Automatic CPU Profiler

This feature automatically captures CPU profiles, which can make it easier to investigate and troubleshoot spikes in CPU usage or erratic CPU load on certain nodes. A CPU profile shows the functions that use the most CPU time, sampled over a window of time. You can collect a CPU Profile manually on the Advanced Debug page. However, it may be difficult to manually capture a profile of a short CPU spike at the right point in time. Automatic CPU profile capture enables the investigation of CPU load in this and other cases, such as periodic high CPU.

Configuration

You can configure automatic CPU profile capture with the following cluster settings:

Cluster Setting	Description	Default Value	Recommended Value
server.cpu_profile.cpu_usage_combined_threshold	The baseline value for when CPU profiles should be taken. Collect profiles from each node that meets threshold. This setting enables and disables automatic cpu profiling. If a value of 0 is set, a profile will be taken every time the server.cpu_profile.interval has passed or the provided usage is increasing. If a value greater than 0 and less than or equal to 100 is set, the profiler is enabled. If a value over 100 is set, the profiler is disabled.	MAX integer, such as 9223372036854775807	80
server.cpu_profile.interval	The period of time after which the high-water mark resets to the baseline value.	5m0s (5 minutes)	1m40s (100 seconds)
server.cpu_profile.duration	The length of time a CPU profile is taken.	10s (10 seconds)	1s or 2s
server.cpu_profile.total_dump_size_limit	Maximum combined disk size for preserving CPU profiles.	128 MiB (128 Mebibytes)

Enabling automatic CPU profile capture

To enable automatic CPU profile capture, you must set server.cpu_profile.cpu_usage_combined_threshold to a value between 0 and 100. Preferably, use the recommended value.

Recommended values

• Set server.cpu_profile.cpu_usage_combined_threshold to 80 for 80%.
• Set server.cpu_profile.duration to a lower value, for example 1s or 2s. This minimizes the impact of overhead on your cluster compared to the current default value.
• Set server.cpu_profile.interval to a lower value, for example 1m40s (1 minute 40 seconds).

High-water mark threshold

The Automatic CPU Profiler runs asynchronously in the background. After every second, the Automatic CPU Profiler checks if the CPU usage exceeds the high-water mark threshold. If so, it captures a CPU profile. If a profile capture is already in progress, a second profile is not taken.

The Automatic CPU Profiler uses the configuration options to determine the high-water mark threshold. For example, with duration set to 2s , interval set to 1m40s, and cpu_usage_combined_threshold set to 80:

• At time0 the CPU usage polled is 82 percent which exceeds the baseline threshold of 80, so a 2s profile is captured and the high-water mark threshold becomes 82.
• After the 2s profile capture, the Automatic CPU Profiler continues to check every second if the CPU usage now exceeds 82 percent.
• At time1 the CPU usage polled is 85 percent, another profile is taken for 2s and the high-water mark threshold becomes 85.
• At time2, the 1m40s interval after time0, the high-water mark threshold is reset to the baseline threshold of 80.
• The Automatic CPU Profiler continues its every second polling and captures profiles when CPU usage exceeds the high-water mark threshold beginning at the 80 percent baseline.

Accessing CPU profiles

• The Automatic CPU Profiler saves the captured CPU profiles to disk on each node's file system in the logging directory. The default path is cockroach-data/logs/pprof_dump.
• The only way to get these CPU profiles is to access the node file system.
• Enabling the Automatic CPU Profiler does not add CPU profiles to debug zips.

Overhead

Enabling the automatic CPU profile capture on a cluster will add overhead to the cluster in the form of potential increases in latency and CPU usage.

• Monitor the following metrics:
  • P99 latency
  • P50 latency by creating a custom chart for the sql.exec.latency-p50 metric
  • CPU usage
• We anticipate a sub-10% regression on these foreground latency metrics. This overhead to your cluster may be deemed acceptable in order to collect CPU profiles that are necessary to troubleshoot problems in your cluster. Please consult with Cockroach Labs Support.
• Overhead only occurs during profile capture, not when it is idle. If the cluster can tolerate a server.cpu_profile.duration (for example, 1 second) increase in latency to capture the CPU profile, consider enabling the Automatic CPU Profiler.