CPU sampling is a profiling technique used to understand how an application spends its processing time.
Understanding CPU Sampling
At its core, CPU sampling works by taking periodic stack snapshots of the application's call stack to identify which functions are currently executing. Think of it like taking pictures of your application's activity at regular intervals.
Each snapshot records the sequence of function calls that led to the current point of execution.
How it Works
- Periodic Snapshots: A profiler tool interrupts the application at fixed time intervals (e.g., every few milliseconds).
- Stack Capture: At each interruption, the tool captures the current call stack. The call stack shows the chain of functions that are currently active, from the entry point of the application down to the function currently being executed by the CPU.
- Function Identification: By examining the captured stack, the tool identifies which specific function or code block is running at that exact moment.
- Pattern Recognition: Over time, a pattern emerges as certain functions are seen to be executing more frequently than others. If a particular function appears on many snapshots, it suggests that the CPU is spending a significant amount of time executing that function or functions called by it.
Why Use CPU Sampling?
CPU sampling is a powerful method for performance analysis because it:
- Identifies Hotspots: Pinpoints functions or code areas where the application spends the most CPU time. These are often referred to as "hotspots" or performance bottlenecks.
- Low Overhead: Generally introduces less performance overhead compared to other profiling methods (like instrumentation, which modifies code) because it relies on infrequent interruptions.
- Provides Overview: Gives a high-level view of where CPU cycles are being consumed across the entire application.
Practical Application
Developers use CPU sampling results, often visualized as a "flame graph" or call tree, to:
- Optimize slow functions.
- Understand execution flow.
- Improve overall application performance.
In summary, CPU sampling is a statistical method that relies on observing the application's state at regular intervals to deduce which parts of the code are responsible for the majority of CPU usage.
