## Java Profiling Practice
**Introduction:**
The document describes a performance monitoring system for a Java service, focusing on measuring and analyzing system performance metrics. It emphasizes the importance of finding bottlenecks and optimizing performance to ensure high throughput and stability.
**Key Concepts:**
* **Throughput:** Number of requests processed per second.
* **Latency:** Time taken for an operation to complete.
* **JVM:** Java Virtual Machine, responsible for running Java applications.
* **Memory:** Memory used by the JVM, including heap, memory usage, and thread stacks.
* **IO:** Input/output operations, which can be CPU-intensive.
**Performance Metrics:**
* **CPU:** CPU utilization by the JVM.
* **Memory:** Memory utilization by the JVM, including heap and memory usage.
* **IO:** Throughput and latency for I/O operations.
* **Network:** Throughput and latency for network operations.
* **Application:** Specific application performance metrics (e.g., battle, login, payment, storage).
**Performance Monitoring Tools:**
* **jmx:** Java Management Extensions for JVM, allows monitoring and control of the JVM.
* **jstat:** Provides performance statistics for the JVM.
* **jstack:** Provides a stack trace of thread execution.
* **jmc:** Java Monitoring and Concurrency framework, a more comprehensive monitoring tool.
**Performance Optimization:**
* Identify bottlenecks in the system.
* Analyze memory usage and optimize memory allocation.
* Optimize I/O performance.
* Reduce network latency.
* Optimize application performance.
**Case Studies:**
* **FullGC issue:** Analysis of full garbage collection taking 120 seconds, identified by jstack.
* **Thread creation optimization:** Investigation of why the system created so many threads, leading to a configuration error.
* **Memory leak:** Analysis of memory usage over time, identified a memory leak in the JVM.
**Conclusion:**
By understanding and monitoring system performance metrics, developers can identify and address performance bottlenecks, improve application performance, and achieve high throughput.
Note that the JVM uses more memory than just the heap. For example Java methods, thread stacks and native handles are allocated in memory separate from the heap, as well as JVM internal data structures
