[sanjay7178]

Summary

Reduce intermediate payload copies in bounded MPSC receives by removing the payload/waker tuple from the receive path. Add matched inline and boxed benchmarks and ownership tests for cancellation, disconnection, and resumed sends.

A balanced four-variant benchmark on Linux x86-64 measured the retained receive-only change at 230.5 ms versus 250.5 ms for the inline ready case (16 samples per variant). Contended inline medians were 730 versus 747 ms with broadly overlapping ranges; this does not establish a significant contended throughput improvement. The send-side change has been removed following review. Full methodology and raw samples are in the benchmark follow-up comment.

Related to #401; further work is needed to close the larger throughput gap.

Validation

• Targeted large-payload ownership test and all 40 MPSC integration tests.
• cargo x lint.
• Baseline, send-only, receive-only, and combined benchmark variants with identical fixtures, fixed rounds, and balanced rotating run order.

[orthur2]

Thanks for working on this. The receive-side change looks good to me, but I'd drop the send-side part.

[sanjay7178]

Benchmark follow-up

Removed the send hunk as requested; the implementation change now only simplifies receive result handling. The earlier combined before/after measurements did not establish the cause of the contended timing change.

Measured on Linux x86-64 (AMD EPYC-Rome VM, 16 CPUs), rustc 1.99.1 / LLVM 23.1.1, optimized Cargo bench builds. All four variants use base 452d858 plus identical PR benchmark fixtures. Baseline has neither hunk; send-only and receive-only isolate each hunk; combined has both. All builds completed before measurement; no tests or builds ran concurrently. Each case has one warmup per variant followed by 16 recorded launches per variant, rotating and reversing order in balanced blocks. Each launch uses --baseline-only --repeat 1 --raw. No outliers were removed.

Each cell is median ms (min–max); lower is faster.

Scenario	Fixed rounds	Baseline	Send only	Receive only (retained)	Combined
ready-roundtrip-inline1k	1,000,000	250.5 (242–283)	264 (245–285)	230.5 (219–243)	243 (228–270)
tasks-4p1c-cap64-4w-inline1k	26	747 (634–832)	793 (712–873)	730 (581–837)	793 (557–877)
tasks-4p1c-cap64-4w	26	84 (74–92)	82.5 (74–91)	81 (77–92)	81 (72–87)

The isolated ready case supports dropping the send change on this host. Contended timings remain noisy and platform-dependent; these data do not establish a lock-timing mechanism or resolve #401. The usize filter also matches the inline and boxed rows: those extra rows were executed consistently for every variant, and the table selects only the exact usize row.

Raw recorded times in chronological order within each variant (ms)

ready-roundtrip-inline1k / baseline: [266, 250, 255, 251, 245, 260, 283, 246, 246, 243, 263, 253, 254, 247, 242, 243]
ready-roundtrip-inline1k / send: [251, 264, 245, 272, 260, 269, 285, 272, 254, 264, 269, 256, 260, 267, 260, 270]
ready-roundtrip-inline1k / receive: [243, 243, 223, 230, 242, 234, 239, 219, 241, 227, 227, 231, 227, 229, 234, 221]
ready-roundtrip-inline1k / combined: [228, 260, 248, 230, 241, 245, 254, 270, 236, 248, 231, 247, 246, 238, 228, 231]
tasks-4p1c-cap64-4w-inline1k / baseline: [777, 667, 643, 634, 801, 769, 697, 762, 832, 807, 722, 829, 769, 693, 732, 729]
tasks-4p1c-cap64-4w-inline1k / send: [871, 768, 754, 840, 712, 873, 771, 725, 822, 818, 825, 802, 757, 784, 780, 833]
tasks-4p1c-cap64-4w-inline1k / receive: [704, 690, 581, 751, 601, 810, 778, 759, 632, 744, 699, 818, 716, 630, 837, 819]
tasks-4p1c-cap64-4w-inline1k / combined: [557, 752, 585, 793, 793, 824, 871, 844, 842, 818, 779, 688, 812, 780, 877, 705]
tasks-4p1c-cap64-4w / baseline: [80, 81, 82, 92, 85, 85, 86, 81, 84, 74, 89, 89, 88, 84, 80, 84]
tasks-4p1c-cap64-4w / send: [88, 85, 91, 74, 75, 82, 82, 80, 77, 78, 83, 84, 80, 86, 84, 83]
tasks-4p1c-cap64-4w / receive: [81, 92, 77, 86, 81, 81, 84, 86, 83, 81, 79, 81, 81, 78, 81, 82]
tasks-4p1c-cap64-4w / combined: [82, 81, 72, 82, 79, 87, 85, 81, 77, 74, 78, 82, 83, 81, 79, 76]