flatten ActivityTreeNodeStats to reduce heap allocations

[mbertrone]

The pprof delta-heap profile showed NewActivityTreeNodeStats as the #1 allocation hotspot: 95.5MB / 1.1M objects (15.95% of total) in a 5-minute window. The child function atomic.NewUint64 alone accounted for 18MB / 1.18M objects (18.72%).

Root cause: each Stats object allocated ~630 individual *atomic.Uint64 pointers via maps — one per event type (MaxKernelEventType ≈ 65) times 10 counters each (1 processedCount + 5 addedCount + 4 droppedCount). The outer map[model.EventType]*statsPerEventType added further overhead.

This commit replaces all heap-allocated pointer-based maps with inline fixed-size arrays of value-type atomics:

• Stats.counts: map[model.EventType]*statsPerEventType → [model.MaxKernelEventType]statsPerEventType (flat array)

• statsPerEventType.processedCount: *atomic.Uint64 → atomic.Uint64 (inline value)

• statsPerEventType.addedCount: map[NodeGenerationType]*atomic.Uint64 → [MaxNodeGenerationType + 1]atomic.Uint64 (5-element array)

• statsPerEventType.droppedCount: map[NodeDroppedReason]*atomic.Uint64 → [nodeDroppedReasonCount]atomic.Uint64 (4-element array)

The entire stats structure is now a single contiguous ~5KB allocation (inline in Stats) instead of 630+ individual heap objects per instance. NewActivityTreeNodeStats() becomes trivial zero-value initialization.

Additional improvements in SendStats:

• Switched from map range to index-based iteration with pointer access to avoid copying non-copyable atomic.Uint64 values.
• Replaced fmt.Sprintf("event_type:%s", evtType) with string concat ("event_type:" + evtType.String()), eliminating ~87k fmt.Sprintf allocations per SendStats cycle.
• Replaced fmt.Sprintf("reason:%s", reason) with reason.Tag() which returns pre-built cached strings.

Migrated from go.uber.org/atomic to sync/atomic (stdlib):

• .Inc() → .Add(1)
• .Swap(0) unchanged

Estimated savings: ~113MB alloc_space, ~2.28M object allocations per 5-minute profiling window.

What does this PR do?

Motivation

Describe how you validated your changes

Additional Notes

[agent-platform-auto-pr]

Static quality checks

✅ Please find below the results from static quality gates
Comparison made with ancestor 562d57d
📊 Static Quality Gates Dashboard
🔗 SQG Job

Error

	Quality gate	Change	Size (prev → curr → max)
⚠️	docker_cluster_agent_amd64 (on disk)	neutral	191.408 MiB → 191.320

Gate failure full details

Quality gate	Error type	Error message
docker_cluster_agent_amd64	StaticQualityGateFailed (non-blocking: size unchanged from ancestor)	�[91mstatic_quality_gate_docker_cluster_agent_amd64 failed! Disk size 191.4 MB exceeds limit of 191.3 MB by 89.9 KB�[0m

Note: Some gates exceeded limits but are non-blocking because the size hasn't increased from the ancestor commit.

Successful checks

Info

	Quality gate	Change	Size (prev → curr → max)
✅	agent_deb_amd64	-4.0 KiB (0.00% reduction)	755.239 → 755.235 → 759.670
✅	agent_deb_amd64_fips	-8.0 KiB (0.00% reduction)	714.527 → 714.519 → 720.180
✅	agent_rpm_amd64	-4.0 KiB (0.00% reduction)	755.222 → 755.218 → 759.640
✅	agent_rpm_amd64_fips	-8.0 KiB (0.00% reduction)	714.511 → 714.503 → 720.160
✅	agent_rpm_arm64	-4.0 KiB (0.00% reduction)	733.594 → 733.590 → 741.840
✅	agent_rpm_arm64_fips	-4.0 KiB (0.00% reduction)	695.759 → 695.755 → 703.390
✅	agent_suse_amd64	-4.0 KiB (0.00% reduction)	755.222 → 755.218 → 759.640
✅	agent_suse_amd64_fips	-8.0 KiB (0.00% reduction)	714.511 → 714.503 → 720.160
✅	agent_suse_arm64	-4.0 KiB (0.00% reduction)	733.594 → 733.590 → 741.840
✅	agent_suse_arm64_fips	-4.0 KiB (0.00% reduction)	695.759 → 695.755 → 703.390
✅	docker_agent_amd64	-4.0 KiB (0.00% reduction)	816.287 → 816.283 → 821.990
✅	docker_agent_arm64	-4.0 KiB (0.00% reduction)	819.371 → 819.367 → 828.520
✅	docker_agent_jmx_amd64	-4.0 KiB (0.00% reduction)	1007.199 → 1007.195 → 1012.870
✅	docker_agent_jmx_arm64	-4.0 KiB (0.00% reduction)	999.065 → 999.061 → 1008.120

16 successful checks with minimal change (< 2 KiB)

	Quality gate	Current Size
✅	agent_heroku_amd64	323.653 MiB
✅	agent_msi	621.319 MiB
✅	docker_cluster_agent_arm64	206.820 MiB
✅	docker_cws_instrumentation_amd64	7.135 MiB
✅	docker_cws_instrumentation_arm64	6.689 MiB
✅	docker_dogstatsd_amd64	38.469 MiB
✅	docker_dogstatsd_arm64	36.812 MiB
✅	dogstatsd_deb_amd64	29.684 MiB
✅	dogstatsd_deb_arm64	27.849 MiB
✅	dogstatsd_rpm_amd64	29.684 MiB
✅	dogstatsd_suse_amd64	29.684 MiB
✅	iot_agent_deb_amd64	42.885 MiB
✅	iot_agent_deb_arm64	39.998 MiB
✅	iot_agent_deb_armhf	40.568 MiB
✅	iot_agent_rpm_amd64	42.885 MiB
✅	iot_agent_suse_amd64	42.885 MiB

On-wire sizes (compressed)

	Quality gate	Change	Size (prev → curr → max)
⚠️	docker_cluster_agent_amd64	neutral	66.745 MiB → 67.220
✅	agent_deb_amd64	-20.55 KiB (0.01% reduction)	184.841 → 184.821 → 186.090
✅	agent_deb_amd64_fips	-14.55 KiB (0.01% reduction)	176.318 → 176.304 → 180.330
✅	agent_heroku_amd64	neutral	87.250 MiB → 88.440
✅	agent_msi	-28.0 KiB (0.02% reduction)	143.320 → 143.293 → 143.470
✅	agent_rpm_amd64	-23.69 KiB (0.01% reduction)	187.109 → 187.086 → 189.170
✅	agent_rpm_amd64_fips	+25.07 KiB (0.01% increase)	178.254 → 178.279 → 181.060
✅	agent_rpm_arm64	-30.04 KiB (0.02% reduction)	169.303 → 169.273 → 170.020
✅	agent_rpm_arm64_fips	+29.25 KiB (0.02% increase)	162.271 → 162.299 → 164.130
✅	agent_suse_amd64	-23.69 KiB (0.01% reduction)	187.109 → 187.086 → 189.170
✅	agent_suse_amd64_fips	+25.07 KiB (0.01% increase)	178.254 → 178.279 → 181.060
✅	agent_suse_arm64	-30.04 KiB (0.02% reduction)	169.303 → 169.273 → 170.020
✅	agent_suse_arm64_fips	+29.25 KiB (0.02% increase)	162.271 → 162.299 → 164.130
✅	docker_agent_amd64	-5.38 KiB (0.00% reduction)	277.432 → 277.426 → 279.410
✅	docker_agent_arm64	-2.47 KiB (0.00% reduction)	264.760 → 264.758 → 267.960
✅	docker_agent_jmx_amd64	-5.42 KiB (0.00% reduction)	346.070 → 346.065 → 348.040
✅	docker_agent_jmx_arm64	-2.42 KiB (0.00% reduction)	329.414 → 329.412 → 332.560
✅	docker_cluster_agent_arm64	neutral	62.815 MiB → 63.640
✅	docker_cws_instrumentation_amd64	neutral	2.995 MiB → 3.330
✅	docker_cws_instrumentation_arm64	neutral	2.726 MiB → 3.090
✅	docker_dogstatsd_amd64	neutral	14.892 MiB → 15.820
✅	docker_dogstatsd_arm64	neutral	14.232 MiB → 14.830
✅	dogstatsd_deb_amd64	neutral	7.848 MiB → 8.790
✅	dogstatsd_deb_arm64	neutral	6.735 MiB → 7.710
✅	dogstatsd_rpm_amd64	neutral	7.860 MiB → 8.800
✅	dogstatsd_suse_amd64	neutral	7.860 MiB → 8.800
✅	iot_agent_deb_amd64	+2.72 KiB (0.02% increase)	11.257 → 11.259 → 12.040
✅	iot_agent_deb_arm64	neutral	9.620 MiB → 10.450
✅	iot_agent_deb_armhf	neutral	9.820 MiB → 10.620
✅	iot_agent_rpm_amd64	neutral	11.273 MiB → 12.060
✅	iot_agent_suse_amd64	neutral	11.273 MiB → 12.060

[cit-pr-commenter-54b7da]

Regression Detector

Regression Detector Results

Metrics dashboard
Target profiles
Run ID: dcf26e87-689e-4f67-9e1c-8dc55b0df57f

Baseline: 562d57d
Comparison: e10fb21
Diff

Optimization Goals: ✅ No significant changes detected

Experiments ignored for regressions

Regressions in experiments with settings containing erratic: true are ignored.

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	docker_containers_cpu	% cpu utilization	+0.10	[-3.04, +3.23]	1	Logs

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI	trials	links
➖	ddot_metrics_sum_delta	memory utilization	+0.39	[+0.19, +0.59]	1	Logs
➖	uds_dogstatsd_20mb_12k_contexts_20_senders	memory utilization	+0.32	[+0.26, +0.37]	1	Logs
➖	otlp_ingest_logs	memory utilization	+0.21	[+0.12, +0.30]	1	Logs
➖	ddot_metrics_sum_cumulativetodelta_exporter	memory utilization	+0.16	[-0.07, +0.39]	1	Logs
➖	docker_containers_memory	memory utilization	+0.11	[+0.03, +0.18]	1	Logs
➖	docker_containers_cpu	% cpu utilization	+0.10	[-3.04, +3.23]	1	Logs
➖	file_to_blackhole_0ms_latency	egress throughput	+0.07	[-0.39, +0.53]	1	Logs
➖	file_to_blackhole_100ms_latency	egress throughput	+0.00	[-0.04, +0.05]	1	Logs
➖	uds_dogstatsd_to_api	ingress throughput	-0.00	[-0.13, +0.12]	1	Logs
➖	tcp_dd_logs_filter_exclude	ingress throughput	-0.00	[-0.09, +0.09]	1	Logs
➖	uds_dogstatsd_to_api_v3	ingress throughput	-0.01	[-0.14, +0.12]	1	Logs
➖	file_to_blackhole_1000ms_latency	egress throughput	-0.01	[-0.44, +0.42]	1	Logs
➖	file_to_blackhole_500ms_latency	egress throughput	-0.03	[-0.41, +0.35]	1	Logs
➖	ddot_metrics_sum_cumulative	memory utilization	-0.04	[-0.20, +0.12]	1	Logs
➖	quality_gate_idle	memory utilization	-0.06	[-0.11, -0.02]	1	Logs bounds checks dashboard
➖	quality_gate_idle_all_features	memory utilization	-0.31	[-0.34, -0.27]	1	Logs bounds checks dashboard
➖	file_tree	memory utilization	-0.51	[-0.56, -0.46]	1	Logs
➖	quality_gate_metrics_logs	memory utilization	-0.53	[-0.73, -0.33]	1	Logs bounds checks dashboard
➖	otlp_ingest_metrics	memory utilization	-0.61	[-0.76, -0.45]	1	Logs
➖	ddot_metrics	memory utilization	-0.76	[-0.97, -0.54]	1	Logs
➖	ddot_logs	memory utilization	-0.79	[-0.86, -0.72]	1	Logs
➖	quality_gate_logs	% cpu utilization	-1.31	[-2.82, +0.19]	1	Logs bounds checks dashboard
➖	tcp_syslog_to_blackhole	ingress throughput	-2.02	[-2.11, -1.94]	1	Logs

Bounds Checks: ✅ Passed

perf	experiment	bounds_check_name	replicates_passed	links
✅	docker_containers_cpu	simple_check_run	10/10
✅	docker_containers_memory	memory_usage	10/10
✅	docker_containers_memory	simple_check_run	10/10
✅	file_to_blackhole_0ms_latency	lost_bytes	10/10
✅	file_to_blackhole_0ms_latency	memory_usage	10/10
✅	file_to_blackhole_1000ms_latency	lost_bytes	10/10
✅	file_to_blackhole_1000ms_latency	memory_usage	10/10
✅	file_to_blackhole_100ms_latency	lost_bytes	10/10
✅	file_to_blackhole_100ms_latency	memory_usage	10/10
✅	file_to_blackhole_500ms_latency	lost_bytes	10/10
✅	file_to_blackhole_500ms_latency	memory_usage	10/10
✅	quality_gate_idle	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_idle	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_idle_all_features	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_logs	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_logs	lost_bytes	10/10	bounds checks dashboard
✅	quality_gate_logs	memory_usage	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	cpu_usage	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	intake_connections	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	lost_bytes	10/10	bounds checks dashboard
✅	quality_gate_metrics_logs	memory_usage	10/10	bounds checks dashboard

Explanation

Confidence level: 90.00%
Effect size tolerance: |Δ mean %| ≥ 5.00%

Performance changes are noted in the perf column of each table:

• ✅ = significantly better comparison variant performance
• ❌ = significantly worse comparison variant performance
• ➖ = no significant change in performance

A regression test is an A/B test of target performance in a repeatable rig, where "performance" is measured as "comparison variant minus baseline variant" for an optimization goal (e.g., ingress throughput). Due to intrinsic variability in measuring that goal, we can only estimate its mean value for each experiment; we report uncertainty in that value as a 90.00% confidence interval denoted "Δ mean % CI".

For each experiment, we decide whether a change in performance is a "regression" -- a change worth investigating further -- if all of the following criteria are true:

1. Its estimated |Δ mean %| ≥ 5.00%, indicating the change is big enough to merit a closer look.

2. Its 90.00% confidence interval "Δ mean % CI" does not contain zero, indicating that if our statistical model is accurate, there is at least a 90.00% chance there is a difference in performance between baseline and comparison variants.

3. Its configuration does not mark it "erratic".

CI Pass/Fail Decision

✅ Passed. All Quality Gates passed.

• quality_gate_idle_all_features, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_idle_all_features, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_idle, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.
• quality_gate_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check memory_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check intake_connections: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check cpu_usage: 10/10 replicas passed. Gate passed.
• quality_gate_metrics_logs, bounds check lost_bytes: 10/10 replicas passed. Gate passed.

[dd-octo-sts]

This pull request has been automatically marked as stale because it has not had activity in the past 15 days.

It will be closed in 30 days if no further activity occurs. If this pull request is still relevant, adding a comment or pushing new commits will keep it open. Also, you can always reopen the pull request if you missed the window.

Thank you for your contributions!

[dd-octo-sts]

This pull request was automatically closed because it has been stale for 15 days with no activity.

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Thanks!