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?

Solution 1

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

Solution 2

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

jcmd <pid> GC.run

Solution 3

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

Solution 4

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

Solution 5

for linux:

$ jcmd $(pgrep java) GC.run

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

View more solutions

Comments

[Dror Bereznitsky]

This will trigger a heap dump not garbage collection

[gtrak]

force a garbage collection on all the javas: ps axf | grep java | grep -v grep | awk '{print "jmap -histo:live " $1}'|sh

[Amin Abbaspour]

at least is solaris it does a force GC.

[nafg]

Where is that documented? What about without :live (e.g. when -F is needed)?

[noahlz]

Why don't you people tell me about these things?! :)

[noahlz]

Hello from the mysterious future of 2014. jcmd is now the right tool for the job.

[Mircea Vutcovici]

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.

[Elad Tabak]

Will not trigger GC, just print the stack trace.

[Thomas Rebele]

if you get an "AttachNotSupportedException: Unable to open socket file", see my addition to this answer

[dhockey]

What about the following? Same thing? java.io.IOException: Operation not permitted

[Thomas Rebele]

I haven't encountered this error message yet. Maybe it works with sudo -u <process_owner> jcmd <pid> GC.run, could you try? The command should be safe

[dhockey]

I would have but I do not have sudo access on that machine.

[Eyal Roth]

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: mail.openjdk.java.net/pipermail/serviceability-dev/2017-Augu‌​st/…

[aled]

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.

[Cas]

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.

[Ali Saleh]

This will work for Oracle JDK only, it will not work for open-jdk.

[renedet]

Run jcmd before to discover all java <pid>

[Geker]

when run "jmap -histo:live <pid> ", the print result will contains the references before gc.

[Admin]

@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 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).