# `[2026-09-11]` MEASURED: two concurrent training jobs on pfi-gx10 are 13% NET SLOWER than running them back to back — VRAM is

⭐ **MEASURED: two concurrent training jobs on pfi-gx10 are 13% NET SLOWER than running them back to back — VRAM is not the constraint and never was.** Operator asked to run the two BabyYarros arms in parallel if VRAM allowed. It does, comfortably: **18.4 GiB per 4B LoRA job, 36 of 121 GiB with both up, 98 GiB free.** But the GB10 is a *capacity* box, not a throughput box, and the binding constraint is memory bandwidth. Solo baseline **37.10 s/it (n=6, 0.05% spread)**; with a second job both arms settled at **~85 s/it — 2.29x each**, so combined throughput 0.0235 vs 0.0270 steps/s solo. Not a clean 2x split: the box is *past* its roofline and pays a contention penalty on top. ⭐ **Control: killing the second job returned the first to 37 s/it on the very next step**, so the slowdown tracked contention and reversed with it. Chaining finished both arms ~43 min earlier than concurrency would have. **General form: on this box, `nvidia-smi` free memory tells you nothing about whether a second job is affordable.** Decision rule was pre-registered before the numbers were read (<55 s/it keep both, ≥2x chain). `scripts/yarros-corpus/{launch-yarros-4b-base,chain-yarros-4b-base}.sh`; the shared-GPU bypass is an explicit argument, never a default.