Arm runs Llama 2 7B at 9.6 tokens per second on three mobile CPU cores

Arm showed a chat assistant running Meta’s Llama 2 7B on existing Android phones using three Cortex-A700 series CPU cores and LPDDR5x memory, at a text generation rate of 9.6 tokens per second. Gian Marco Iodice and Adnan AlSinan published the demo video and the measurement on January 17, 2024, and state that the video runs at actual speed and that everything runs locally on the device. Arm says the rate is faster than average human reading speed.

Integer quantisation is what brought a 7B model into phone memory, according to Arm’s write-up. Arm puts the RAM needed for Llama 2 7B with 16-bit floating point weights at about 14 GB, and says quantising those weights to 4-bit shrinks the model four times, to roughly 4 GB. Arm warns that going down to 3-bit or 2-bit costs accuracy. Pinning each worker thread to a specific core with a thread affinity mask, rather than letting Android’s scheduler place it, improved latency by over 10 percent in Arm’s tests.

Arm then wrote its own 4-bit matrix-by-vector and matrix-by-matrix routines, the two functions that dominate the work in a language model, tuned for Cortex-A700 series cores. The routines are built on two instructions the cores already carry, SDOT for dot products and SMMLA for 8-bit integer matrix multiply and accumulate. Arm reports that they cut the time to the first token by over 50 percent and raised text generation by 20 percent against the native implementation in llama.cpp, and said at the time that the routines would be available soon.

Arm makes the case for the CPU on the shape of the workload rather than on peak throughput, arguing that the typical batch size and the balance of compute and bandwidth in language model inference suit a CPU, and that its programmability lets developers try new compression and quantisation schemes. The post names no NPU or GPU and offers no comparison against one. Arm puts the reach of the approach at around 99 percent of smartphones worldwide and says 70 percent of AI in third-party applications already runs on Arm CPUs.

Arm later extended the same post with a second demo, a teaching assistant chatbot built on Microsoft’s Phi-3 3.8B, which Arm measures at just over 15 tokens per second on the same software optimisations. Arm names the Scalable Matrix Extension for Armv9-A as the next hardware step for these workloads.