[kixelated]

Completes quest/m1/quic/shard.md (deleted here, with its references). Sharding stays opt-in. Flipping the relay default waits on a saturating benchmark, tracked in the new quest/m2/perf/shard-default.md quest.

What changes

• moq-quic names the shard itself. RandomConnectionIdGenerator::set_shard(index, count) starts every CID with a byte congruent to index mod count, which is the byte the SO_REUSEPORT filter reads. It also sets fix(sock): steer only recognizable QUIC packets #5638's non-SRT marker (0x18 in the fourth byte), which the filter checks on short headers. This replaces moq_sock::shard::cid and its wrappers (moq-tokio's ShardIdGenerator, moq-uring's Cids), so one generator owns the whole encoding.
  • The encoding is the same as fix(sock): steer only recognizable QUIC packets #5638's. Every other bit stays random, about 54 bits per shard, so a client that steers every connection to one shard can't exhaust its IDs.
  • validate refuses a CID without the shard's prefix or the marker. A stray packet is dropped instead of drawing a stateless reset for a connection that a sibling owns.
• moq-sock forms the group in one call. shard::bind(addr, count) -> Bound replaces Group / Member / Claim. Both runtimes ran the same acquire/bind/complete loop on one thread, so the cross-thread ordering machinery and its out-of-order checks are gone. The port lock, the bind probe and fix(sock): steer only recognizable QUIC packets #5638's BPF filter are unchanged.
• moq-tokio's worker group shrinks. SocketRetainer is gone, since Bound already keeps every member socket in the kernel array.
• A stopped worker sends its closes. A worker used to drop its runtime as soon as it was told to stop, and the endpoint driver went with it. Any close still queued, such as a drain's abort, never reached the wire, so the peer only noticed at its idle timeout. The shared server avoids this by awaiting Listener::close. Each worker now closes its endpoint and waits for every connection to send its close before the runtime drops. This surfaced as moq-cli's fails_when_the_session_ends timing out in CI while test relays ran on workers.
• Workers default to unpinned. runtime.pin and moq_tokio::worker::Config::new now default to off. Every group pins worker i to the ith core, so two groups on one host would crowd onto the first cores, and so would one limited to fewer CPUs than the host has. Pinned and unpinned runs were already within noise.
• Benchmark. just bench-shard [SHARDS] [CONNECTIONS] sweeps the shard count (0 = the shared runtime) against the connection count, for the room (video) and fanout workloads. It reports CPU per connection, so a cost that grows on either axis shows up as a slope. It runs nightly.

Public API and wire impact

• moq-quic: adds RandomConnectionIdGenerator::set_shard, whose validate checks the shard's prefix and marker. HashedConnectionIdGenerator is unchanged from upstream.
• moq-sock (breaking):
  • removes shard::{Group, Member, Claim, cid, CID_LEN};
  • adds shard::bind and shard::Error::{Bind, Steer};
  • moves rand to dev-dependencies.
• moq-uring (breaking): removes quic::endpoint::CID_LEN. Endpoints still issue 8 random bytes, sharded or not.
• moq-tokio (behavior): worker::Config::new(count) is unpinned; with_pin(true) restores pinning. No signatures change.
• moq-relay (behavior): runtime.pin defaults to false. runtime.workers is unchanged: unset or 0 keeps the shared runtime.
• Wire:
  • No MoQ wire change, and QUIC CIDs stay 8 bytes.
  • The CID layout is unchanged from main: a sharded CID is the prefix, random bytes, and the marker in byte 3; an unsharded one is 8 random bytes (moq-uring) or quinn's hashed CID (moq-tokio, as before).

Tests

• moq-quic: a shard CID reduces to its shard at every group size up to 256, carries the non-SRT marker and validates. Shards refuse each other's CIDs and CIDs without the marker, and out-of-range shards or CIDs too short for the marker panic. A shard of a 256-way group keeps 54 varying bits, and an unsharded CID keeps all 64.
• moq-sock: the existing tests run against bind: steering (short, v1, v2), non-QUIC flows, overlap, ephemeral ports and retention. A reuseport address that is already held is refused with AddrInUse instead of being joined. Off Linux, a group is refused with Unsupported.
• moq-tokio: stopping_the_group_closes_its_sessions checks that a client with a 600s idle timeout sees the close within 5s of Group::shutdown. It fails without the worker close.
• moq-relay: the existing worker and io_uring relay tests pass, and they can only pass if the generator's encoding matches the filter.

Benchmark

just bench-shard ran on a 24-core shared host at load average 24 to 52, so the numbers are noisy. At these loads the relay never uses even one full core, so the sweep shows no cost growing with either axis, but it also can't show the multi-core win. That gap is why the default stays shared.

workload	conns	shared CPU	1 shard	4 shards	16 shards
video	16	0.092	0.071	0.100	0.114
video	256	0.946	0.737	0.837	0.970
fanout	16	0.022	0.016	0.020	0.025
fanout	256	0.169	0.171	0.165	0.201

• Throughput, fps and loss match across all shard counts. Loss is 0 everywhere.
• 16 shards costs about 15 to 25% more CPU than the shared runtime at low load, from the idle per-core runtimes.

Decided

• Sharding stays opt-in, and the default flip is a follow-up quest.
• runtime.pin is off by default, and worker::Config matches it.
• The CID encoding lives in moq-quic's generator, while moq-sock's filter reads it.
• moq_sock::shard::Error::{Bind, Steer} are public.

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📝 Code Review	✅ Completed 2026-10-16T20:07:22.852017Z	13155a1	Manual request

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📒 Files selected for processing: 30

• .github/workflows/nightly.yml
• bench/README.md
• bench/run.sh
• doc/bin/relay/config.md
• justfile
• quest/m1/quic/README.md
• quest/m1/quic/shard.md
• quest/m2/perf/README.md
• quest/m2/perf/kio-channel-contention/README.md
• quest/m2/perf/shard-default.md
• rs/moq-quic/src/cid_generator.rs
• rs/moq-relay/src/config.rs
• rs/moq-relay/src/runtime.rs
• rs/moq-relay/src/uring.rs
• rs/moq-sock/Cargo.toml
• rs/moq-sock/README.md
• rs/moq-sock/src/lib.rs
• rs/moq-sock/src/shard.rs
• rs/moq-tokio/src/listen.rs
• rs/moq-tokio/src/quic.rs
• rs/moq-tokio/src/quic/backend.rs
• rs/moq-tokio/src/server.rs
• rs/moq-tokio/src/worker/group.rs
• rs/moq-tokio/src/worker/mod.rs
• rs/moq-tokio/tests/worker.rs
• rs/moq-uring/Cargo.toml
• rs/moq-uring/src/quic/driver/mod.rs
• rs/moq-uring/src/quic/endpoint.rs
• rs/moq-uring/src/udp.rs
• rs/moq-uring/tests/workers.rs

Walkthrough

The change adds shard-aware QUIC connection IDs and replaces staged reuseport group setup with direct binding. Worker shutdown now closes and drains QUIC connections. Relay and worker pinning defaults change to off. A Linux-only benchmark sweeps shard and connection counts for video and fanout workloads, with a nightly workflow and performance quest documentation.

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to 8472b

The relay is mergeable, but the documentation could mislead operators deciding whether to pin workers. Clarify the core-affinity claim.

Pre-merge checks: 5

Check name	Status	Explanation
Docstring Coverage		Docstring coverage is 86.21% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 58 functions across 17 files. (9 skipped: 9…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title accurately refers to QUIC endpoint sharding, but its “by default” claim conflicts with the changes, which keep sharding opt-in.
Description check		The description is detailed and directly covers the sharding implementation, API changes, worker shutdown behavior, defaults, tests, and benchmark results.

Full details: Docstring Coverage

Explanation

Docstring coverage is 86.21% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 58 functions across 17 files. (9 skipped: 9 unsupported.)

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Reviewed commit: 61f21018f8

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• 🪄 Fix comments: Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @doc/bin/relay/config.md:
- Line 120: Update the packet-handling description to say packets remain on one
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📒 Files selected for processing: 30

• .github/workflows/nightly.yml
• bench/README.md
• bench/run.sh
• doc/bin/relay/config.md
• justfile
• quest/m1/quic/README.md
• quest/m1/quic/shard.md
• quest/m2/perf/README.md
• quest/m2/perf/kio-channel-contention/README.md
• quest/m2/perf/shard-default.md
• rs/moq-quic/src/cid_generator.rs
• rs/moq-relay/src/config.rs
• rs/moq-relay/src/runtime.rs
• rs/moq-relay/src/uring.rs
• rs/moq-sock/Cargo.toml
• rs/moq-sock/README.md
• rs/moq-sock/src/lib.rs
• rs/moq-sock/src/shard.rs
• rs/moq-tokio/src/listen.rs
• rs/moq-tokio/src/quic.rs
• rs/moq-tokio/src/quic/backend.rs
• rs/moq-tokio/src/server.rs
• rs/moq-tokio/src/worker/group.rs
• rs/moq-tokio/src/worker/mod.rs
• rs/moq-tokio/tests/worker.rs
• rs/moq-uring/Cargo.toml
• rs/moq-uring/src/quic/driver/mod.rs
• rs/moq-uring/src/quic/endpoint.rs
• rs/moq-uring/src/udp.rs
• rs/moq-uring/tests/workers.rs

💤 Files with no reviewable changes: 4

• rs/moq-tokio/src/listen.rs
• rs/moq-uring/Cargo.toml
• quest/m1/quic/README.md
• quest/m1/quic/shard.md

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Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: 13155a1476

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[kixelated]

Merge summary for 13155a147.

• Review: Codex reviewed this head clean (no findings, thumbs-up). Both earlier threads are fixed and resolved:
  • the Codex P1 on CID entropy (fixed in d34978c);
  • the CodeRabbit wording in doc/bin/relay/config.md.
• CID fix: set_shard lives on RandomConnectionIdGenerator, so a sharded CID keeps about 54 random bits and an unsharded moq-uring CID keeps all 64. CIDs stay 8 bytes with main's layout, so there is no wire change.
  • Exhaustion in Endpoint::new_cid stays a loop rather than a loud failure: at 54 bits per shard it is out of reach, and failing it loudly would need error plumbing through every caller.
• Merges: main is merged through 7d187c6, including fix(quic): fail a dial at once when the peer has no route #5668, feat(quic): hold connection credit until the receiver releases it #5675 and docs(quest): narrow the next release and reconcile stale plans #5679.
  • fix(quic): fail a dial at once when the peer has no route #5668's per-socket unreachable routing composes with sharding: each member socket's endpoint calls route_unreachable and fails only its own dials.
  • docs(quest): narrow the next release and reconcile stale plans #5679 moved udp-demux to m2/p2p, so shard-default.md now links the new path.
• Decided:
  • sharding stays opt-in;
  • runtime.pin and worker::Config default to unpinned;
  • the CID encoding lives in moq-quic, and moq-sock's filter reads it;
  • moq_sock::shard::Error::{Bind, Steer} is public.
• Follow-up: quest/m2/perf/shard-default.md runs a saturating sweep and flips the default only on a measured win.

(Written by Claude Opus 5.5)