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Johnathan Corgan
2bc0345174
cost-avoidance and mixed-technology decide a parent by measured link cost (etx * (1 + srtt_ms/100)). Under the local CI's 4-way-parallel chaos the fast fiber link's measured srtt spikes from host scheduling and can exceed the Bluetooth link's, flipping the choice and reddening a run that proves nothing about the daemon. The old margin was ~0.4 cost units, which a ~24ms one-sided scheduling blip closes. Widen the Bluetooth delay to 150-250ms so the margin dwarfs any plausible one-sided spike. The link still establishes well within the handshake budget (a ~500ms RTT against a 30s stale-handshake window), so the losing candidate is still a real, established peer rather than an absent one. Add a netem mutation exclude_edges option and use it in mixed-technology to pin the two links n08 chooses between, so a random degradation can't flip the asserted comparison either. An unknown excluded edge is a hard error, since a silent no-op would reintroduce the flakiness it exists to remove. Validated by running both scenarios repeatedly under heavier-than-CI concurrent load: the parent choice holds every time.
97 lines
3.1 KiB
YAML
97 lines
3.1 KiB
YAML
# Cost-Based Parent Selection: Bottleneck Avoidance Test
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#
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# Topology (explicit 4-node diamond):
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#
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# n01 (root — smallest addr)
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# / \
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# fiber fiber
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# / \
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# n02 n03
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# \ /
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# BT fiber
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# \ /
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# n04 (test subject)
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#
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# n04 has two candidate parents at depth 1: n02 (via Bluetooth L2CAP)
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# and n03 (via fiber). Cost-based selection should pick n03 because
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# link_cost = etx * (1 + srtt_ms/100), so the far slower Bluetooth link
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# costs far more:
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# effective_depth(n02) = 1 + ~4.5 (Bluetooth, ~150-250ms) = ~5.5
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# effective_depth(n03) = 1 + ~1.05 (fiber, ~1-5ms) = ~2.05
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#
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# The Bluetooth delay was widened deliberately, and the size is set by load,
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# not by realism. At the original 15-40ms the cost margin was ~0.4, and even
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# 80-120ms was not enough: under the CI's parallel-chaos contention the
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# *measured* srtt of the fiber link spikes (host scheduling, not netem) and
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# was observed to exceed a 120ms Bluetooth base, flipping n04 to n02. At
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# 150-250ms the gap dwarfs any plausible one-sided fiber spike, so the
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# assertion below is a reliable verdict rather than a coin toss under load.
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# n04 still establishes the n02 link (it was parented to it when this flaked),
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# so the choice remains real rather than vacuous.
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#
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# Validation: tree snapshot shows n04's parent is n03.
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scenario:
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name: "cost-avoidance"
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seed: 42
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duration_secs: 45
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topology:
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algorithm: explicit
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num_nodes: 4
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params:
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adjacency:
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- [n01, n02]
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- [n01, n03]
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- [n02, n04]
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- [n03, n04]
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subnet: "172.20.0.0/24"
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ip_start: 10
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netem:
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enabled: true
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default_policy:
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# Fiber-like
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delay_ms: [1, 5]
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jitter_ms: [0, 1]
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loss_pct: [0, 0.5]
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link_policies:
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# Bluetooth (L2CAP) link from n02 to n04. Delay widened to 150-250ms so the
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# fiber-vs-Bluetooth cost margin survives CI load spikes on the fiber
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# probe's measured latency (see header). Well within handshake timeouts, so
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# the n02 link still establishes and the choice stays real.
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- edges: ["n02-n04"]
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policy:
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delay_ms: [150, 250]
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jitter_ms: [15, 30]
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loss_pct: [2, 8]
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link_flaps:
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enabled: false
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traffic:
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enabled: true
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max_concurrent: 2
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interval_secs: {min: 15, max: 30}
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duration_secs: {min: 5, max: 10}
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parallel_streams: 2
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# The validation line at the top of this file, encoded. It existed only as
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# that comment and was checked by nothing, so the scenario could not fail
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# on the one thing it was built to test.
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#
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# From the archived corpus: of the six runs that carry a status.txt and are
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# therefore provably completed, n04's parent is n03 in all six. Among the
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# 176 older runs the picture is quite different -- n04 is absent from the
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# snapshot in 107 of them and is its own root in 49 -- but those are runs
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# whose tree had not converged when the snapshot was taken rather than runs
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# that chose the wrong parent, which is why the assertion fails an absent
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# or self-parented node explicitly instead of skipping it.
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assertions:
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tree_parents:
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n04: n03
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logging:
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rust_log: "info"
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output_dir: "./sim-results"
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