Problems / LLM serving, medium

A/B test of a new model version

An experiment splits chats 50/50 between the current model (A) and a new version (B). B was deployed with only 60 concurrent streams; 40 chats arrive a second.

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Your goals

Success ≥ 99%.

Test cases

Like a coding problem, a solution has to pass every case, not just the one it was built for. Submit in the lab runs all 6; 2 of them are hidden until then.

  1. As given. The problem’s own traffic, scheduled failures and seed.
  2. Peak hour. 25% more traffic than the problem states, with the same failures.
  3. A slow dependency. The main data store or model answers twice as slowly for part of the run.
  4. A server dies. One replica of the service with the most replicas is killed for part of the run.

The traffic

40 requests a second for 30 seconds: 100% request (reads).

The system

What each component does in this design, and what calls it. The simulator mimics each one from these settings.

ComponentIts job hereSettings
Experiment splitModel routerTraffic enters here.Calls Variant A app, Variant B app.A router sends easy requests to small, cheap models and hard ones to larger models.100000 slots
Variant A appApp serviceExperiment split forwards to it 50% of traffic.Calls Model A (current).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.3 replicas × 500 slots · timeout 30 s
Model A (current)LLM endpointVariant A app calls it.Latency has two parts: time to the first token, then a steady stream of tokens per second.400 streams
Variant B appApp serviceExperiment split forwards to it 50% of traffic.Calls Model B (candidate).Little's law: throughput ≈ concurrency ÷ latency. 200 slots at 50 ms serve about 4,000 req/s.3 replicas × 500 slots · timeout 30 s
Model B (candidate)LLM endpointVariant B app calls it.Latency has two parts: time to the first token, then a steady stream of tokens per second.60 streams

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