Jacky's Blog

Set VM configuration to get full GC dump files

2022/07/25

Background

If full GC is frequently made in a service, it will impact availability and performance, the root cause must be identified.

Using VM configuration, e.g, PrintGCDetails, PrintHeapAtGC, the detail log will be printed in gc log file.

But as full gc is the most focused, there are 2 points which are not fit for troubleshooting full gc.

  • The format of gc log file is text, which is not GUI type.
  • All gc logs, including full gc, are printed in 1 log file. Full gc logs are hard to be found.

Set configuration in static way

Add below in VM configuration

-XX:+HeapDumpOnOutOfMemoryError
-XX:+HeapDumpBeforeFullGC
-XX:+HeapDumpAfterFullGC
-XX:HeapDumpPath=/tmp/heap-dump/

Set configuration in dynamic way

jinfo prints and set Java configuration information for a given Java process.

// Print all VM flags
jinfo -flags 25070

// Print VM flag by name
jinfo -flag HeapDumpOnOutOfMemoryError 25070
jinfo -flag HeapDumpBeforeFullGC 25070
jinfo -flag HeapDumpAfterFullGC 25070
jinfo -flag HeapDumpPath 25070

// Set(Enable) VM flag by name
jinfo -flag +HeapDumpOnOutOfMemoryError 25070
jinfo -flag +HeapDumpBeforeFullGC 25070
jinfo -flag +HeapDumpAfterFullGC 25070

// Set(Disable) VM flag by name
jinfo -flag -HeapDumpOnOutOfMemoryError 25070
jinfo -flag -HeapDumpBeforeFullGC 25070
jinfo -flag -HeapDumpAfterFullGC 25070

// Set the value of VM flag by name
jinfo -flag HeapDumpPath=/tmp/heap-dump/ 25070

Full GC dump files generation

If above confiiguration has been made, the before/after full gc dump files will be generated in HeapDumpPath.

These 2 dump files can be downloaded, then analysed in VM tools, e.g, jvisualvm, MAT.

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