fix(hybrid): budget the egress relay loop so parallel flows share the grant - #349
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Included review availability: Your plan provides up to 10 included reviews per hour; 8 remain after this review. 📝 WalkthroughWalkthroughThe TCP egress relay now validates and enforces a 65536-byte fairness budget per readiness pass. The test suite verifies that excess data remains queued for a subsequent readable event. ChangesTCP egress relay fairness
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Bug fix · Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant Reactor
participant UplinkRelay
participant EgressSocket
participant H3Stream
Reactor->>UplinkRelay: dispatch readable event
UplinkRelay->>EgressSocket: read queued data
UplinkRelay->>H3Stream: relay data
UplinkRelay->>Reactor: yield at 65536 bytes
Reactor->>UplinkRelay: dispatch next readable event
UplinkRelay->>EgressSocket: read remaining data
Suggested reviewers: Merge Risk: ⚪ Minimal · up to The relay enforces its per-event byte limit and preserves remaining queued data for the next readable event. No actionable merge risk remains. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@src/hybrid/tcp_egress.c`:
- Around line 1276-1277: Update the relay loop’s recv call to request no more
than TCP_EGRESS_RELAY_BUDGET - relayed bytes, while preserving the existing
relayed accounting and termination check so each iteration stays within the
65,536-byte maximum.
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src/hybrid/tcp_egress.csrc/hybrid/tcp_egress.htests/test_tcp_egress.c
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… grant svr_tcp_egress_on_relay_ready() reads its egress socket with no byte budget: the loop exits on recv() <= 0, on a send_body error, or on a partial accept. With a fast peer behind that socket it does not run out of bytes -- the peer refills it about as fast as the loop drains it -- so what stops the loop is xquic's send queue instead. That queue is not released smoothly. xqc_engine_process_conn() latches sndq_full and does not call xqc_process_write_streams() again until sndq_packets_used_max / XQC_SNDQ_RELEASE_ENOUGH_SPACE_TH packets have been freed (third_party/xquic/src/transport/xqc_send_queue.h; both call sites of xqc_process_write_streams sit under that check in xqc_engine.c). mqvpn asks for sndq_packets_used_max = 16384 (src/mqvpn_conn_settings.c), so the room comes back in steps of 1638 packets. The first CONNECT-TCP flow the reactor reaches after a release therefore puts that whole step back into the send queue before any other flow on the connection runs. The next flow's send_body accepts nothing, so it stashes at most one TCP_EGRESS_RELAY_CHUNK, goes uplink_withheld -- which drops want_read via the interest helper -- and from there moves only what svr_tcp_egress_on_h3_writable() pushes out of that stash, until a release lets it read again. The release is spent either way, so the connection's total does not move: what moves is the split between its flows, which is why nothing above this layer sees it. Stop the loop after TCP_EGRESS_RELAY_BUDGET bytes and let the reactor come back. Each read asks for no more than the budget has left, so a pass relays at most TCP_EGRESS_RELAY_BUDGET bytes even when a read returns less than a chunk; without that cap a short read early in the pass lets the last read overshoot by up to one chunk less a byte. break rather than return: n > 0 on that path, so neither post-loop recv()-status branch applies, control reaches svr_tcp_egress_update_fd_interest() at the tail, and want_read stays set -- the flow is out of budget, not withheld. `readable` is a level-triggered signal by that function's own contract (the comment above it says so), and the only two places that set cbs.egress_fd_register deliver it that way (EV_PERSIST | EV_READ in src/platform/linux/platform_linux.c, POLLIN in tests/test_tcp_egress.c's harness_pump), so the remainder is reported again on the next pass. The budget is at least one chunk -- a _Static_assert beside TCP_EGRESS_RELAY_CHUNK pins that, and that it is a whole number of chunks -- so bytes have always moved before the yield. Why 65536, four chunks: the budget is a round-robin quantum, so one number sets both sides of the trade. It has to stay well under one release divided by the flows sharing a connection or it never binds; and a flow with nobody to share with pays a reactor round trip per budget for fairness it gains nothing from. One chunk is the tightest quantum this loop can have and yields four times as often. The comment on the stash-and-return exit above loses the word ONLY, which the new break would otherwise falsify. tests/test_tcp_egress.c gains mqvpn_tcp_uplink_relay_stops_at_budget: it queues a budget plus 8 KiB on a real egress socket, dispatches exactly one readable event, and counts what left the socket with FIONREAD. Without the budget that count is the whole 72 KiB. It also pins the two properties that make the budget a yield rather than a stall -- want_read still armed, and a second event taking the remainder. The existing hybrid-lane coverage cannot see this: the iperf3 probes in tests/test_e2e_hybrid_h2.sh run a single flow at a time (-P 1), and one flow has nothing to be starved by. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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Fixes #348.
Why
Capacity comes back from xquic in steps, not continuously, and this loop has no budget —
so the first flow the reactor reaches after a step puts the whole step back into the send
queue before any other flow on the connection runs. The others get one stash each. The
connection's total does not move; the split between its flows does, which is why nothing
above this layer sees it.
The change
A few executable lines in
svr_tcp_egress_on_relay_ready(): count what has been relayedin this invocation, ask each read for no more than the budget has left, and
breakatTCP_EGRESS_RELAY_BUDGET, plus a_Static_assertnextto the existing chunk assert, the constant with its rationale in
tcp_egress.h, and onetest.
breakrather thanreturn:n > 0on that path, so neither post-looprecv()-statusbranch applies, control reaches
svr_tcp_egress_update_fd_interest()at the tail, andwant_readstays set — the flow is out of budget, not withheld.readableislevel-triggered by this function's own contract (
:1203-1207), and the only two placesthat set
cbs.egress_fd_registerdeliver it that way, so the remainder is reported on thenext pass. The budget is at least one chunk, so bytes have always moved before the yield.
Budget 65536, four chunks. One chunk is fairer (ratio 1.03 against 1.14 measured
downstream) but yields four times as often, and the yield is the only thing that costs
anything — it costs it on the single-flow case, which gains nothing from fairness.
Before / after
Measured on a downstream deployment running a fork of this project, not here and not in
this repository's CI, with the budget but before the per-read cap was added (see the
review thread); the cap was not re-measured there. Eight equal parallel downloads over two
150 Mbit paths, 40 s:
Aggregate unchanged, ICMP latency unchanged (p50 600 → 601 ms), CPU unchanged at eight
flows. A single flow costs about +18% CPU on the server — roughly 0.03 of a core at
273 Mbit/s — for a round trip per budget it gains nothing from; its throughput is
unchanged.
How to verify
59/59. The test binds: restoresrc/hybrid/tcp_egress.cfrom main, keep the header andthe test, rebuild, and it fails with
one readable event relayed 73728 bytes, budget is 65536.Whole suite on
b9227aa9+ this commit:100% tests passed, 0 tests failed out of 38,and the xquic unit suite
230 230 230 0 0.What this does not fix
The scheduling is still first-come: a flow that wins the race still goes first, it just
cannot take the whole step. And nothing in-tree covers the lane with parallel flows — the
hybrid probes are
-P 1attests/test_e2e_hybrid_h2.sh:591and:629— so this test isa unit test, not the netns coverage that gap really wants.
Summary by CodeRabbit
Performance
Reliability
Tests