to CPU registers first. Thereby improving latency and power consumption, and possibly sidestepping the great “memory wall”.

Notably to compare ASIC and FPGA hardware across varying semiconductor process sizes, the paper uses a fitted polynomial to extrapolate to a common denominator of 16nm:

> Based on the article by Aaron Stillmaker and B.Baas titled ”Scaling equations for the accurate prediction of CMOS device performance from 180 nm to 7nm,” we extrapolated the performance and the energy efficiency on a 16nm technology to make a fair comparison

But extrapolation for CIM/PIM is not done because they claim:

> As the in-memory accelerators the performance is not based only on the process technology, the extrapolation is performed only on the FPGA and ASIC accelerators where the process technology affects significantly the performance of the systems.

Which strikes me as an odd claim at face value, but perhaps others here could offer further insight on that decision.

Links below for further reading.

https://arxiv.org/abs/2403.14123

https://en.m.wikipedia.org/wiki/In-memory_processing

http://vcl.ece.ucdavis.edu/pubs/2017.02.VLSIintegration.Tech...

https://news.ycombinator.com/item?id=41069685

I like the one that’s in RAM sticks with an affordable price. I could imagine cramming a bunch of them into a 1U board with high-speed interconnects. Or just PCI cards full of them.

NVMeOF has been scaling with fabric performance and it's been great.

400GB/s interconnect on the H100 DGX today.

Has anyone else heard of this idea or have any insight on it?

Sara Acour has some answers, e.g. https://scholar.google.com/citations?view_op=view_citation&h...

I've always been partial to the idea of two parallel surfaces with optical links, Make a connection machine style hypercube where the bit of the ID of every processor indicates its location in the hypercube. Place all of the even parity CPUs on one surface and the odd parity CPUs on the other surface, every CPU would have line of sight on its neighbour in the hypercube (as well as the diametrically opposed CPU with all the ID bits flipped)

Groq and Cerebras are probably that kind of architecture

"rapid growth of AI models is being constrained by a growing disparity between compute performance and memory bandwidth. While next-generation models like GPT-5 are expected to reach an unprecedented scale of 3-5 trillion parameters, the technical bottleneck of memory bandwidth is becoming a critical obstacle to realizing their full potential."

https://www.lycee.ai/blog/2024-09-04-samsung-memory-bottlene...

Each cell would have 4 input bits, 1 each from the neighbors, and 4 output bits, again, one to each neighbor. In the middle would be 64 bits of shift register from a long scan chain, the output of which goes to 4 16:1 multiplexers, and 4 bits of latch.

Through the magic of graph coloring, a checkerboard pattern would be used to clock all of the cells to allow data to flow in any direction without preference, and without race conditions. All of the inputs to any given cell would be stable.

This allows the flexibility of an FPGA, without the need to worry about timing issues or race conditions, glitches, etc. This also keeps all the lines short, so everything is local and fast/low power.

What it doesn't do is be efficient with gates, nor give the fastest path for logic. Every single operation happens effectively in parallel. All computation is pipelined.

I've had this idea since about 1982... I really wish someone would pick it up and run with it. I call it the BitGrid.

The BitGrid is bit oriented, not bytes or words, which makes if flexible for doing FP8, FP16, or whatever other type of calculation is required. Since everything is clocked at the same rate/step, data flows continuously, without need for queue/dequeue, etc.

Ideally, a chip could be made that's a billion cells. The structure is simple, and uniform. An emulator exists, and that would return the exact same answer (albeit much slower) than a billion cell chip. You could divide up simulation among a network of CPUs to speed it up.

https://arxiv.org/pdf/2406.08413 Memory Is All You Need: An Overview of Compute-in-Memory Architectures for Accelerating Large Language Model Inference

Here’s an example of visualizing CNN layers; https://www.researchgate.net/figure/Visualization-of-example...

You seldom see it.

For new foundation models it's even worse, because there's some fancy experiment every time and the llama.cpp team needs two weeks to figure out how to implement it so the model can even run.

https://arxiv.org/abs/2406.02528

https://news.ucsc.edu/2024/06/matmul-free-llm.html

> ..avoid using matrix multiplication using two main techniques. The first is a method to force all the numbers within the matrices to be ternary, meaning they can take one of three values: negative one, zero, or positive one. This allows the computation to be reduced to summing numbers rather than multiplying.. Instead of multiplying every single number in one matrix with every single number in the other matrix.. the matrices are overlaid and only the most important operations are performed.. researchers were able to maintain the performance of the neural network by introducing time-based computation in the training of the model. This enables the network to have a “memory” of the important information it processes, enhancing performance.

> ... On standard GPUs.. network achieved about 10 times less memory consumption and operated about 25 percent faster.. could provide a path forward to enabling the algorithms to run at full capacity on devices with smaller memory like smartphones.. Over three weeks, the team created a [FPGA] prototype of their hardware.. surpasses human-readable throughput.. on just 13 watts of power. Using GPUs would require about 700 watts of power, meaning that the custom hardware achieved more than 50 times the efficiency of GPUs.

To quote from an old article about such acceleration which sees 19x improvements over DRAM + GPU:

   Since MAC operations consume the dominant part of most ML workload runtime,
   we propose in-subarray multiplication coupled with intra-bank accumulation.
   The multiplication operation is performed by performing AND operations and
   addition in column-based fashion while only adding less than 1% area overhead.

Combining memory with computation seems good in theory, but it is difficult to do in practice.

The fabrication technologies for DRAM and for computational devices are very different. If you implement computational units on a DRAM chip, they will have a much worse performance than those implemented with a dedicated fabrication process, so for instance their performance per watt and per occupied area will be worse, leading to higher costs than for using separate memories and computational devices.

The higher cost might be acceptable in certain cases if a much higher performance is obtained. However it is unavoidable that unlike with a CPU/GPU/FPGA, where you can easily reprogram the device to implement a completely different algorithm, a device with in-memory computation would be much less flexible, so it either will implement extremely simple operations, like adding to memory or multiplying the memory, which would not increase much the performance due to communication overheads, or it would implement some more complex operations, which might implement some ML/AI algorithm that is popular for the moment, but which would be hard to use to implement better algorithms when such algorithms are discovered.

https://arxiv.org/pdf/2406.08413

> At their core, NVM arrays are arranged in two dimensions and programmed to discrete conductances (Fig. 5). Each crosspoint in the array has two terminals connected to a word line and a bit line. Digital inputs are converted to voltages, which then activate the word lines. The multiplication operation is performed between the conductance gij and the voltage Vi by applying Ohm’s law at each cell, while currents Ij accumulate along each column according to Kirchhoff’s current law

Sounds like the compute element is embedded within the DRAM but instead of doing a digital computation it's done in analog space (which feels a bit wrong since the DAC+ADC combo would eat quite a bit of power but maybe it's easier to manufacture or other reasons to do it in analog space).

Or you're saying it would be better with flash storage because it could be used for even larger models. I think that's right but my overall point holds - removal of the DRAM controller could free up significant amounts of DRAM bandwidth (like 20x IIRC) and reduction in power (by 100x IIRC). There's value in that regardless and it would just be a free speedup and would significantly benefit existing LLMs that rely on RAM. An analog compute circuit embedded within flash would be usable basically only for today's LLMs architecture and not be very flexible and require a huge change in how this stuff works to take advantage. Might still make sense if the architecture remains largely unchanged and other approaches can't be as competitive, but it does lock you into a design more than something more digitally programmable that can also do other things.

Yes, you are. NVM stands for 'non-volatile memory", which is literally the opposite of DRAM.

Analog computation can be done using any memory cell technology (transistor, capacitor, memristor, etc), but the result will always go through ADC to be stored in a digital buffer.

Flash does not provide any advantages as far as model size, the size of crossbar is constrained by other factors (e.g. current leakage), and typically it's in the ballpark of 1kx1k matmuls. You simply put more of them on a chip, and try to parallelize as much as possible.

But I largely agree with your conclusion.

The existing CPUs, GPUs and FPGAs are full of SRAM that is intimately mixed with the computational parts of the chips and you could not find any structure improving on that.

All the talk about in-memory computing is strictly about DRAM, because only DRAM could increase the amount of memory from the up to hundreds of MB of memory that is currently contained inside the biggest CPUs or GPUs to the hundreds of GB that might be needed by the biggest ML/AI applications.

All the other memory technologies mentioned in the paper linked by you are many years or even decades away from being usable as simple memory devices. In order to be used for in-memory computing, one must first solve the problem of making them work as memories. For now, it is not even clear if this simpler problem can be solved.

Which companies are using DRAM for in-memory computing?

Flash cannot be used for in-memory computing, because writing it is too slow.

According to what they say, they have an inference device that uses analog computing for inference. They have a flash memory, but that stores only the weights of the model, which are constant during the computation, so the flash is not a working memory, it is used only for the reconfiguration of the device, when a new model is loaded.

Analog computing for inference is actually something that is much more promising than in-memory computing, so Mythic might be able to develop useful devices.

d-Matrix appears to do true in-memory computing, but the price of their devices for an amount of memory matching a current GPU will be astronomical.

Perhaps there will be organizations willing to pay huge amounts of money for a very high performance, like those which are buying Cerebras nowadays, but such an expensive technology will always be a niche too small to be relevant for most users.

You can remove all the flash memory and replace all its bits with suitable connections to ground or the supply voltage, corresponding to the weights of the model.

Then the device without any flash memory will continue to function exactly like before, computing the inference algorithm without changes. Therefore it should be obvious that this is not in-memory computing, if you can remove the memory without affecting the computing.

The memory is needed only if you want to be able to change the model, by loading another set of weights.

The flash memory is a configuration memory, exactly like the configuration memories of logic devices like FPGAs or CPLDs. In FPGAs or CPLDs you do the same thing, you load the configuration memory with a new set of values, then the FPGA/CPLD will implement a new logic device, until the next reloading of the configuration memory.

Exactly like in this device, the configuration memory of the FPGAs/CPLDs, which may be a flash memory too, is not counted as a working memory. The FPGAs/CPLDs contain memories and registers, but those are distinct from the configuration memory and they cannot be implemented with flash memory, like the configuration memory.

In this inference device with analog computing there must also be a working memory, which contains mutable state, but that must be implemented with capacitors that store analog voltages.

You might talk about an in-memory computing only with reference to the analog memory with capacitors, but even this description is likely to be misleading, because from the point of view of the analog memory it is more probable that the structure of the inference device is some kind of dataflow structure, where the memory capacitors implement some kind of analog shift registers and not anything resembling memory cells in which information is stored for later retrieval.

(Context in the abstract is "First, we present the accelerators based on FPGAs, then we present the accelerators targeting GPUs and finally accelerators ported on ASICs and In-memory architectures" and the section title in the paper body is "V. In-Memory Hardware Accelerators")

Every time I land on that site I'm so confused / lost in it's interface (or lack there of) I usually end up leaving without getting to the content.

But I find it quite interesting that I've managed to completely miss the big obvious blue buttons every time, I just immediately scan down to the first paragraph.

The cynic in me guesses it's because I'm so used to extraneous content taking up space that I instinctively skim past, but maybe that's too pessimistic & there's another UX/psychological reason for it.