What factors are causing the sudden increase in memory usage for ClickHouse's distributed query execution feature this week?

Introduction

The sudden increase in memory usage for ClickHouse's distributed query execution feature this week presents a critical issue that demands immediate attention. As we delve into this problem, we'll employ a systematic approach to identify, validate, and address the root cause while considering both short-term fixes and long-term implications for the product.

Our analysis will follow a structured framework, beginning with clarifying questions to establish context, ruling out external factors, understanding the product and user journey, breaking down the metric, gathering relevant data, forming hypotheses, conducting root cause analysis, and finally proposing validation methods and next steps.

Step 1

Clarifying Questions (3 minutes)

• Looking at the timing, I'm thinking this could be related to a recent deployment. Has there been any significant update to the ClickHouse codebase or configuration in the past week?

Why it matters: Recent changes often correlate with sudden performance shifts. Expected answer: Yes, there was a deployment last Tuesday. Impact on approach: If confirmed, we'd focus on changes in that deployment.

• Considering the nature of distributed queries, I'm wondering about data volume changes. Have we seen any unusual spikes in data ingestion or query complexity recently?

Why it matters: Increased data volume or complexity could strain memory resources. Expected answer: Data volume has been steady, but query complexity has increased. Impact on approach: We'd investigate query optimization and resource allocation.

• Given that it's a distributed system, I'm curious about cluster changes. Has there been any modification to the cluster configuration or node count in the past week?

Why it matters: Cluster changes can significantly impact resource utilization. Expected answer: No changes to the cluster configuration. Impact on approach: We'd shift focus to software-level issues rather than infrastructure.

• Thinking about potential monitoring issues, has there been any change in how we measure or report memory usage?

Why it matters: Ensures we're dealing with an actual issue, not a measurement anomaly. Expected answer: No changes to monitoring systems. Impact on approach: Confirms the issue is real, not a reporting artifact.