pub static ATTN_PHASE_US: [AtomicU64; 4]Expand description
2026-09-25: Per-phase host time in µs for the prefill attention path, written
only by prefill/cache_skip.rs: 0 = the Q/K/V projections, 1 = from the
projections to the BR=64 attention call, 2 = that attention call. No code
writes slot 3. Summed across layers; the layer loop takes them once per
prefill when METRALE_PREFILL_HOST_TIMING=1.