More proactive cooling for heavy Mac workloads.

I've been using a number of LLMs on my M4 Max MacBook Pro using the amazing LM Studio. I noticed some sustained workloads seemed to get slower and slower. This is easy to detect in an LLM harness like LM Studio where it can show you the tokens per second (t/s) rate after every response. When I ran a bunch of queries back to back on some models, the fans would spin up as expected, but I noticed my t/s falling off substantially. Using the outstanding mactop tool (brew install mactop), I immediately saw that GPU performance was starting at 100%, and then progressively dropping several minutes into long-running jobs. While I had paid an extra $300 to jump from the available 32-core GPU to 40 cores, under a few minutes of heavy load, I was sometimes left with a 24-core GPU's worth of performance.

I experimented with some of the available Mac apps for adjusting the fans. What I found was that for my GPU-heavy LLM workloads, the set-it-and-forget-it fan options were sometimes noisy, and still would not prevent all thermal throttling in cases where blasting the fans at 100% did help. Nobody wants a noisy machine, and Apple did a fantastic job with their tradeoffs, to a point. But I wanted the performance I bought, and if achieving that was sometimes louder, that was the tradeoff I was willing to make.

After a lengthy investigation of how my system handled power, heat, performance, and fans, I found that the fixed-fan-percent-by-temperature model is simply unsuited for some heavy loads lasting more than a few minutes. The system has an inherent notion of the thermal soak properties of the SoC. In practice this means my Mac was quite lazy about ramping up the fans initially even when the CPU and GPU shot past 90°C, and it did not throttle speeds early. But if that heat load continued for a couple minutes, the fans would happily sit at a middling speed while reducing performance.

That is the problem I set out to solve for at least some workloads with MacTC. It defers to the system's fan choices by default, but when it observes power and heat rising, it should try to front-run accumulated heat with more fan earlier, while still being as quiet as possible. And when thermal throttling begins, it will take the fans higher only for as long as it takes to end the throttling, and then attempt to fall off to the lowest fan speed that can sustain full performance.

When MacTC does increase fan speed, it deliberately takes a few seconds. Ideally, the end result at your desk is the difference between noticing "oh, my fans are on," and "wow, my fans just came on." That small whoosh turns out to be really aggravating all by itself. As it came together, I immediately understood this was an app worth sharing with others. I hope it can benefit you as much as it benefitted me during testing.