In evaluating recall vs latency trade-offs, what is a good methodology to determine the optimal operating point for a system? (e.g., plotting a recall-vs-QPS curve and choosing a target recall)

To determine the optimal operating point for a system balancing recall and latency, start by systematically measuring how changes to the system’s configuration affect both metrics. For example, if the system uses a machine learning model for search or recommendation, you might adjust parameters like the number of candidates retrieved, the model’s complexity, or the use of approximate algorithms (e.g., ANN for vector search). Run controlled experiments where you vary one parameter at a time while keeping others fixed, then record the resulting recall (e.g., percentage of relevant results retrieved) and latency (e.g., average response time). Plotting these results on a recall-vs-latency curve reveals how the two metrics trade off. For instance, increasing the number of candidates retrieved might improve recall but slow down responses, while using a simpler model could reduce latency at the cost of missing relevant results.

Next, analyze the curve to identify the point that aligns with your application’s priorities. If the system serves users who value speed over completeness—like a real-time chat app’s search feature—you might prioritize lower latency, even if it means accepting 80% recall. Conversely, a medical diagnosis tool might require 95% recall, even if responses take longer. To make this decision concrete, define explicit requirements based on user needs or business goals. For example, set a maximum acceptable latency (e.g., 200ms) and choose the highest recall achievable within that constraint. Alternatively, use a cost function that assigns weights to recall and latency (e.g., 0.7 * recall - 0.3 * latency) to quantify the trade-off. If there’s no clear priority, test different operating points with A/B experiments to measure user engagement or satisfaction, then select the configuration that maximizes the desired outcome.

Finally, validate the chosen operating point under realistic conditions and iterate as needed. For example, deploy the system with the selected configuration and monitor its performance in production, checking for discrepancies between lab tests and real-world behavior (e.g., latency spikes during peak traffic). If the system’s recall or latency drifts over time—due to data distribution shifts or increased user load—re-run the evaluation process to update the operating point. Tools like canary deployments or shadow testing can help assess changes safely. For instance, if a new caching layer reduces latency but harms recall, compare the cached and uncached results for a subset of traffic before rolling it out fully. This iterative approach ensures the system remains optimized as requirements evolve.