lm-Meter times on-device inference and finds prefill, not decode, is the bottleneck

Researchers at Georgia State University and Toyota InfoTech Labs published lm-Meter, a latency profiler that runs inside the inference engine on the phone and splits each generation into embedding, prefill, decode, softmax and sampling. Measuring the Pythia models on a Google Pixel 8 Pro, they report that scaling from 70M to 1.4B parameters raises prefill latency from 0.012 s to 1.9 s per input token, a 158x slowdown, while decode latency per output token grows from 0.015 s to 0.15 s, a 10x slowdown. The authors write that this inverts the server picture, where decode is usually the limiting phase for single-request inference.

Below the phase level the profiler times individual GPU kernels through OpenCL event timestamps, which give queue, submit, start and end times without access to the closed-source driver. Running a 4-bit quantised Gemma-2-2B-it on a Pixel 8 Pro, the authors report that fused matrix-multiplication kernels dominate a decode step and that the GPU sits idle for more than 21% of it, the second-largest contributor to the step, which they attribute to host-side data preparation and I/O stalls. The paged attention kernel that scans the growing key-value cache is the only one whose cost rises with position in the sequence, climbing from roughly 0.2 ms to about 0.8 ms per token over 250 decode steps, and idle time drops from about 21% to 12% when the model generates 256 tokens instead of 16.

Whether those measurements mean anything depends on what the profiler itself costs. lm-Meter sits in the MLC LLM runtime and TVM in about 3,500 lines of code and needs no host machine attached, and under the Powersave CPU governor, the most constrained setting they tested, the authors measure a throughput loss of 2.58% in prefill and 0.99% in decode. They put the same figures for MELTing Point, the on-device profiler they compare against, at 22% for prefill and more than 93% for decode. Checked against traces from Android GPU Inspector, they report end-to-end phase accuracy of at least 99.99% and mean kernel-level accuracy of 96.82% on the Pixel 8 Pro and 96.61% on a Pixel 7.

The code is on GitHub under the MIT license, with the MLC LLM path released for Android GPUs through OpenCL and support for llama.cpp, vLLM, iOS Metal and Nvidia Jetson listed as unfinished. The work was accepted to the ACM/IEEE Symposium on Edge Computing 2025 and funded by Toyota Motor North America. The measurements come from three phones, the Pixel 8 Pro, Pixel 7 and Pixel 6, and the authors state that other edge platforms such as Jetson boards and Intel NPUs may show different bottlenecks.