unkin-agent
bc8a72e5cc
## Why We added a mirror load-balancing strategy (round_robin / least_conn) in #122 and need hard numbers on whether mirror *selection* adds meaningful request latency. This PR adds microbenchmarks that isolate the selection cost (no network/DB/redis) and commits the results as a permanent regression guard. ## How `internal/proxy/selection_bench_test.go` builds a zero-value `Engine` (same setup as `leastconn_test.go` / `multibaseurl_test.go`) and calls the selection functions directly: - `BenchmarkBaseURLAttemptOrder_SingleURL` — the `len<=1` early-return no-op path - `BenchmarkBaseURLAttemptOrder_RoundRobin` — pools of 3 and 8 - `BenchmarkBaseURLAttemptOrder_LeastConn` — pools of 3 and 8, with in-flight skew preloaded on the gauges - `BenchmarkBeginEndAttempt` — the gauge inc/dec pair - `_Parallel` (`RunParallel`) variants of RR / least_conn / begin-end to surface sync.Map + atomic contention Run: `go test -run=^$ -bench='BaseURLAttemptOrder|BeginEndAttempt' -benchmem -benchtime=1s -count=6 -cpu=8 ./internal/proxy/` `internal/proxy/BENCHMARKS.md` has the median-of-6 table (Ryzen 7 4700U, go1.26.5) plus the raw runs. ## Headline numbers (median of 6, ns/op — sequential | parallel@8) | selection | seq ns/op | parallel ns/op | allocs/op | |---|---:|---:|---:| | single-URL | ~133 | — | 1 | | round_robin (3) | ~462 | ~67 | 4 | | round_robin (8) | ~682 | ~130 | 4 | | least_conn (3) | ~2690 | ~292 | 22 | | least_conn (8) | ~15200 | ~1673 | 132 | | beginAttempt+endAttempt | ~514 | ~72 | 4 | **Key caveat:** `baseURLAttemptOrder` is only called from `headUpstream` / `fetchFromUpstream` / `checkUpstream` — the cache-miss/upstream path. A cache hit returns `Source: "cache"` before any selection runs, so the **cache-hit hot path pays zero** selection cost regardless of strategy. **Verdict:** even the worst case (least_conn across 8 mirrors, ~15 µs) is <1% of the multi-millisecond upstream round-trip it accompanies; round_robin (~0.5 µs) is negligible. The strategy adds no meaningful latency. least_conn's cost scales super-linearly because the `sort.SliceStable` comparator re-derives each mirror's gauge via `inflightCounter` (string-concat key + speculative `new(atomic.Int64)`) O(n·log n) times — a possible future micro-opt (snapshot loads before sorting), out of scope here. Reviewed-on: #123 Co-authored-by: unkin-agent <unkin-agent@unkin.net> Co-committed-by: unkin-agent <unkin-agent@unkin.net>
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Mirror-selection benchmarks
These benchmarks (selection_bench_test.go) isolate the mirror load-balancing
selection overhead — no network, no DB, no Redis. They build a zero-value
Engine and call baseURLAttemptOrder / beginAttempt / endAttempt
directly, the same way leastconn_test.go and multibaseurl_test.go do.
Goal: quantify how much latency the load-balancing strategy (round_robin vs
least_conn) adds versus a plain single-URL remote, and give a permanent
regression guard.
Key context: selection is cache-miss-only
baseURLAttemptOrder is called from exactly three places — headUpstream,
fetchFromUpstream, and checkUpstream — all on the upstream / cache-miss
path. A cache hit returns Source: "cache" from GetArtifact / store.Stat
before any selection code runs. So none of the numbers below apply to the hot
cache-hit path: cache hits pay zero selection cost regardless of strategy.
The overhead here is paid once per upstream fetch, alongside a network round-trip
measured in milliseconds.
How to run
go test -run=^$ -bench='BaseURLAttemptOrder|BeginEndAttempt' -benchmem \
-benchtime=1s -count=6 -cpu=8 ./internal/proxy/
Results
Machine: AMD Ryzen 7 4700U (8 threads), linux/amd64, go1.26.5.
-benchtime=1s -count=6; figures below are the median of 6 runs.
Sequential (single-goroutine)
| Benchmark | ns/op | B/op | allocs/op |
|---|---|---|---|
| BaseURLAttemptOrder_SingleURL | ~133 | 16 | 1 |
| BaseURLAttemptOrder_RoundRobin/3 | ~462 | 120 | 4 |
| BaseURLAttemptOrder_RoundRobin/8 | ~682 | 280 | 4 |
| BaseURLAttemptOrder_LeastConn/3 | ~2690 | 474 | 22 |
| BaseURLAttemptOrder_LeastConn/8 | ~15200 | 2688 | 132 |
| BeginEndAttempt (gauge inc/dec) | ~514 | 104 | 4 |
Parallel (RunParallel, GOMAXPROCS=8) — ns/op is wall-time across 8 cores
| Benchmark | ns/op | B/op | allocs/op |
|---|---|---|---|
| BaseURLAttemptOrder_RoundRobin_Parallel/3 | ~67.5 | 120 | 4 |
| BaseURLAttemptOrder_RoundRobin_Parallel/8 | ~130 | 280 | 4 |
| BaseURLAttemptOrder_LeastConn_Parallel/3 | ~292 | 474 | 22 |
| BaseURLAttemptOrder_LeastConn_Parallel/8 | ~1673 | 2688 | 132 |
| BeginEndAttempt_Parallel | ~71.5 | 104 | 4 |
Reading the numbers
- Single-URL is a near-no-op (~133 ns, 1 alloc): the
len(urls) <= 1early return just returns the pool slice. Every non-mirrored remote takes this path. - round_robin is cheap: ~462 ns for a 3-mirror pool, ~682 ns for 8. Cost is
one atomic cursor increment plus building the rotated
[]string. Allocs are constant at 4 (the ordered slice + its backing string headers), size grows with pool length. - least_conn is more expensive and scales super-linearly: ~2.7 µs / 22
allocs at 3 mirrors, ~15 µs / 132 allocs at 8. The cost is the per-call
sort.SliceStable, whose comparator callsinflightCounter(async.Map.LoadOrStorewith aremoteName\x00urlstring-concat key plus a speculativenew(atomic.Int64)) O(n·log n) times. That is where the alloc count and the time come from — not the sort itself. A future optimization could snapshot each mirror's load once before sorting; out of scope for this measurement PR. - beginAttempt/endAttempt (~514 ns seq, ~72 ns parallel) is one
LoadOrStore+ two atomic adds; it only runs for least_conn multi-mirror remotes, once per upstream attempt. - Under concurrency the atomics/sync.Map do not collapse: every parallel
variant reports lower ns/op than its sequential twin because work spreads
across 8 cores (RunParallel reports aggregate wall-time-per-op). No contention
cliff on the shared rrCounters cursor, the inflight
sync.Map, or the per-mirroratomic.Int64gauges.
Verdict
At the per-request scale that matters (a cache-miss that is already doing a multi-millisecond network fetch), even the worst case here — least_conn across 8 mirrors at ~15 µs — is <1% of a single upstream round-trip, and round_robin (~0.5 µs) is negligible. The strategy adds no meaningful latency, and it adds exactly zero to the cache-hit hot path because selection never runs there.
Raw output (all 6 runs)
goos: linux
goarch: amd64
pkg: git.unkin.net/unkin/artifactapi/internal/proxy
cpu: AMD Ryzen 7 4700U with Radeon Graphics
BenchmarkBaseURLAttemptOrder_SingleURL-8 8635875 138.4 ns/op 16 B/op 1 allocs/op
BenchmarkBaseURLAttemptOrder_SingleURL-8 9673108 126.7 ns/op 16 B/op 1 allocs/op
BenchmarkBaseURLAttemptOrder_SingleURL-8 8001002 147.3 ns/op 16 B/op 1 allocs/op
BenchmarkBaseURLAttemptOrder_SingleURL-8 11303490 135.0 ns/op 16 B/op 1 allocs/op
BenchmarkBaseURLAttemptOrder_SingleURL-8 10125138 132.0 ns/op 16 B/op 1 allocs/op
BenchmarkBaseURLAttemptOrder_SingleURL-8 8025687 130.1 ns/op 16 B/op 1 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool3-8 2706013 453.2 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool3-8 2498718 444.4 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool3-8 2605516 471.6 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool3-8 2799928 487.6 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool3-8 2463375 418.6 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool3-8 2472265 474.3 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool8-8 1741789 689.4 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool8-8 1775467 602.1 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool8-8 1829398 688.4 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool8-8 1781149 679.3 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool8-8 1795680 599.4 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin/pool8-8 1739122 684.5 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool3-8 424184 2675 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool3-8 426796 2498 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool3-8 427116 2716 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool3-8 430540 2681 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool3-8 418156 2700 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool3-8 423009 2711 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool8-8 163642 14007 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool8-8 78501 15457 ns/op 2688 B/op 131 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool8-8 76380 14928 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool8-8 183634 16091 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool8-8 73809 15561 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn/pool8-8 74546 13962 ns/op 2688 B/op 132 allocs/op
BenchmarkBeginEndAttempt-8 2350348 519.7 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt-8 2321659 514.0 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt-8 2284635 438.0 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt-8 2287051 513.9 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt-8 2286481 520.3 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt-8 2837775 512.9 ns/op 104 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool3-8 16052568 67.78 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool3-8 17452791 66.07 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool3-8 17549858 69.25 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool3-8 18845167 64.55 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool3-8 16285608 69.81 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool3-8 17382639 67.12 ns/op 120 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool8-8 9734368 120.6 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool8-8 10154736 133.3 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool8-8 10061422 131.8 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool8-8 10212364 127.4 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool8-8 10259030 132.5 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_RoundRobin_Parallel/pool8-8 10069576 122.4 ns/op 280 B/op 4 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool3-8 4288112 292.7 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool3-8 4009249 295.9 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool3-8 4176378 291.3 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool3-8 4104871 289.9 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool3-8 4245262 296.4 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool3-8 4079778 290.3 ns/op 474 B/op 22 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool8-8 748200 1636 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool8-8 763029 1653 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool8-8 663717 1772 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool8-8 739677 1676 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool8-8 763148 1669 ns/op 2688 B/op 132 allocs/op
BenchmarkBaseURLAttemptOrder_LeastConn_Parallel/pool8-8 610597 1684 ns/op 2688 B/op 132 allocs/op
BenchmarkBeginEndAttempt_Parallel-8 17266058 69.46 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt_Parallel-8 17151303 72.05 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt_Parallel-8 16919542 74.17 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt_Parallel-8 16948015 72.49 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt_Parallel-8 16918693 69.52 ns/op 104 B/op 4 allocs/op
BenchmarkBeginEndAttempt_Parallel-8 17376012 70.99 ns/op 104 B/op 4 allocs/op