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stadium: wire STADIUM_CAPACITY_TICK in as a flat threshold, not a scheduler

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Closes FABRIC-2.md's last open §12 Q5 question. fleet_heartbeat_tick_count
is fed by every live VM's own vm_tick(), not one VM's, so it was reaching
HEARTBEAT_INFERENCE_FREQUENCY (shared/borrowed from the per-VM inference
gate) several times faster than intended with more than one VM live -
backwards from FABRIC.md §22.4's required ~1000:1 separation.

What's actually gated turned out to be low-stakes: vm_physics_tick()
(capsule_vm_physics.c:397) is a passive statistics refit - re-sorts a
window of past heat-transfer samples and recomputes a median rate
estimate. It doesn't move heat or arbitrate capacity. Firing too often
just meant a noisier statistic recomputed more frequently than planned,
not incorrect behavior.

Considered and explicitly rejected: scaling the threshold by live VM
count at the check site. That's the first brick of a scheduler - reading
fleet state to adjust a rate dynamically - which this project has
deliberately avoided building. Implemented instead: STADIUM_CAPACITY_TICK
(existing Kconfig symbol, defined but never read by any code path) now
gates vm_physics_heartbeat_tick()'s call directly, replacing the borrowed
HEARTBEAT_INFERENCE_FREQUENCY. Default bumped 1000 -> 4000, a flat
constant picked once for Tripod's known 4-VM topology, same kind of
placeholder as every other frequency knob in Kconfig.kernel - not
computed from anything at runtime. Renamed fleet_last_inference_tick ->
fleet_last_capacity_tick to match. Still one clock, one counter
(fleet_heartbeat_tick_count) - just a bigger flat divisor on it.

Three-arch QEMU acceptance: all clean to ok>, identical Stadium
conservation invariant on all three (resident_sum=43691 reservoir=21845
sum=65536). logs/20260815-093425/amd64, logs/20260815-093521/aarch64,
logs/20260815-093641/riscv64.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
This commit is contained in:
Robert Allan James
2026-08-15 09:37:58 -04:00
co-authored by Claude Sonnet 5
parent c5442c8377
commit 89d8c08582
10 changed files with 25755 additions and 46 deletions
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include/starkernel/vm/stadium.h
+6 -4
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@@ -109,10 +109,12 @@ typedef char stadium_cell_size_check[(sizeof(StadiumCell) == STADIUM_CELL_BYTES)
/*
* Items 1.1 and 1.4 named this item as where their Kconfig symbols would be
* implemented. Neither has a consumer yet (item 3.5 for the depth cap,
* capacity arbitration -- not yet on the punch list -- for the tick); these
* checks only prove the symbols are defined and sane, the same discipline
* already applied to the byte-count checks above.
* implemented. STADIUM_CONTAINS_DEPTH_MAX still has no consumer (item 3.5
* for the depth cap, not yet implemented). STADIUM_CAPACITY_TICK was wired
* in 2026-08-15 (capsule_vm_physics.c's vm_physics_heartbeat_tick(), see
* FABRIC-2.md F.2/§12 Q5) -- this check now proves a real, live constant
* is sane, not just a placeholder, same discipline already applied to the
* byte-count checks above.
*/
typedef char stadium_contains_depth_configured_check[(STADIUM_CONTAINS_DEPTH_MAX > 0) ? 1 : -1];
typedef char stadium_capacity_tick_configured_check[(STADIUM_CAPACITY_TICK > 0) ? 1 : -1];