JVM Tutorials - Herong's Tutorial Examples - Version 4.23, by Dr. Herong Yang
"-XX:+PrintGCDetails" - Garbage Collection Logging
This section describes HotSpot JVM options to generate garbage collection logging messages. It also describes log message format to help us understand what the JVM is telling us.
Now we have a good idea of what is Serial Collector, and how it works. Let's see if we watch it doing the memory management job for my sample application, GarbageCollection.java.
To do this, we are going to use some HotSpot command line options related to garbage collection logging:
Now let's try it with HotSpot 1.8:
C:\herong>\progra~1\java\jdk1.8.0\bin\javac GarbageCollection.java C:\>\progra~1\java\jdk1.8.0\bin\java -Xms2m -Xmx64m -XX:+UseSerialGC -XX:+PrintGCDetails GarbageCollection Step/TotalMemory/FreeMemory/UsedMemory: [GC (Allocation Failure) 0.054: [DefNew: 649K->64K(960K), 0.0008674 secs] 649K->505K(1984K), 0.0009833 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] 1 2031616 697016 1334600 ...
Okay. We got our first log message, which indicates a Minor Collection, from the Serial Collector. We need to understand the format of the message first to be able to read the it:
Continue with additional output in the console:
... [GC (Allocation Failure) 1.055: [DefNew: 861K->64K(960K), 0.0008510 secs] 1.056: [Tenured: 1226K->1279K(1280K), 0.0009817 secs] 1303K->1290K(2240K), [Metaspace: 44K->44K(4480K)], 0.0019995 secs] [Times: user=0.00 sys=0.00, real=0.00 secs] ...
This log message indicates a Full Collection in the Heap (both Young Generation and Tenured Generation). It also did a Method Area (Metaspace) garbage collection. The Full Collection log is larger than the Minor Collection log and contains additional information:
The table below summarizes different types of information included in a GC log message:
[GC (Allocation Failure) ^- GC reason v- GC type - Young Generation 1.055: [DefNew: 861K->64K(960K), 0.0008510 secs] ^ ^ ^ ^ ^- Time spent on GC ^ ^ ^ ^- Total area size ^ ^ ^- Object size aftter GC ^- Timestamp ^- Object size before GC v- GC type - Tenured Generation 1.056: [Tenured: 1226K->1279K(1280K), 0.0009817 secs] v- Heap Summary 1303K->1290K(2240K), ^ ^ ^- Total area size ^ ^- Object size aftter GC ^- Object size before GC v- GC type - Method Area (Metaspace) [Metaspace: 44K->44K(4480K)], v- Total time spent on the entire GC operation 0.0019995 secs] v- CPU time report of the entire GC operation [Times: user=0.00 sys=0.00, real=0.00 secs] ^ ^ ^- Elapsed time ^ ^- CPU time spent on OS kernel ^- CPU time spent on JVM
When Java application is ended or terminated, the JVM also generate a memory usage summary in the GC log:
Heap def new generation total 19648K, used 8770K [0x03c00000, 0x051... eden space 17472K, 50% used [0x03c00000, 0x04490bb0, 0x04d1000... from space 2176K, 0% used [0x04f30000, 0x04f30000, 0x05150000... to space 2176K, 0% used [0x04d10000, 0x04d10000, 0x04f30000... tenured generation total 43712K, used 32730K [0x05150000, 0x07... the space 43712K, 74% used [0x05150000, 0x07146ba8, 0x07146c0... Metaspace used 45K, capacity 2242K, committed 2368K, reserved 4480K
Now we now how big each data area is and how much space was actually used in each area.
Last update: 2014.
Table of Contents
Downloading and Installing JDK 1.8.0 on Windows
Downloading and Installing JDK 1.7.0 on Windows
java.lang.Runtime Class - The JVM Instance
java.lang.System Class - The Operating System
ClassLoader Class - Class Loaders
Class Class - Class Reflections
JRockit JVM 28.2.7 by Oracle Corporation
►Memory Management and Garbage Collectors
Memory Management General Rules
Java Exception: "java.lang.OutOfMemoryError: Java heap space"
OutOfMemoryError Comparison of HotSpot and JRockit
Garbage Collection Demonstration
JVM Memory Manager - Garbage Collector
Generational Garbage Collection in HotSpot
Young Generation Collection - Minor Collection
Tenured Generation Collection - Full Collection
HotSpot Default Garbage Collector - Serial Collector
►"-XX:+PrintGCDetails" - Garbage Collection Logging
GC Log Messages on GarbageCollection.java
Serial, Parallel, Concurrent, and Regionalized Collectors
Parallel Collector GC Log Message Format
Parallel Compacting Collector GC Log Message Format
Concurrent Mark-Sweep Collector GC Log Message Format
Garbage First GC Log Message Format
JVM Stack, Frame and Stack Overflow
Thread Testing Program and Result
CPU Impact of Multi-Thread Applications
I/O Impact of Multi-Thread Applications
Micro Benchmark Runner and JVM Options
Micro Benchmark Tests on "int" Operations
Micro Benchmark Tests on "long" Operations
Micro Benchmark Tests in JIT Compilation Mode