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Stability & soak testing

This document records the stability/performance testing campaign run against xtcp2 — the methods used, the bugs it found (and their fixes), the measured performance wins, the OS-thread scaling model it uncovered, and the resulting operator guidance. It is meant to give other developers the full picture of what has been validated and how to reproduce it.

Table of contents

What we tested and how

Three complementary layers, all driven from the Nix flake (see integration-testing.md for the microVM harness):

  1. Go micro-benchmarks + race detector — pkg/recordfmt (marshalling), pkg/xtcp (envelope size-cap), pkg/xtcpnl (netlink parse). Run with benchstat for before/after deltas and go test -race for concurrency.
  2. Live CPU/alloc profiling — the daemon's net/http/pprof endpoint (:9088/debug/pprof) captured under a synthetic ~2,000-socket load to find the real hot paths (see performance.md).
  3. MicroVM integration soaks — real xtcp2 in a QEMU/KVM guest under real load:
    • clickhouse-pipeline — the production path (protobufList → Kafka/Redpanda → ClickHouse), with end-to-end row reconciliation.
    • tcp-stress — 20 docker containers × ~100 sockets (per-container namespace discovery under load).
    • soak — continuous ip netns add/del churn (~200 namespaces) plus a persistent TCP socket population — the leak/thread shake-out.

Performance optimizations (measured)

The profile showed the daemon's CPU was dominated by reflective protobuf work (proto.Size ≈ 40% of non-idle samples) and netlink syscalls (≈ 26%). Two structural fixes removed essentially all of the marshalling cost:

Optimization PR Result
Envelope size-cap → O(1) accumulator (was proto.Size over the whole growing envelope every 64 appends, ~O(rows²)) #46 filling a 10k-row envelope: 366 ms → 5 ms (~74×)
Reflection-free protobufList via vtprotobuf (MarshalVT/SizeVT) #48 protobufList marshal: 68.5 µs → 13.1 µs (~5.2×), 4 allocs/6 KB → 0 allocs
Profile-guided optimization (PGO) #44 recordfmt geomean −7.3% (now small post-vtproto; kept as free hygiene)

vtprotobuf's wire output is guarded as byte-identical to the protobuf runtime by a differential conformance test (#49) — so a future vtprotobuf bump can't silently change the ClickHouse wire format.

End-to-end effect: in the clickhouse-pipeline soak, a mid-run CPU profile under live Kafka→ClickHouse ingest showed the daemon at ~0.1% CPU, 100% netlink syscall, zero marshalling samples — i.e. xtcp2 feeds a real pipeline essentially for free, and is now I/O-bound rather than marshalling-bound.

Bugs found and fixed

# Bug Found by Fix
1 Envelope size-cap O(rows²) proto.Size (~40% CPU) pprof #46
2 OS-thread leak: namespace deleted during instance init left its cancel unreachable → goroutine blocked forever holding a locked thread soak + crash-dump #52
3 Data race in kafka_to_clickhouse produce-promise callback (wg.Done() before kgoRecordPool.Put()) go test -race #53
4 tcp-stress hard-coded host :9090 hostfwd → qemu won't start on a box already running Prometheus tcp-stress soak #50
5 soak VM under-sized (1024 MiB) → nsTest load-gen OOM-loops, degrading churn soak #51
6 Soak runner under-reported xtcp2 restarts (missed Go fatal error exits → would falsely PASS) soak crash-loop analysis tracked (#54 plan)

Note on #2: that fix is correct but was not the dominant thread consumer — see the scaling model below. It was the soak finding the real limit that mattered most.

The OS-thread scaling model

The most important finding. Under sustained namespace churn the daemon crash-looped on fatal error: thread exhaustion (the Go -maxThreads cap, default 2000). Crash-dump analysis showed where the threads were:

Each per-namespace netlinker blocks in syscall.Recvfrom, and a blocked syscall pins one OS thread. So:

OS threads ≈ namespaces × (netlinkers + 1)

(the +1 is the per-namespace instance goroutine, which holds one LockOSThread'd thread for the namespace's lifetime). With the default -netlinkers 4, ~200 namespaces ≈ ~1,000 threads steady-state; churn-init backlog pushes the total past the 2,000 cap → crash.

io_uring does not avoid this. The io_uring netlinker also runtime.LockOSThread()s for the ring's lifetime (one pinned thread per netlinker) and its wait is a blocking io_uring_enter — same thread cost. (And as the io_uring evaluation below shows, it doesn't reduce kernel CPU either.)

The only approach that decouples thread count from ns × netlinkers is reading netlink non-blocking through Go's runtime poller (readers park instead of pinning threads) — designed in design-nonblocking-netlink.md.

Soak results

Soak Duration Result
clickhouse-pipeline 3 h PASS — envelopeRows 1,975→47,795 monotonic, ClickHouse reconcile within ~0.1%, 0 panics/restarts, mid-run CPU ~0.1%
tcp-stress 3 h PASS — 20/20 containers + per-container netns discovered, ~10.7 M packets, 0 panics/restarts
soak (churn), -netlinkers 4 ~10 min FAIL — thread-exhaustion crash-loop (~25 restarts) → root-caused the scaling model
soak (churn), -netlinkers 1 + -maxThreads 8000 12 h PASS — 242,100 ns-churn events, 0 panics, 0 restarts, 0 thread-exhaustion, single xtcp2 process throughout

The 12 h soak ran the worst case: ~200 namespaces churned at 100 ms add/delete. pollDuration under that storm stayed bounded at ~6–10 s (one reader draining many churning sockets) — a steady-state degradation, not a leak. Real deployments with stable containers and low churn do far less per-poll work.

io_uring evaluation

We implemented and benchmarked the optional io_uring netlink reader (-ioUring) to test the hypothesis that its shared-memory ring + batched syscalls would reduce kernel load. A controlled A/B on the stable tcp-stress workload — 1 h each, io_uring the only variable, -d 1 so logging doesn't confound CPU — showed it does not help this workload:

per netlink packet syscall io_uring Δ
kernel CPU (stime) 743 µs 733 µs −1.4% (noise)
context switches 0.086 0.083 −3.4%
kernel/user CPU ratio 4.14 4.10 −0.8%
RSS ~56 MB ~186 MB +232%
dominant syscall (strace -c) recvfrom 92.5% (2,115 calls) io_uring_enter 92.4% (3,467 calls) replaced, not reduced

Why: io_uring cleanly replaces recvfrom with io_uring_enter, but per-packet kernel CPU is unchanged because the cost is dominated by the kernel generating the inet_diag dump (walking the socket table + serializing tcp_info/cong/ meminfo — roughly ~10 µs/socket, ~99.9% of the per-packet cost), not by syscall entry/exit overhead (~1 µs/packet, ~0.1%). io_uring optimizes the 0.1%. It also gives no thread benefit — the io_uring netlinker still LockOSThreads per netlinker (same ns × netlinkers scaling). Net: same CPU, same thread count, 3× the memory for the ring buffers.

io_uring wins for high-frequency small I/O where syscall overhead dominates; netlink inet_diag dumps are the opposite (infrequent, large, kernel-generation- heavy). No io_uring config rescues it: bigger batches don't touch dump-gen, and SQPOLL would burn a dedicated kernel thread per ring (more system CPU, not less). The real lever for kernel load is reducing what the kernel dumps (-deserializers, poll -frequency).

Verdict: leave -ioUring off. The code/flag are kept (tested, iouring-audit guarded) for completeness; the default is correct. See performance.md.

Operator guidance

For a deployment of up to ~200 containers (namespaces):

  • Set -netlinkers 2 and -maxThreads 4000. This keeps OS threads ≈ 200 × (2+1) ≈ 600 — comfortably under the cap — while preserving read parallelism. -maxThreads is only a safety backstop; 4000 is ample for 200 namespaces (raise it proportionally if you run more).
  • -netlinkers 1 is also validated (the 12 h soak config) and has the smallest footprint (~400 threads); choose it if your per-namespace socket volume is low. -netlinkers 2 is the recommended balance for production.
  • Leave -ioUring off. It was tested and gives no benefit for this workload — same kernel CPU/packet, same ns × netlinkers thread cost, and +232% RSS (see io_uring evaluation).
  • Rule of thumb for any config: keep -maxThreads (and systemd TasksMax/LimitNPROC, default 8192 in the microVMs) above namespaces × (netlinkers + 1) with healthy margin.

These settings are validated stable; the blocking-read path is fine at this scale. The non-blocking redesign is only needed if you head toward ~1,000 namespaces or sustained heavy churn.

Known limitation & future work

  • Thread scaling at very high namespace counts / churn — addressed by the non-blocking-netlink design (design-nonblocking-netlink.md); not required for the ≤200-container target.
  • Soak runner restart detection — it greps for a log pattern that misses Go fatal error exits, so it reported restarts=0 while the daemon crash-looped. Harden it (assert a thread-count ceiling, detect fatal-error exits) so a future regression can't pass silently.

How to reproduce

# Micro-benchmarks + race
go test -bench=. -benchmem -count=8 ./pkg/recordfmt/... ./pkg/xtcp/... ./pkg/xtcpnl/...
go test -race ./pkg/xtcp/...
nix build .#test-go-race          # full race suite (CI gate)

# Live profile under synthetic load (daemon on :9088)
curl -s 'http://127.0.0.1:9088/debug/pprof/profile?seconds=45' > cpu.pprof
go tool pprof -top cpu.pprof

# MicroVM soaks (KVM; no sudo)
nix run .#microvm-x86_64-clickhouse-pipeline           # production pipeline (interactive)
nix run .#microvm-x86_64-tcp-stress -- --duration 3h   # container netns under load
nix run .#microvm-x86_64-soak -- --duration 12h        # namespace churn stability

The soak flavor uses -netlinkers 1 / a raised -maxThreads when validating the high-namespace thread budget; adjust nix/microvms/mkVm.nix (xtcp2BasicArgs) to test other values.