You Asked, So I Measured: What Concurrency Actually Costs On One Local Model
Reddit pushed back on my batching post with 3 sharp, testable claims: decode is memory-bandwidth-bound, prefill batches better than decode, and prompt length changes the whole story. So I built a measurement matrix, ran 36 barrier-synchronized loads against 1 local Qwen 3.8 on my Macbook Pro M3 Max, and let the numbers settle it. 2 of the 3 predictions held. 1 did not show up the way I expected, and I am keeping the correction in. Then a real agent handoff stalled on the exact wall these charts describe, and I show how a one-day-old runtime was already a first-class citizen in my receipts governance framework.
Over last week and these last 2 days in this week I posted measured answers to readers both in Twitter and Reddit who asked whether my experiments of 2 local agents on 1 Mac run in parallel or take turns. The answers was that they share one continuous batch, batching is real, and 3 quiet choices collapse it. I thought that was the end of it. It was not. The comments were better than my post, WHICH IS AWESOME, becaue this is the crowdsourcing of brain power I love. I not trying to be a KNOW IT…
