Load Testing and Capacity Planning
Learn to simulate realistic traffic against an LLM application, find its breaking point, and plan capacity so production stays fast and within budget under load.
Load Testing and Capacity Planning is a free LLM Apps in Production (RAG + Vector DB + Caching) lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the LLM Apps in Production (RAG + Vector DB + Caching) learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Load Test LLM Apps?
LLM apps behave differently under load than typical web services: token generation is slow, requests are long-lived, and upstream provider rate limits add a hard ceiling.
Load testing reveals how your system degrades before real users do.
Key Metrics
Track these under load:
- Throughput — requests or tokens per second
- Latency percentiles — p50, p95, p99
- Error rate — timeouts, 429s
- Time to first token for streaming
Open vs Closed Load Models
Two ways to generate load:
- Closed — fixed number of virtual users, each waits for a response before sending the next
- Open — requests arrive at a fixed rate regardless of responses
Open-model tests better expose queue buildup.
Realistic Workloads
Use realistic prompts. A test with tiny prompts hides cost; production prompts include long retrieved context. Sample real queries and vary input length to mimic actual token distributions.
Percentile Latency in Code
Averages lie; percentiles tell the truth about tail latency.
def percentile(values, p):
s = sorted(values)
idx = int(round((p/100) * (len(s)-1)))
return s[idx]
lat = [120, 130, 140, 900, 150]
print('p95 =', percentile(lat, 95))Finding the Breaking Point
Ramp the request rate gradually until latency or error rate crosses your SLO. That inflection point is your saturation capacity. Run below it in production with headroom.
Estimating Required Capacity
Use Little's Law: concurrency = arrival rate x average latency. Estimate how many concurrent slots you need for peak traffic.
def concurrency(rps, avg_latency_s):
return rps * avg_latency_s
print('Need', concurrency(50, 2.0), 'concurrent slots')Accounting for Provider Limits
Your effective capacity may be capped by the LLM provider's tokens-per-minute and requests-per-minute limits, not your servers. Plan around those quotas and request increases ahead of launches.
Headroom and Autoscaling
Run at a target utilization (often 60-70 percent) so spikes do not immediately saturate. Configure autoscaling on a leading signal like queue depth, since CPU is a poor proxy for LLM load.
Soak and Spike Tests
Beyond steady ramps, run:
- Soak — sustained load for hours to catch leaks
- Spike — sudden surge to test autoscaling reaction
From Test to Plan
Turn results into a capacity plan: peak rps, required concurrency, provider quota needs, scaling rules, and a cost estimate. Re-test after major changes since model and prompt changes shift the numbers.
Quick Check
Test your understanding of capacity planning.
Recap
You learned to load test LLM apps with realistic workloads, track latency percentiles and error rate, find the saturation point, and size capacity with Little's Law. Account for provider quotas, keep headroom, autoscale on queue depth, and run soak and spike tests.
Frequently asked questions
Is the “Load Testing and Capacity Planning” lesson free?
Yes — the full text of “Load Testing and Capacity Planning” is free to read here on the web, and the LLM Apps in Production (RAG + Vector DB + Caching) course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the LLM Apps in Production (RAG + Vector DB + Caching) course, upgrade to CoddyKit PRO.
What will I learn in “Load Testing and Capacity Planning”?
Learn to simulate realistic traffic against an LLM application, find its breaking point, and plan capacity so production stays fast and within budget under load. You practise LLM Apps in Production (RAG + Vector DB + Caching) with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start LLM Apps in Production (RAG + Vector DB + Caching)?
No prior experience is required. LLM Apps in Production (RAG + Vector DB + Caching) on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Load Testing and Capacity Planning” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this LLM Apps in Production (RAG + Vector DB + Caching) lesson?
Yes. Every LLM Apps in Production (RAG + Vector DB + Caching) lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Horizontal Scaling of RAG Components
- Observability: Logging, Metrics, Tracing
- Alerting and Incident Response for LLM Ops
- Load Testing and Capacity Planning