pkg/trace/api: limit simultaneous otlp requests, do not drop payloads

[knusbaum]

What does this PR do?

This causes the OTLP Receiver to not drop traces when the payload channel is full.

This commit also limits the number of simultaneous RPC requests, forwarding the backpressure from the pipeline to the client, and limiting our memory to O(MaxConnections * MaxStreams * MaxPayloadSize) in the receiver, with MaxStreams being 1.

Motivation

This should improve the trace dropping situation without exploding memory as badly as having a large channel.

By default, a gRPC server will allow practically unlimited concurrent RPC requests, spawning a goroutine for each. The routine will read and deserialize the payload before calling the handler.

This means that our memory usage is not reasonably bounded, since we could be holding hundreds of payloads in memory waiting to be processed.

Additional Notes

Possible Drawbacks / Trade-offs

Adding this means that traces will not be dropped when the payload buffer to the processor is full, meaning GRPC will not send a response to the sender right away.

Describe how to test/QA your changes

We will use the new OTel load testing environment to exercise this change.

We should compare:

• throughput (are traces being dropped)
• memory usage (is memory usage better or worse)

See @knusbaum for details about more specifics regarding infrastructure.

Reviewer's Checklist

• [x] If known, an appropriate milestone has been selected; otherwise the Triage milestone is set.
• [x] Use the major_change label if your change either has a major impact on the code base, is impacting multiple teams or is changing important well-established internals of the Agent. This label will be use during QA to make sure each team pay extra attention to the changed behavior. For any customer facing change use a releasenote.
• [x] A release note has been added or the changelog/no-changelog label has been applied.
• [ ] Changed code has automated tests for its functionality.
• [ ] Adequate QA/testing plan information is provided. Except if the qa/skip-qa label, with required either qa/done or qa/no-code-change labels, are applied.
• [ ] At least one team/.. label has been applied, indicating the team(s) that should QA this change.
• [ ] If applicable, docs team has been notified or an issue has been opened on the documentation repo.
• [ ] If applicable, the need-change/operator and need-change/helm labels have been applied.
• [ ] If applicable, the k8s/<min-version> label, indicating the lowest Kubernetes version compatible with this feature.
• [ ] If applicable, the config template has been updated.

[pr-commenter[bot]]

Bloop Bleep... Dogbot Here

Regression Detector Results

Run ID: 194b73be-5bad-4308-a665-9da05e85b2e8 Baseline: a82a8f97aa98a0654fa09dfd295c5f01a3162423 Comparison: ff19778d477d7edb772be9753f064a2f3e96282c Total CPUs: 7

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

Experiments with missing or malformed data

• basic_py_check

Usually, this warning means that there is no usable optimization goal data for that experiment, which could be a result of misconfiguration.

No significant changes in experiment optimization goals

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

There were no significant changes in experiment optimization goals at this confidence level and effect size tolerance.

Experiments ignored for regressions

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

perf	experiment	goal	Δ mean %	Δ mean % CI
➖	file_to_blackhole	% cpu utilization	-0.61	[-7.16, +5.94]

Fine details of change detection per experiment

perf	experiment	goal	Δ mean %	Δ mean % CI
➖	otel_to_otel_logs	ingress throughput	+0.42	[-0.20, +1.04]
➖	idle	memory utilization	+0.30	[+0.27, +0.34]
➖	process_agent_standard_check_with_stats	memory utilization	+0.21	[+0.18, +0.24]
➖	process_agent_standard_check	memory utilization	+0.16	[+0.12, +0.19]
➖	trace_agent_json	ingress throughput	+0.02	[-0.03, +0.06]
➖	tcp_dd_logs_filter_exclude	ingress throughput	+0.00	[-0.00, +0.00]
➖	uds_dogstatsd_to_api	ingress throughput	+0.00	[-0.00, +0.00]
➖	trace_agent_msgpack	ingress throughput	-0.00	[-0.02, +0.01]
➖	process_agent_real_time_mode	memory utilization	-0.05	[-0.10, -0.01]
➖	tcp_syslog_to_blackhole	ingress throughput	-0.42	[-0.48, -0.37]
➖	file_tree	memory utilization	-0.44	[-0.53, -0.36]
➖	file_to_blackhole	% cpu utilization	-0.61	[-7.16, +5.94]
➖	uds_dogstatsd_to_api_cpu	% cpu utilization	-1.52	[-2.94, -0.09]

Explanation

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".

[knusbaum]

Dropped in favor of #23085