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+---
+status: assigned
+kind: defect
+opened: 2026-11-02
+---
+
+# A spawned thread starts in any LLC, not its spawner's
+
+Spawn placement (`CpuHandles::place`, from `kernel/src/sched/driver.rs`'s
+`placement`) ranks CPUs by queue, then by whether they sleep with no kick
+coming, by busy SMT siblings and by wake depth, and breaks ties by a rotation
+over the whole machine. It never weighs the LLC of the thread doing the
+spawning. A thread that works with the thread that spawned it, sharing its
+address space and often its data, therefore starts split from it across LLCs
+whenever the rotation lands in the other one. Nothing brings it back
+afterwards while neither side parks often.
+
+On the AMD bench (LLCs 0–7 and 8–15), ab9: six `benchcase` boots, base #1117
+`c9f88d36e` and head #1129 `cebbdedd5`, each with a per-round diagnostic
+([judgment](https://github.com/ToyOSOrg/evidence/blob/6b481f7d84081c39c3d9ee8160ea1bda0d1bb687/pr-1129/r8/judgment.md),
+[rows](https://github.com/ToyOSOrg/evidence/tree/6b481f7d84081c39c3d9ee8160ea1bda0d1bb687/pr-1129/r8/runs)):
+
+- `pipe.throughput`'s writer is a new thread each round. At 16384 its CPU
+  followed the rotation 14, 15, 0, 1, 2, 3, 4, 5 in every boot of both arms,
+  whatever LLC the reader, osbench's main thread, was in.
+- Every round whose writer and reader ran in different LLCs read slow, in both
+  arms and with no exception: about 2.0, 3.5 and 4.8 GB/s at 4096, 16384 and
+  65536 bytes per write, against about 5.0, 8.4 and 10.0 in one LLC.
+- So the row's p50 reports which LLC the reader happened to sit in: 2 of 8
+  rounds split in two base boots and 6 of 8 in the third.
+
+Open observation: one same-LLC round (base boot 5, round 1, reader cpu13 and
+writer cpu15) read 5.6 GB/s, as did its warmup on 13 and 14. Its record shows
+no move, no other process on those cores and nothing lost. It is not explained.
+
+Assigned to the change that places a spawned thread on a free core in its
+spawner's LLC first.