Analysis With its newly formed partnership with Arm server processor designer Ampere Computing, Qualcomm is slowly establishing itself as AI infrastructure startups’ best friend.
Announced during Ampere’s annual strategy and roadmap update on Thursday, the duo promised a 2U machine that includes eight Qualcomm AI 100 Ultra accelerators for performing machine-learning inference and 192 Ampere CPU cores. “In a typically 12.5kW rack, this equates to hosting up to 56 AI accelerators with 1,344 computation cores, while eliminating the need for expensive liquid cooling,” Ampere beamed.
Ampere and its partner Oracle have gone to great lengths to demonstrate that running the large language models (LLMs) behind many popular chatbots is entirely possible on CPUs, provided you set your expectations appropriately. We’ve explored this concept at length, but in a nutshell limited memory bandwidth means that CPUs are generally best suited to running smaller models between seven and eight billion parameters in size and usually only at smaller batch sizes — that is to say fewer concurrent users.
This is where Qualcomm’s AI 100 accelerators come in, as their higher memory bandwidth allows them to handle inferencing on larger models or higher batch sizes. And remember that inferencing involves running operations over the whole model; if your LLM is 4GB, 8GB, or 32GB in size, that’s a lot of numbers to repeatedly crunch every time you want to generate the next part of a sentence or piece of source code from a prompt.
Why Qualcomm?
When it comes to AI chips for the datacenter, Qualcomm isn’t a name that tends to come up all that often.
Most of the focus falls on GPU giant Nvidia with the remaining attention split between Intel’s Gaudi and AMD’s Instinct product lines. Instead, most of the attention Qualcomm has garnered has centered around its AI smartphone and notebook strategy.
However, that’s not to say Qualcomm doesn’t have a presence in the datacenter. In fact, its AI 100 series accelerators have been around for years, with its most recent Ultra-series parts making their debut last fall.
The accelerator is a slim, single slot PCIe card aimed at inferencing on LLMs. At 150W the card’s power requirements are rather sedate compared to the 600W and 700W monsters from AMD and Nvidia that are so often in the headlines.
Despite its slim form factor and relatively low-power draw, Qualcomm claims a single AI 100 Ultra is capable of running 100 billion parameter models while a pair of them can be coupled to support GPT-3 scale models (175 billion parameters).
In terms of inference performance, the 64-core card pushes 870 TOPs [PDF] at INT8 precision and is fueled by 128GB of LPDDR4x memory capable of 548GB/s of bandwidth.
Memory bandwidth is a major factor for scaling AI inferencing to larger batch sizes.
Generating the first token which, with chatbots we experience as the delay between submitting a prompt and the first word of the response appearing, is often compute bound. However, beyond that, each subsequent word generated tends to be memory bound.
This is part of the reason that GPU vendors like AMD and Nvidia have been moving to larger banks of faster HBM3 and HBM3e memory. The two silicon slingers’ latest chips boast memory bandwidths in excess of 5TB/s, roughly ten times that of Qualcomm’s part.
To overcome some of these limitations, Qualcomm has leaned heavily on software optimizations, adopting technologies like speculative decoding and micro-scaling formats (MX).
If you’re not familiar, speculative decoding uses a small, lightweight model to generate the initial response and then uses a larger model to check and correct its accuracy. In theory, this combination can boost the throughput and efficiency of an AI app.
Formats like MX6 and MX4, meanwhile, aim to reduce the memory footprint of models. These formats are technically a form of quantization that compresses model weights to lower precision, reducing the memory capacity and bandwidth required.
By combining MX6 and speculative decoding, Qualcomm claims these technologies can achieve a fourfold improvement in throughput over a FP16 baseline.
For Ampere, Qualcomm offers an alternative to Nvidia GPUs, which already work with its CPUs, for larger scale AI inferencing.
AI upstarts amp Qualcomm’s accelerators
Ampere isn’t the only one that’s teamed up with Qualcomm to address AI inferencing. There’s a missing piece to this puzzle that hasn’t been addressed: Training.
Waferscale AI startup Cerebras, another member of Ampere’s AI Platform Alliance, announced a collaboration with Qualcomm back in March alongside the launch of its WSE-3 chips and CS-3 systems.
Cerebras is unique among AI infrastructure vendors for numerous reasons, the most obvious being their chips are literally the size of dinner plates and now each boast 900,000 cores and 44GB of SRAM — and no, that’s not a typo.
As impressive as Cerebra’s waferscale chips may be, they’re designed for training models, not running them. This isn’t as big a headache as it might seem. Inferencing is a far less vendor-specific endeavor than training. This means that models trained on Cerebra’s CS-2 or 3 clusters can be deployed on any number of accelerators with minimal tuning.
The difference with Qualcomm is that the two are making an ecosystem play. As we covered at the time, Cerebras is working to train smaller, more accurate, and performant models that can take full advantage of Qualcomm’s software optimizations around speculative decoding, sparse inference, and MX quantization.
Building the ecosystem
Curiously, Qualcomm isn’t listed as a member of the AI Platform Alliance, at least not yet anyway. Having said that, the fact that Qualcomm’s AI 100 Ultra accelerators are already on the market may mean they’re just a stop gap while other smaller players within the alliance catch up.
And in this regard, the AI Platform Alliance has a number of members working on inference accelerators at various stages of commercialization. One of the more interesting we’ve come across is Furiosa — and yes, that is a Mad Max reference. The chip startup even has a computer vision accelerator codenamed Warboy, if there was any doubt.
Furiosa’s 2nd-gen accelerator codenamed RNGD — pronounced Renegade because in the post-AI world, who needs vowels — is fabbed on a TSMC 5nm process and boasts up to 512 teraFLOPS of 8-bit performance or 1,024 TOPS at INT4. So, for workloads that can take advantage of lower 4-bit precision, the 150W chip has a modest advantage over Qualcomm’s AI 100.
The chip’s real bonus is 48GB of HBM3 memory which, while lower in capacity than Qualcomm’s part, boasts nearly three times more bandwidth at 1.5TB/s.
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When we might see the RNGD in the wild remains to be seen. However, the key takeaway from the AI Platform Alliance seems to exist so that individual startups can focus on tackling whatever aspect of the AI spectrum they’re best at and lean on the others for the rest, whether that’s through direct collaborations or standardization.
In the meantime, it seems Qualcomm has picked up a few new friends along the way.
Filling a gap
Ampere’s reliance on Qualcomm for larger models at higher batch sizes may be short lived, thanks to architectural improvements introduced in the Armv9 instruction set architecture.
As we previously reported, the custom cores the CPU vendor developed for its Ampere One family of processors utilized elements of both the older v8 and newer v9 architectures. As we understand it, the v9-A spec introduced Scalable Matrix Extension 2 (SME2) support aimed at accelerating the kinds of matrix mathematics common in machine learning workloads. However for the moment, we’re told Ampere’s current chips are handling AI inferencing jobs using its twin 128-bit vector units.
It’s reasonable to believe future Arm-compatible chips from Ampere and others could make use of SME2. In fact, on the client side, Apple’s new M4 SoC is Armv9-compatible with SME2 acceleration baked into its cores, The Register has learned from trusted sources.
Qualcomm was actually one of the first to adopt Armv9, in some of its Snapdragon system-on-chips. However, the chip biz appears to be going back to Armv8, when using CPU designs from its Nuvia acquisition, a decision we have little doubt has become a point of contention with Arm. While Arm would like its customers to pick v9 with SME2 for CPU-based AI inference, Qualcomm is instead taking the line that v8 is fine with inference offloaded from the CPU to another processing unit.
In datacenter land, memory bandwidth will remain a bottleneck regardless of Armv9 or SME2. The introduction of speedier multiplexer combined rank (MCR) DIMMs should help, with 12 channel platforms capable of achieving 825GB/s of bandwidth.
As we’ve seen from Intel’s Xeon 6 demos, this bandwidth boost should allow models up to 70 billion parameters to run reasonably at 4-bit precision on a single CPU. ®
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