Failback Fails. Massive Failure!

By _Quadric Blog

May 30, 2024

Not just a little slow down. A massive failure!

Conventional AI/ML inference silicon designs employ a dedicated, hardwired matrix engine – typically called an “NPU” – paired with a legacy programmable processor – either a CPU, or DSP, or GPU. You can see this type of solution from all of the legacy processor IP licensing companies as they try to reposition their cash-cow processor franchises for the new world of AI/ML inference. Arm offers an accelerator coupled to their ubiquitous CPUs. Ceva offers an NPU coupled with their legacy DSPs. Cadence’s Tensilica team offers an accelerator paired with a variety of DSP processors. There are several others, all promoting similar concepts.

The common theory behind these two-core (or even three core) architectures is that most of the matrix-heavy machine learning workload runs on the dedicated accelerator for maximum efficiency and the programmable core is there as the backup engine – commonly known as a Fallback core – for running new ML operators as the state of the art evolves.

As Cadence boldly and loudly proclaims in a recent blog : “The AI hardware accelerator will provide the best speed and energy efficiency, as the implementation is all done in hardware. However, because it is implemented in fixed-function RTL (register transfer level) hardware, the functionality and architecture cannot be changed once designed into the chip and will not provide any future-proofing.” Cadence calls out NPUs – including their own! - as rigid and inflexible, then extols the virtues of their DSP as the best of the options for Fallback. But as we explain in detail below, being the best at Fallback is sorta like being the last part of the Titanic to sink: you are going to fail spectacularly, but you might buy yourself a few extra fleeting moments to rue your choice as the ship sinks.

In previous blogs, we’ve highlighted the conceptual failings of the Fallback concept. Let’s now drill deeper into an actual example to show just how dreadfully awful the Fallback concept is in real life!

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