analyzing-test-results

Analyzing k6 Test Results

Interpret k6 test output, identify performance bottlenecks, and provide optimization recommendations.

Analysis Workflow

1. Review summary output — Check overall pass/fail, iteration count, VU count
2. Check thresholds — Which passed/failed? By how much?
3. Analyze response times — Look at p50, p90, p95, p99 distribution
4. Break down request duration — Examine blocked, connecting, TLS, waiting, receiving
5. Identify bottlenecks — Map timing components to root causes
6. Suggest optimizations — Script improvements, threshold tuning, infrastructure changes

Understanding k6 Output

End-of-Test Summary

     data_received..................: 148 kB  4.9 kB/s
     data_sent......................: 17 kB   564 B/s
     http_req_blocked...............: avg=23.38ms  min=1µs    med=5µs    max=235.44ms p(90)=10µs   p(95)=210.47ms
     http_req_connecting............: avg=11.12ms  min=0s     med=0s     max=117.4ms  p(90)=0s     p(95)=99.38ms
   ✓ http_req_duration..............: avg=131.01ms min=116.4ms med=127.6ms max=196.38ms p(90)=146.21ms p(95)=176.91ms
     http_req_failed................: 0.00%  ✓ 0  ✗ 200
     http_req_receiving.............: avg=5.11ms   min=26µs   med=4.88ms max=14.82ms  p(90)=9.31ms p(95)=10.82ms
     http_req_sending...............: avg=27µs     min=4µs    med=23µs   max=240µs    p(90)=42µs   p(95)=66µs
     http_req_tls_handshaking.......: avg=11.7ms   min=0s     med=0s     max=116.76ms p(90)=0s     p(95)=105.06ms
     http_req_waiting...............: avg=125.86ms min=116.27ms med=122.6ms max=185.5ms p(90)=137.21ms p(95)=170.53ms
     http_reqs......................: 200    6.63/s
     iteration_duration.............: avg=1.16s    min=1.12s  med=1.13s  max=1.43s    p(90)=1.15s  p(95)=1.41s
     iterations.....................: 200    6.63/s
     vus............................: 10     min=10  max=10
     vus_max........................: 10     min=10  max=10

Reading the Output

• ✓ = threshold passed, ✗ = threshold failed
• Each metric shows: avg, min, med (median), max, p(90), p(95)
• http_reqs rate = requests per second (RPS)

HTTP Request Duration Breakdown

The total http_req_duration is composed of:

blocked → connecting → tls_handshaking → sending → waiting → receiving
                                                      ↑
                                              (TTFB / server time)

Component	What It Measures	Typical Cause of High Values
http_req_blocked	Time in queue before request starts	Connection pool exhaustion, DNS lookup
http_req_connecting	TCP connection time	Network latency, server far away
http_req_tls_handshaking	TLS/SSL handshake	Missing keep-alive, new connections per request
http_req_sending	Time to send request body	Large request body, slow upload
http_req_waiting	Server processing time (TTFB)	Slow backend, DB queries, server overload
http_req_receiving	Time to download response	Large response body, slow network
http_req_duration	sending + waiting + receiving	Overall request time

See reference/bottleneck-patterns.md for detailed bottleneck analysis patterns.

Key Metrics Quick Reference

See reference/metrics.md for the complete k6 metrics reference.

Most Important Metrics to Monitor

Metric	Type	What to Watch
http_req_duration	Trend	p95 and p99 — are they within SLA?
http_req_failed	Rate	Error rate — should be < 1% for normal load
http_reqs	Counter	Request rate (RPS) — is target throughput achieved?
iteration_duration	Trend	Full iteration time including sleeps
checks	Rate	Assertion pass rate — should be > 99%
vus	Gauge	Active VU count — did it reach target?

Percentile Interpretation

Percentile	Meaning
p(50) / med	Half of requests are faster. Typical user experience.
p(90)	90% of requests are faster. Most users' experience.
p(95)	95% of requests are faster. Common SLA target.
p(99)	99% of requests are faster. Worst-case for most users.
p(99.9)	1 in 1000 requests slower. Tail latency.

Rule of thumb: If p95 is 2x+ the median, there's likely a bottleneck affecting some requests.

handleSummary() Customization

Override the default summary output:

import { textSummary } from 'https://jslib.k6.io/k6-summary/0.1.0/index.js';

export function handleSummary(data) {
  return {
    'summary.json': JSON.stringify(data, null, 2),
    stdout: textSummary(data, { indent: ' ', enableColors: true }),
  };
}

Available output targets:

• stdout — Console output
• stderr — Error output
• 'filename.json' — Write to file
• 'filename.html' — Write HTML report

Results Output Options

JSON Lines (Real-Time)

k6 run --out json=results.json script.js

CSV Output

k6 run --out csv=results.csv script.js

Web Dashboard (Built-in)

K6_WEB_DASHBOARD=true k6 run script.js
# Dashboard at http://localhost:5665

Grafana Cloud k6

k6 cloud run --local-execution script.js
# Or set K6_CLOUD_TOKEN environment variable

See reference/visualization.md for detailed visualization options.

Related Skills

• For designing test scenarios and thresholds: /k6:designing-test-scenarios
• For generating test scripts: /k6:generating-api-load-tests