How do you Force Garbage Collection from the Shell?

Question

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?

*Not* the same as http://stackoverflow.com/questions/1481178/how-to-force-garbage-collection-in-java — Raedwald, Mar 31 '16 at 16:02

score 400 · Answer 1 · answered Jan 15 '14 at 14:10

400

Since JDK 7 you can use the JDK command tool 'jcmd' such as:

jcmd <pid> GC.run

answered Jan 15 '14 at 14:10

user3198490

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Why don't you people tell me about these things?! :) – noahlz Sep 16 '14 at 14:11
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if you get an "AttachNotSupportedException: Unable to open socket file", see [my addition to this answer](https://stackoverflow.com/a/48646359/1562506) – Thomas Rebele Feb 06 '18 at 15:23
And if you're getting `Explicit GC is disabled, no GC has been performed` that might be due to the `-XX:+DisableExplicitGC` VM argument. See: http://mail.openjdk.java.net/pipermail/serviceability-dev/2017-August/021763.html – Eyal Roth Aug 28 '18 at 15:03
This will work for Oracle JDK only, it will not work for open-jdk. – Ali Saleh Apr 05 '20 at 11:03
Run jcmd before to discover all java – renedet Nov 13 '20 at 19:52
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@AliSaleh: It is also working with openjdk (in Debian and Ubuntu) for me. I installed it with `sudo apt install openjdk-11-jdk-headless` then ran with `jcmd 0 GC.run` (Process ID "0" means: "do it for all java processes") and all `java` processes had less RAM allocated (Resident Set Size RSS; see here https://stackoverflow.com/a/21049737/14972917 ). Virtual Memory Size (VSZ) is still the same as before (checked with `ps -A u` or `ps -A u | grep java`). – Nov 13 '21 at 09:01

score 109 · Answer 2 · edited Feb 11 '19 at 20:55

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If you run jmap -histo:live <pid>, that will force a full GC on the heap before it prints anything out.

edited Feb 11 '19 at 20:55

Erica Kane

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answered Aug 19 '10 at 16:39

Will Hartung

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force a garbage collection on all the javas: ps axf | grep java | grep -v grep | awk '{print "jmap -histo:live " $1}'|sh – gtrak Nov 19 '13 at 23:18
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Where is that documented? What about without :live (e.g. when -F is needed)? – nafg Aug 29 '14 at 09:25
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Hello from the mysterious future of 2014. `jcmd` is now the right tool for the job. – noahlz Sep 16 '14 at 14:12
when run "jmap -histo:live ", the print result will contains the references before gc. – Geker Aug 24 '21 at 12:13

score 26 · Accepted Answer · edited Jun 06 '14 at 12:47

You can do this via the free jmxterm program.

Fire it up like so:

java -jar jmxterm-1.0-alpha-4-uber.jar

From there, you can connect to a host and trigger GC:

$>open host:jmxport
#Connection to host:jmxport is opened
$>bean java.lang:type=Memory
#bean is set to java.lang:type=Memory
$>run gc
#calling operation gc of mbean java.lang:type=Memory
#operation returns: 
null
$>quit
#bye

Look at the docs on the jmxterm web site for information about embedding this in bash/perl/ruby/other scripts. I've used popen2 in Python or open3 in Perl to do this.

UPDATE: here's a one-liner using jmxterm:

echo run -b java.lang:type=Memory gc | java -jar jmxterm-1.0-alpha-4-uber.jar -n -l host:port

score 20 · Answer 4 · answered Feb 06 '18 at 15:22

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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

answered Feb 06 '18 at 15:22

Thomas Rebele

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What about the following? Same thing? `java.io.IOException: Operation not permitted` – dhockey Apr 09 '18 at 19:03
I haven't encountered this error message yet. Maybe it works with `sudo -u jcmd GC.run`, could you try? The command should be safe – Thomas Rebele Apr 10 '18 at 12:16
I would have but I do not have sudo access on that machine. – dhockey Apr 10 '18 at 14:50
The tool works correctly. You just don't have the right permissions in the operating system to execute it. The same applies to other applications, even not using Java. – aled Sep 03 '18 at 20:14

score 12 · Answer 5 · edited Sep 25 '19 at 20:48

12

for linux:

$ jcmd $(pgrep java) GC.run

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

edited Sep 25 '19 at 20:48

Andrew Regan

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answered Sep 25 '19 at 20:06

Xuejun Liu

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This will only work when there's one java process running it seems. Otherwise it will interpret the second PID as the command for jcmd which is obviously not recognized and throw an error. – Cas Mar 05 '20 at 13:11

The Alchemist · Answer 6 · 2017-05-04T12:40:18.843

There's a few other solutions (lots of good ones here already):

Write a little code to access the MemoryMBean and call gc().
Using a command-line JMX client (like cmdline-jmxclient, jxmterm) and run the gc() operation on the MemoryMBean

The following example is for the cmdline-jmxclient:

$ java -jar cmdline-jmxclient-0.10.3.jar - localhost:3812 'java.lang:type=Memory' gc

This is nice because it's only one line and you can put it in a script really easily.

score 1 · Answer 7 · answered Jul 14 '21 at 07:19

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.

YoK · Answer 8 · 2010-08-19T16:43:31.607

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I don't think there is any command line option for same.

You will need to use jvisualvm/jconsole for same.

I would rather suggest you to use these tools to identity , why your program is high on memory.

Anyways you shouldn't force GC, as it would certainly disturb GC algorithm and make your program slow.

edited Aug 19 '10 at 16:43

answered Aug 19 '10 at 16:30

YoK

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I'm sure there was a way to do it in 2010, but even if it wasn't, at 2022 this is an incorrect answer. One of the ways to do it is using jcmd – gortiz Aug 12 '22 at 10:57

score 0 · Answer 9 · answered Dec 22 '17 at 19:21

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If you are using jolokia with your application, you can trigger a garbage collection with this command:

curl http://localhost:8558/jolokia/exec/java.lang:type=Memory/gc

answered Dec 22 '17 at 19:21

dustmachine

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score 0 · Answer 10 · answered Apr 06 '21 at 04:00

0

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

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

answered Apr 06 '21 at 04:00

San Emmanuel James

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score 0 · Answer 11 · answered Aug 03 '21 at 00:54

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

score -16 · Answer 12 · answered Jan 17 '13 at 03:05

-16

just:

kill -SIGQUIT <PID>

answered Jan 17 '13 at 03:05

Amin Abbaspour

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This will trigger a heap dump not garbage collection – Dror Bereznitsky May 01 '13 at 20:18
at least is solaris it does a force GC. – Amin Abbaspour Feb 04 '14 at 23:27
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Not even in Solaris, SIGQUIT will trigger neither a GC or a heap dump. SIGQUIT will trigger a thread dump only for HotSpot. For IBM JVM it is configurable. – Mircea Vutcovici Aug 28 '15 at 19:15
Will not trigger GC, just print the stack trace. – Elad Tabak Nov 09 '16 at 08:03

How do you Force Garbage Collection from the Shell?

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