How do you Force Garbage Collection from the Shell?

user_39110 0
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So I am looking at a heap with jmap on a remote box and I want to force garbage collection on it. How do you do this without popping into jvisualvm or jconsole and friends?

I know you shouldn't be in the practice of forcing garbage collection -- you should just figure out why the heap is big/growing.

I also realize the System.GC() doesn't actually force garbage collection -- it just tells the GC that you'd like it to occur.

Having said that is there a way to do this easily? Some command line app I'm missing?

jmx jmap garbage-collection java
12 Answers

Addition to user3198490's answer. Running this command might give you the following error message:

$ jcmd 1805 GC.run    
[16:08:01]
1805:
com.sun.tools.attach.AttachNotSupportedException: Unable to open socket file: target process not responding or HotSpot VM not loaded
...

This can be solved with help of this stackoverflow answer

sudo -u <process_owner> jcmd <pid> GC.run

where <process_owner> is the user that runs the process with PID <pid>. You can get both from top or htop

user_15625060

for linux:

$ jcmd $(pgrep java) GC.run

jcmd is packaged with the JDK, $(pgrep java) gets the process ID of java

user_95291750

In addition to user3198490's answer, if nothing really changes after you run jcmd <pid> GC.run, the reason could be:

GC.run essentially calls java.lang.System.gc(), which is just a hint to gc and the JVM is free to ignore it.

If you want to ensure a full GC is FORCED, a choice is to use:

jcmd <pid> GC.heap_dump filename.hprof

The original purpose of this command is to create a heap dump file named filename.hprof. But as a side effect, in order to reach all the live objects, it "request a full GC unless the -all option is specified".

There are some other commands like jmap -histo:live <PID> mentioned in this answer triggers GC as a side effect in the same way.

user_77083920

Consider using GNU parallel with jcmd as below for multiple processes;

parallel 'jcmd {} GC.run' ::: $(pgrep java)

user_84134000

In addition:

  • To run GC for multi process with the same application, jar file, etc. Use:
jcmd your_application.jar GC.run
  • To run GC on windows, try:
cd C:\"Program Files"\Java\jdk-13.0.2\bin
.\jcmd.exe your_application.jar GC.run
  • MacOS / Linux:
jcmd your_application.jar GC.run
# or
/usr/bin/jcmd your_application.jar GC.run
user_85722050
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