[hawkingrei]

Summary

Every HTTP/3 discovery lookup currently prunes all cached origins while holding the cache mutex, including lookups for an absent origin. With 256 origins and eight alternatives each, this scans up to 2048 alternatives before looking up one key.

Prune only the requested origin during AltSvcCache::candidates. Cache writes, len(), snapshot(), and network changes retain their full-cache cleanup. Lookups preserve origin normalization, expiration checks, candidate metadata, and failure backoff. Entries remain bounded by the configured capacity.

Expired unrelated entries can remain resident until a cache mutation or observer reclaims them. Regression coverage checks that insertion reclaims those entries before evicting live origins, and that observers still report only live entries.

Measurements

The benchmark includes the production cache source directly to exercise the private lookup without adding a public API. Both binaries use the same benchmark source, locked dependencies, Cargo bench profile, and --no-default-features --features http3 on macOS aarch64 with Rust 1.99.1.

• Baseline: b03ffd33e6e1a46efaee8edec6552f5fb17d6ae0 (benchmark fixture only, production code unchanged from main).
• Optimized: 5706f5f589bc6c8bfe97cc6318ddaa642a51371d.
• Execution order: baseline, optimized, optimized, baseline.
• Nine samples per round, 100000 lookups per thread per sample: 18 samples per variant and scenario.
• Values below are medians of aggregate wall time divided by all lookups. Population and thread creation are excluded. These are cache microbenchmarks; HTTP network throughput and request latency were not measured.

Scenario	Before (ns/lookup)	After (ns/lookup)	Reduction
empty_miss	76.87	72.49	5.7%
single_hit	97.66	100.32	-2.7%
64_origins_hit	207.34	100.96	51.3%
256_origins_hit	500.23	100.03	80.0%
full_hit	1396.29	133.13	90.5%
full_miss	1290.10	72.09	94.4%
shared_full_hit	2955.45	591.39	80.0%
shared_full_miss	3081.72	106.41	96.5%

full means 256 origins with eight alternatives per origin. shared uses four threads sharing one cache. The single-origin median increases 2.7%, with overlapping interquartile ranges; the improvement is concentrated in populated-cache lookups.

Reproduce each revision with:

cargo bench --locked -j 2 --no-default-features --features http3 --bench alt_svc_lookup -- --iterations 100000 --samples 9

Validation

Added regressions for target expiration, retained backoff metadata, reclamation before capacity eviction, and exact len()/snapshot() results after misses. CI now compiles the benchmark in addition to the existing native, feature, MSRV, protocol, and package checks. Both push CI and PR CI passed all eight jobs at 5706f5f589bc6c8bfe97cc6318ddaa642a51371d.

Formatting and git diff --check completed. Regression suites are delegated to CI; local execution was limited to the benchmark.

Related: #1.

Raw benchmark samples, grouped by process invocation

[
  {
    "variant": "baseline",
    "round": 1,
    "ns_per_lookup_samples": {
      "empty_miss": [
        71.39792,
        71.905,
        73.20625,
        74.84458,
        77.07209,
        79.23958,
        79.43666,
        103.59125,
        226.97167
      ],
      "single_hit": [
        98.16875,
        100.36625,
        101.30792,
        102.83834,
        104.21792,
        104.80875,
        105.29417,
        105.88166,
        106.38333
      ],
      "64_origins_hit": [
        197.57417,
        199.585,
        199.77958,
        206.55583,
        207.35708,
        208.74541,
        209.85791,
        211.65958,
        213.47459
      ],
      "256_origins_hit": [
        472.13167,
        496.52125,
        498.91,
        501.92375,
        504.39542,
        511.81833,
        516.19834,
        518.07834,
        527.12083
      ],
      "full_hit": [
        1350.97209,
        1377.82583,
        1442.45708,
        1744.52666,
        1919.75792,
        1941.63125,
        1959.22208,
        1970.0075,
        1993.80459
      ],
      "full_miss": [
        1247.33667,
        1260.5025,
        1262.34208,
        1267.6075,
        1279.90666,
        1281.2399999999998,
        1286.1275,
        1312.70959,
        1322.50959
      ],
      "shared_full_hit": [
        2726.3714575,
        2764.6520825,
        2773.2460425,
        2778.858645,
        2779.63646,
        2794.496145,
        2837.841665,
        2855.63354,
        3101.65
      ],
      "shared_full_miss": [
        2418.311145,
        2430.908335,
        2450.3372925,
        2472.950105,
        2473.6003125,
        2488.642605,
        2500.260625,
        2525.683125,
        2539.42302
      ]
    }
  },
  {
    "variant": "optimized",
    "round": 1,
    "ns_per_lookup_samples": {
      "empty_miss": [
        68.52875,
        69.46667,
        69.73834,
        70.83416,
        74.2675,
        80.17333,
        88.24917,
        96.455,
        114.975
      ],
      "single_hit": [
        99.1075,
        99.45333,
        99.62625,
        99.93417,
        99.95292,
        100.22417,
        100.57125,
        100.96208,
        102.10584
      ],
      "64_origins_hit": [
        99.55209,
        100.48792,
        100.82958,
        101.05375,
        101.47791,
        101.61084,
        101.71666,
        101.71708,
        103.26917
      ],
      "256_origins_hit": [
        99.09917,
        99.31791,
        100.14458,
        100.76417,
        101.09083,
        101.13,
        101.25458,
        102.30042,
        103.32334
      ],
      "full_hit": [
        132.94167,
        133.07416,
        133.18792,
        134.17875,
        134.765,
        135.14291,
        135.25417,
        135.53084,
        136.0325
      ],
      "full_miss": [
        70.04625,
        71.79875,
        71.955,
        72.22,
        72.22167,
        72.4625,
        72.59458,
        73.03042,
        74.37958
      ],
      "shared_full_hit": [
        575.7083325,
        579.915,
        586.87823,
        587.8871875,
        589.996875,
        592.763125,
        593.2309375,
        596.9579175,
        599.0421875
      ],
      "shared_full_miss": [
        105.29125,
        105.531355,
        105.73021,
        106.0629175,
        106.0871875,
        106.3395825,
        106.3790625,
        106.438335,
        107.3151025
      ]
    }
  },
  {
    "variant": "optimized",
    "round": 2,
    "ns_per_lookup_samples": {
      "empty_miss": [
        67.03167,
        67.3925,
        67.41791,
        67.45,
        72.38084,
        72.59041,
        73.53833,
        75.05542,
        75.45375
      ],
      "single_hit": [
        98.00875,
        99.32583,
        99.71375,
        100.41708,
        102.3475,
        103.1725,
        103.38333,
        104.4325,
        105.43542
      ],
      "64_origins_hit": [
        98.72583,
        99.43208,
        99.975,
        100.59167,
        100.62792,
        100.87542,
        102.14542,
        102.61666,
        102.765
      ],
      "256_origins_hit": [
        98.38833,
        99.1375,
        99.23958,
        99.6475,
        99.7675,
        99.87208,
        99.97709,
        100.0825,
        100.43416
      ],
      "full_hit": [
        130.47375,
        130.83459,
        131.07166,
        131.11542,
        132.35792,
        132.52,
        132.59291,
        133.66083,
        135.43042
      ],
      "full_miss": [
        71.23666,
        71.27542,
        71.38333,
        71.54417,
        71.58708,
        71.85667,
        72.59375,
        72.81667,
        73.38458
      ],
      "shared_full_hit": [
        563.011355,
        581.0104175,
        582.369585,
        590.0071875,
        596.5191675,
        603.194375,
        604.366875,
        614.5578125,
        622.4014575
      ],
      "shared_full_miss": [
        105.587085,
        106.341145,
        106.548855,
        106.66229,
        107.0239575,
        107.0253125,
        107.1209375,
        107.5758325,
        107.6841675
      ]
    }
  },
  {
    "variant": "baseline",
    "round": 2,
    "ns_per_lookup_samples": {
      "empty_miss": [
        73.95583,
        75.19208,
        75.80417,
        76.0775,
        76.66833,
        77.35042,
        77.74833,
        78.8925,
        79.99083
      ],
      "single_hit": [
        95.19958,
        95.27833,
        95.4025,
        95.53125,
        96.63958,
        96.73708,
        96.73917,
        97.01333,
        97.15083
      ],
      "64_origins_hit": [
        189.04375,
        198.28917,
        203.89208,
        206.51125,
        207.33083,
        208.81042,
        209.09542,
        211.24958,
        216.31375
      ],
      "256_origins_hit": [
        464.10625,
        472.73792,
        476.32041,
        482.79583,
        487.60167,
        488.05084,
        501.55792,
        506.9675,
        507.48084
      ],
      "full_hit": [
        1348.77584,
        1358.49792,
        1369.54375,
        1375.22083,
        1384.31959,
        1385.50709,
        1389.85084,
        1402.72625,
        1409.2475
      ],
      "full_miss": [
        1247.99625,
        1262.0637500000003,
        1294.07583,
        1350.38791,
        1363.23083,
        1405.28083,
        1426.20875,
        1577.08083,
        1714.90375
      ],
      "shared_full_hit": [
        2818.0784375,
        3055.2578125,
        3106.24302,
        3107.051665,
        3194.2504175,
        3342.4760425,
        3671.4670825,
        3817.8232300000004,
        3822.1465625
      ],
      "shared_full_miss": [
        3624.00979,
        3701.916665,
        3712.568855,
        3837.7475,
        3876.475729999999,
        3923.7982275,
        4014.1990625,
        4029.5464575,
        4149.4392725
      ]
    }
  }
]