JVM startup parameters
JAVA_OPTS = "-Xms4G -Xmx4G -Xmn2G -XX:MetaspaceSize=512M -XX:MaxMetaspaceSize=512M -XX:+UseParallelGC -XX:+PrintGCDetails -Xloggc:/home/gc.log -XX:+PrintGCDateStamps -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC"
Setting JVM startup parameters
- -XX:+PrintGCDetails: Output detailed information of GC, including heap memory usage after each garbage collection and the status before and after collection.
- -Xloggc:/home/gc.log: Save GC logs to the file /home/gc.log instead of outputting to the console. This allows the logs to be analyzed after the application has run.
- -XX:+PrintGCDateStamps: Add a date stamp to each GC log record to facilitate viewing the time when garbage collection occurred.
- -XX:+PrintGCTimeStamps: Add the elapsed time after JVM startup to each GC log, in seconds.
- -XX:+PrintHeapAtGC: Print the detailed status of the heap before and after garbage collection, including the memory allocation of each generation (young generation, old generation, metaspace).
- -XX:+UseParallelGC: Use Parallel GC (parallel garbage collector) to improve performance by executing garbage collection in parallel with multiple threads.
GC log snippet
OpenJDK 64-Bit Server VM (25.342-b07) for linux-amd64 JRE (1.8.0_342-b07), built on Jul 16 2022 09:19:19 by "openjdk" with gcc 4.4.7 20120313 (Red Hat 4.4.7-23) Memory: 4k page, physical 8388608k(8386904k free), swap 0k(0k free) CommandLine flags: -XX:CompressedClassSpaceSize=528482304 -XX:InitialHeapSize=4294967296 -XX:MaxHeapSize=4294967296 -XX:MaxMetaspaceSize=536870912 -XX:MaxNewSize=2147483648 -XX:MetaspaceSize=536870912 -XX:NewSize=2147483648 -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC 2024-09-24T10:43:44.094+0800: 21.703: Total time for which application threads were stopped: 0.0015655 seconds, Stopping threads took: 0.0000978 seconds {Heap before GC invocations=11 (full 0): PSYoungGen total 1494528K, used 1466389K [0x0000000790800000, 0x00000007f0800000, 0x00000007f0800000) eden space 1414656K, 100% used [0x0000000790800000,0x00000007e6d80000,0x00000007e6d80000) from space 79872K, 64% used [0x00000007eba00000,0x00000007eec85748,0x00000007f0800000) to space 78336K, 0% used [0x00000007e6d80000,0x00000007e6d80000,0x00000007eba00000) ParOldGen total 2621440K, used 72408K [0x00000006f0800000, 0x0000000790800000, 0x0000000790800000) object space 2621440K, 2% used [0x00000006f0800000,0x00000006f4eb60e0,0x0000000790800000) Metaspace used 114761K, capacity 120350K, committed 120576K, reserved 1155072K class space used 13324K, capacity 14260K, committed 14336K, reserved 1048576K 2024-09-24T10:43:44.149+0800: 21.759: [GC (Allocation Failure) [PSYoungGen: 1466389K->41915K(1492992K)] 1538798K->152295K(4114432K), 0.0591035 secs] [Times: user=0.16 sys=0.01, real=0.06 secs] Heap after GC invocations=11 (full 0): PSYoungGen total 1492992K, used 41915K [0x0000000790800000, 0x00000007f0800000, 0x00000007f0800000) eden space 1414656K, 0% used [0x0000000790800000,0x0000000790800000,0x00000007e6d80000) from space 78336K, 53% used [0x00000007e6d80000,0x00000007e966ee58,0x00000007eba00000) to space 78336K, 0% used [0x00000007ebb80000,0x00000007ebb80000,0x00000007f0800000) ParOldGen total 2621440K, used 110380K [0x00000006f0800000, 0x0000000790800000, 0x0000000790800000) object space 2621440K, 4% used [0x00000006f0800000,0x00000006f73cb088,0x0000000790800000) Metaspace used 114761K, capacity 120350K, committed 120576K, reserved 1155072K class space used 13324K, capacity 14260K, committed 14336K, reserved 1048576K } 2024-09-24T10:43:44.209+0800: 21.818: Total time for which application threads were stopped: 0.0593724 seconds, Stopping threads took: 0.0000333 seconds {Heap before GC invocations=906 (full 2): PSYoungGen total 2065920K, used 2041627K [0x0000000760800000, 0x00000007e0800000, 0x00000007e0800000) eden space 2034688K, 100% used [0x0000000760800000,0x00000007dcb00000,0x00000007dcb00000) from space 31232K, 22% used [0x00000007de980000,0x00000007df046e10,0x00000007e0800000) to space 31232K, 0% used [0x00000007dcb00000,0x00000007dcb00000,0x00000007de980000) ParOldGen total 2097152K, used 2080736K [0x00000006e0800000, 0x0000000760800000, 0x0000000760800000) object space 2097152K, 99% used [0x00000006e0800000,0x000000075f7f8060,0x0000000760800000) Metaspace used 176015K, capacity 189742K, committed 191104K, reserved 1218560K class space used 19024K, capacity 21351K, committed 21632K, reserved 1048576K 2024-09-24T14:14:45.047+0800: 11571.644: [GC (Allocation Failure) [PSYoungGen: 2041627K->26414K(2061312K)] 4122363K->2109278K(4158464K), 0.0208053 secs] [Times: user=0.07 sys=0.00, real=0.02 secs] Heap after GC invocations=906 (full 2): PSYoungGen total 2061312K, used 26414K [0x0000000760800000, 0x00000007e0800000, 0x00000007e0800000) eden space 2034688K, 0% used [0x0000000760800000,0x0000000760800000,0x00000007dcb00000) from space 26624K, 99% used [0x00000007dcb00000,0x00000007de4cba70,0x00000007de500000) to space 33792K, 0% used [0x00000007de700000,0x00000007de700000,0x00000007e0800000) ParOldGen total 2097152K, used 2082863K [0x00000006e0800000, 0x0000000760800000, 0x0000000760800000) object space 2097152K, 99% used [0x00000006e0800000,0x000000075fa0be58,0x0000000760800000) Metaspace used 176015K, capacity 189742K, committed 191104K, reserved 1218560K class space used 19024K, capacity 21351K, committed 21632K, reserved 1048576K } {Heap before GC invocations=907 (full 3): PSYoungGen total 2061312K, used 26414K [0x0000000760800000, 0x00000007e0800000, 0x00000007e0800000) eden space 2034688K, 0% used [0x0000000760800000,0x0000000760800000,0x00000007dcb00000) from space 26624K, 99% used [0x00000007dcb00000,0x00000007de4cba70,0x00000007de500000) to space 33792K, 0% used [0x00000007de700000,0x00000007de700000,0x00000007e0800000) ParOldGen total 2097152K, used 2082863K [0x00000006e0800000, 0x0000000760800000, 0x0000000760800000) object space 2097152K, 99% used [0x00000006e0800000,0x000000075fa0be58,0x0000000760800000) Metaspace used 176015K, capacity 189742K, committed 191104K, reserved 1218560K class space used 19024K, capacity 21351K, committed 21632K, reserved 1048576K 2024-09-24T14:14:45.068+0800: 11571.665: [Full GC (Ergonomics) [PSYoungGen: 26414K->0K(2061312K)] [ParOldGen: 2082863K->300700K(2097152K)] 2109278K->300700K(4158464K), [Metaspace: 176015K->174750K(1218560K)], 0.6404574 secs] [Times: user=1.88 sys=0.00, real=0.64 secs] Heap after GC invocations=907 (full 3): PSYoungGen total 2061312K, used 0K [0x0000000760800000, 0x00000007e0800000, 0x00000007e0800000) eden space 2034688K, 0% used [0x0000000760800000,0x0000000760800000,0x00000007dcb00000) from space 26624K, 0% used [0x00000007dcb00000,0x00000007dcb00000,0x00000007de500000) to space 33792K, 0% used [0x00000007de700000,0x00000007de700000,0x00000007e0800000) ParOldGen total 2097152K, used 300700K [0x00000006e0800000, 0x0000000760800000, 0x0000000760800000) object space 2097152K, 14% used [0x00000006e0800000,0x00000006f2da7140,0x0000000760800000) Metaspace used 174750K, capacity 187618K, committed 191104K, reserved 1218560K class space used 18827K, capacity 20997K, committed 21632K, reserved 1048576K }
Application pause time
During the GC process, all threads of the application are suspended, which is called the "Stop-The-World" (STW) event. Pause time directly affects the performance and responsiveness of the application.
Log example:
2024-09-24T10:43:44.094+0800: 21.703: Total time for which application threads were stopped: 0.0015655 seconds, Stopping threads took: 0.0000978 seconds
Total time for which application threads were stopped:
- The total time the application thread was suspended, here is 0.0015655 seconds (~1.57 milliseconds).
Stopping threads took:
- The time spent suspending the thread, here 0.0000978 seconds (approximately 0.098 milliseconds).
GC (Minor GC)
Log example
2024-09-24T10:43:44.149+0800: 21.759: [GC (Allocation Failure) [PSYoungGen: 1466389K->41915K(1492992K)] 1538798K->152295K(4114432K), 0.0591035 secs]
- GC type: Minor GC, triggered by memory allocation failure (Allocation Failure).
- Young generation changes: from 1466389K to 41915K, total capacity 1492992K.
- Total change in heap memory: from 1538798K to 152295K, total capacity 4114432K.
- Time taken: 0.0591035 seconds (about 59 milliseconds).
Full GC
Log example:
2024-09-24T14:14:45.068+0800: 11571.665: [Full GC (Ergonomics) [PSYoungGen: 26414K->0K(2061312K)] [ParOldGen: 2082863K->300700K(2097152K)] 2109278K->300700K(4158464K), [Metaspace: 176015K->174750K(1218560K)], 0.6404574 secs]
- GC type: Full GC, triggered by JVM adaptive mechanism (Ergonomics).
- Young generation change: from 26414K to 0K, indicating that the young generation has been fully recycled.
- Old generation changes: reduced from 2082863K to 300700K, freeing up a lot of memory.
- Total change in heap memory: from 2109278K to 300700K, total capacity 4158464K.
- Metaspace change: reduced from 176015K to 174750K.
- Time taken: 0.6404574 seconds (about 640 milliseconds).
Detailed status of heap memory
Due to the use of the -XX:+PrintHeapAtGC parameter, the GC log will print the detailed status of the heap memory before and after each GC, including the capacity and usage of each memory area.
Log example:
Heap before GC invocations=11 (full 0): PSYoungGen total 1494528K, used 1466389K [0x0000000790800000, 0x00000007f0800000, 0x00000007f0800000) eden space 1414656K, 100% used [0x0000000790800000,0x00000007e6d80000,0x00000007e6d80000) from space 79872K, 64% used [0x00000007eba00000,0x00000007eec85748,0x00000007f0800000) to space 78336K, 0% used [0x00000007e6d80000,0x00000007e6d80000,0x00000007eba00000) ParOldGen total 2621440K, used 72408K [0x00000006f0800000, 0x0000000790800000, 0x0000000790800000) object space 2621440K, 2% used [0x00000006f0800000,0x00000006f4eb60e0,0x0000000790800000) Metaspace used 114761K, capacity 120350K, committed 120576K, reserved 1155072K class space used 13324K, capacity 14260K, committed 14336K, reserved 1048576K
-
PSYoungGen (young generation):
- total (total capacity): 1494528K.
- used: 1466389K.
- eden space (Eden area): capacity 1414656K, usage rate 100%.
- from space (Survivor area): Capacity 79872K, usage rate 64%.
- to space (Survivor area): capacity 78336K, unused.
-
ParOldGen (old generation):
- total (total capacity): 2621440K.
- used: 72408K, usage rate is about 2%.
-
Metaspace:
- used: 114761K.
- capacity: 120350K.
Correspondence between JVM startup parameters and GC logs
Due to manually setting the JVM startup parameters, the GC logs contain more detailed information that facilitates in-depth analysis:
-
Memory allocation and usage:
- With -XX:+PrintHeapAtGC, you can view the heap memory status before and after each GC and understand the memory allocation and usage of each area.
-
Time stamp information:
- -XX:+PrintGCDateStamps and -XX:+PrintGCTimeStamps provide precise time information to facilitate analysis of the time points and intervals of GC events.
-
GC event details:
- -XX:+PrintGCDetails outputs detailed information such as the type of GC event, triggering reason, memory recycling status and time consumption.
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