Hands-on Practical: Setting up Basic LLM Monitoring

Here is a hands-on guide to setting up basic monitoring for a simulated LLM inference endpoint, focusing on two fundamental performance indicators: latency and throughput. Monitoring LLM-specific metrics is important for operational stability and performance. While a full-scale LLM is not deployed, the principles and techniques demonstrated are directly applicable to various scenarios.

We'll use Python with the FastAPI framework to create a simple web server simulating an LLM endpoint and the prometheus_client library to instrument it, exposing metrics in a format that can be scraped by monitoring systems like Prometheus.

1. Setting Up a Simulated LLM Endpoint

First, ensure you have FastAPI and Uvicorn (an ASGI server) installed, along with the Prometheus client library:

pip install fastapi uvicorn prometheus-client requests

Now, let's create a simple FastAPI application (main.py) that mimics an LLM endpoint. It will include an artificial delay to simulate processing time and expose a /metrics endpoint for Prometheus.

# main.py
import time
import random
from fastapi import FastAPI, Request
from prometheus_client import Counter, Histogram, Summary, generate_latest, REGISTRY
from prometheus_client import start_http_server # For standalone metric server if not using framework integration

# --- Prometheus Metrics ---
# Using Histogram for request latency (allows calculating quantiles)
REQUEST_LATENCY = Histogram(
    'llm_request_latency_seconds',
    'Latency of requests to the LLM endpoint',
    ['endpoint']
)

# Using Counter for total requests
REQUEST_COUNT = Counter(
    'llm_request_total',
    'Total number of requests to the LLM endpoint',
    ['endpoint', 'method', 'http_status']
)

# Using Summary for request latency (alternative, calculates quantiles client-side)
# REQUEST_LATENCY_SUMMARY = Summary(
#     'llm_request_latency_summary_seconds',
#     'Latency Summary of requests to the LLM endpoint',
#     ['endpoint']
# )

app = FastAPI()

# Middleware to capture metrics for all requests
@app.middleware("http")
async def track_metrics(request: Request, call_next):
    start_time = time.time()
    endpoint = request.url.path

    try:
        response = await call_next(request)
        status_code = response.status_code
    except Exception as e:
        status_code = 500 # Internal Server Error
        raise e from None # Re-raise the exception
    finally:
        latency = time.time() - start_time
        REQUEST_LATENCY.labels(endpoint=endpoint).observe(latency)
        # REQUEST_LATENCY_SUMMARY.labels(endpoint=endpoint).observe(latency) # If using Summary
        REQUEST_COUNT.labels(
            endpoint=endpoint,
            method=request.method,
            http_status=status_code
        ).inc()

        # Important: If an exception occurred before response is set,
        # ensure we still record the request count with the error status.
        # The try/finally block helps manage this. Note that if call_next
        # raises an exception, the response object might not be available,
        # hence setting status_code in the except block.

    return response

@app.post("/predict")
async def predict(payload: dict):
    """Simulates an LLM prediction endpoint."""
    # Simulate LLM processing time (e.g., 50ms to 500ms)
    processing_time = random.uniform(0.05, 0.5)
    time.sleep(processing_time)

    # Simulate a simple response
    input_text = payload.get("text", "")
    response_text = f"Simulated response for: {input_text[:20]}..."
    return {"prediction": response_text}

@app.get("/metrics")
async def metrics():
    """Exposes Prometheus metrics."""
    from starlette.responses import Response
    return Response(generate_latest(REGISTRY), media_type="text/plain")

# Optional: If running without Uvicorn's multiple workers or outside FastAPI context,
# you might start a separate server for metrics. Generally integrated is better.
# if __name__ == "__main__":
#     start_http_server(8001) # Start Prometheus client server on port 8001
#     # Run FastAPI app separately using uvicorn main:app --reload --port 8000

Explanation:

• We define two primary metrics: REQUEST_LATENCY (a Histogram) and REQUEST_COUNT (a Counter). Histograms are suitable for latency because they allow calculating quantiles (e.g., p95, p99) on the server side (Prometheus), which is often preferred for performance monitoring. Counters track cumulative totals.
• We use FastAPI's middleware mechanism (@app.middleware("http")) to intercept every request. Before processing the request (call_next), we record the start time. After processing, we calculate the duration (latency) and observe it using our REQUEST_LATENCY histogram. We also increment the REQUEST_COUNT, labeling it with the endpoint, HTTP method, and status code.
• The /predict endpoint simulates work using time.sleep().
• The /metrics endpoint uses generate_latest(REGISTRY) from the prometheus_client library to return all registered metrics in the text format expected by Prometheus.

2. Running the Simulated Endpoint

Save the code above as main.py and run it using Uvicorn:

uvicorn main:app --reload --port 8000

Your simulated LLM API is now running on http://localhost:8000.

3. Generating Load and Observing Metrics

Now, let's send some requests to our /predict endpoint. You can use a simple Python script (load_test.py) or tools like curl or hey.

# load_test.py
import requests
import time
import random
import concurrent.futures

API_URL = "http://localhost:8000/predict"
METRICS_URL = "http://localhost:8000/metrics"

def send_request(i):
    payload = {"text": f"This is test input number {i}."}
    try:
        start = time.time()
        response = requests.post(API_URL, json=payload, timeout=2) # Add timeout
        latency = time.time() - start
        if response.status_code == 200:
            print(f"Request {i}: Status {response.status_code}, Latency: {latency:.4f}s")
        else:
            print(f"Request {i}: Status {response.status_code}")
        return True
    except requests.exceptions.RequestException as e:
        print(f"Request {i}: Failed - {e}")
        return False
    time.sleep(random.uniform(0.01, 0.1)) # Slight delay between requests

if __name__ == "__main__":
    num_requests = 100
    max_workers = 10 # Simulate concurrent users

    print(f"Sending {num_requests} requests with {max_workers} workers...")

    with concurrent.futures.ThreadPoolExecutor(max_workers=max_workers) as executor:
        futures = [executor.submit(send_request, i) for i in range(num_requests)]
        concurrent.futures.wait(futures)

    print("\nLoad generation finished.")
    print(f"Check metrics at: {METRICS_URL}")

Run the load generator:

python load_test.py

While the load generator is running (or after it finishes), access the metrics endpoint in your browser or using curl: http://localhost:8000/metrics or curl http://localhost:8000/metrics.

You will see output similar to this (values will differ):

# HELP llm_request_latency_seconds Latency of requests to the LLM endpoint
# TYPE llm_request_latency_seconds histogram
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.005"} 0.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.01"} 0.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.025"} 0.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.05"} 2.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.075"} 10.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.1"} 25.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.25"} 68.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.5"} 100.0
llm_request_latency_seconds_bucket{endpoint="/predict",le="0.75"} 100.0
# ... more buckets ...
llm_request_latency_seconds_bucket{endpoint="/predict",le="+Inf"} 100.0
llm_request_latency_seconds_sum{endpoint="/predict"} 28.7345...
llm_request_latency_seconds_count{endpoint="/predict"} 100.0
# HELP llm_request_total Total number of requests to the LLM endpoint
# TYPE llm_request_total counter
llm_request_total{endpoint="/predict",http_status="200",method="POST"} 100.0
llm_request_total{endpoint="/metrics",http_status="200",method="GET"} 5.0
# ... other metrics ...

This text-based format is designed to be scraped by Prometheus.

4. Monitoring with Prometheus & Grafana

In a full setup, you would configure Prometheus to periodically "scrape" (fetch) metrics from the /metrics endpoint of your application instances. Grafana would then be configured with Prometheus as a data source to visualize these metrics.

Prometheus Configuration (Snippet):

# prometheus.yml (example snippet)
scrape_configs:
  - job_name: 'llm-service'
    static_configs:
      - targets: ['localhost:8000'] # Replace with your actual endpoint(s)

Example PromQL Queries (for Grafana or Prometheus UI):

• Request Rate (per second) for the /predict endpoint: rate(llm_request_total{endpoint="/predict", method="POST"}[5m]) (Calculates the per-second average rate of increase of the counter over the last 5 minutes)

• 95th Percentile Latency (p95) for the /predict endpoint: histogram_quantile(0.95, sum(rate(llm_request_latency_seconds_bucket{endpoint="/predict"}[5m])) by (le)) (Calculates the 95th percentile latency based on the histogram buckets over the last 5 minutes)

• Average Latency: sum(rate(llm_request_latency_seconds_sum{endpoint="/predict"}[5m])) / sum(rate(llm_request_latency_seconds_count{endpoint="/predict"}[5m]))

Visualization:

You could create dashboards in Grafana displaying these metrics over time. For instance, a dashboard might show:

Basic monitoring pipeline: Clients interact with the instrumented API, which exposes metrics via an endpoint scraped by Prometheus. Grafana queries Prometheus to display visualizations on a dashboard.

Example visualization showing simulated p95 latency, average latency, and request throughput over time. Note the latency spike around 10:15, corresponding with a dip in throughput.

5. Interpretation and Next Steps

By observing these basic metrics, you can start answering important questions:

• Is latency increasing? This could indicate infrastructure saturation, changes in request patterns, or issues with the model server itself.
• Is throughput dropping? This often correlates with increased latency or errors.
• Are error rates (e.g., 5xx status codes from llm_request_total) increasing? This signals backend problems.

This practical demonstrated instrumenting an application for basic performance monitoring. LLMOps monitoring extends significantly further, incorporating:

• Infrastructure Metrics: GPU utilization, GPU memory usage, network bandwidth (often collected via node exporters or cloud provider agents).
• Cost Metrics: Tying usage back to cloud billing APIs or internal cost allocation systems.
• Output Quality Metrics: Implementing sampling, automated checks (toxicity, PII), and potentially human feedback loops to assess the quality and safety of LLM responses.
• Drift Detection: Monitoring input prompt distributions and output characteristics over time.

Setting up foundational latency and throughput monitoring provides the initial visibility needed to operate LLMs reliably and forms the basis upon which more advanced observability can be built.