java 8 find out size of metaspace at runtime
Java 8 uses metaspace that is capable of expanding dynamically. GC will run on metaspace when its getting full. Does that mean GC will never run on metaspace?
My Java 8 application is using a LOT of memory. I would like to know what is the size of my metaspace at the run time. How do I do that?
I am thinking of setting MaxMetaspaceSize. What should I set it to? Any recommendations?
Answer
Option 1:
Execute
jstat -gc PID
(with PID replaced by the PID of the JVM to monitor) which will return sth. like:
S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT
103936.0 107008.0 0.0 41618.3 820736.0 401794.3 444928.0 183545.7 181888.0 137262.8 28544.0 20386.6 313 16.024 8 3.706 19.729
According to this of interest are:
MC: Metaspace capacity (kB)
MU: Metaspace utilization (kB)
So in this case about 181mb of Metaspace are committed while 137mb are used currently.
Option 2:
If you have garbage collection logs enabled you can also find this out from there, e.g. after the application crashed already or an issue was reported. Search for lines like
2016-04-06T01:50:04.842+0200: 7.795: [Full GC (Metadata GC Threshold)
[PSYoungGen: 7139K->0K(177152K)]
[ParOldGen: 18396K->22213K(101888K)] 25535K->22213K(279040K),
[Metaspace: 34904K->34904K(1081344K)], 0.1408610 secs]
[Times: user=0.45 sys=0.00, real=0.14 secs]
This constitutes a resizing of the Metaspace as the previous threshold was reached.
[Metaspace: 34904K->34904K(1081344K)], 0.1408610 secs]
contains the relevant information: 34,9mb were used before as well as after GC. The latest of these log entries you can find will show the current size (that is: after GC).
Keep in mind that full GC is run whenever Metaspace is resized. So its a good idea to configure a good start value for this when you already know that the default value of ~21mb (depending on Host configuration) is not enough.
See this for more information on tuning Metaspace size.