feat: --latency-window sliding-window tail latency for --realtime-latencies

[fcostaoliveira]

Summary

Adds a load1/5/15-style sliding window to the live latency block under --realtime-latencies. The per-tick output now emits three labeled rows per configured percentile:

• inst — last tick only
• win<N>s — sliding window (default 60s, configurable via --latency-window=SECS, set to 0 to disable, max 3600)
• overall — full run

Example with --latency-window=60 --print-percentiles=50,99,99.9:

[RUN #1 50%  2s] throughput 178,155 (avg: 172,043) ops/sec   …   miss 49.88% (avg: 49.87%)
[RUN #1 50%  2s] latency [inst    ]  p50 0.111  p99 0.223  p99.9 0.367
[RUN #1 50%  2s] latency [win60s  ]  p50 0.119  p99 0.239  p99.9 0.479
[RUN #1 50%  2s] latency [overall ]  p50 0.119  p99 0.239  p99.9 0.479 ms

Implementation

• Ring buffer of cloned hdr_histogram snapshots, one per second (memtier_benchmark.cpp:2125-2143).
• Per tick: clone inst_hist_agg into the next slot, freeing the displaced snapshot. O(W) per tick (microseconds for W ≤ 3600).
• To render: hdr_reset a reused scratch histogram, hdr_add every live slot, then hdr_value_at_percentile.
• Empty ticks still consume a slot, so the window measures wall-clock seconds, not "seconds with traffic".
• Long --print-percentiles lists wrap to multiple lines per row with deterministic line count, so the TTY cursor-up redraw stays stable.

Memory / CPU

• Each snapshot is one hdr_histogram (~tens of KB at default precision). Default 60s window ≈ 1-2 MB; capped at 3600s.
• O(W) hdr_add per tick, once per second. Negligible.
• No effect when --realtime-latencies is off or --latency-window=0.

Test plan

• make format-check clean
• Builds cleanly (release + --enable-sanitizers)
• ASAN smoke run with deep-tail percentiles: exit 0, no leaks, no errors
• RLTest test_realtime_latencies.py: 5/5 PASS (incl. 2 new tests: test_latency_window_zero_drops_window_row, test_latency_window_ring_smooths_spikes)
• Existing --realtime-latencies tests updated for new label-row layout and still pass
• --help and man page show --latency-window
• bash-completion lists --latency-window under options_no_comp

🤖 Generated with Claude Code

---

Note

Medium Risk
Changes --realtime-latencies progress output format and adds a per-second histogram ring buffer, which could affect log parsers and introduce memory/CPU overhead if misconfigured (bounded to 3600s).

Overview
Adds a new --latency-window=SECS option that computes sliding-window tail latency for --realtime-latencies by keeping a ring of per-second HDR histogram snapshots (default 60s, 0 disables, max 3600).

Updates the live progress block to emit labeled latency rows per tick (always inst + overall, plus win<N>s when enabled) and adjusts TTY redraw/wrapping logic accordingly.

Wires the flag through CLI parsing/help/man page and bash completion, extends benchmark_config with latency_window_secs, and updates/adds RLTest coverage to validate the new rows, wrapping behavior, and the --latency-window=0 disable contract.

^{Reviewed by Cursor Bugbot for commit 10285ab. Bugbot is set up for automated code reviews on this repo. Configure here.}

[jit-ci]

🛡️ Jit Security Scan Results

✅ No security findings were detected in this PR

---

^{Security scan by Jit}