How to get heap usage using jstat?

Question

I'm running jstat -gc (from OpenJDK):

# jstat -gc 1
 S0C    S1C    S0U    S1U      EC       EU        OC         OU       MC     MU    CCSC   CCSU   YGC     YGCT    FGC    FGCT     GCT
287744.0 290304.0 88368.6  0.0   1469440.0 787186.5 2162176.0  1805969.7  945432.0 923880.4 136576.0 133284.0    268   32.797  21     30.089   62.886

How to read:

1. used heap

2. heap size

3. max heap

from this output, just like shown by VisualVM?

Tonguing

If you are insterested only in those three values, maybe jmap -heap <pid> would better fit your needs. – Wellchosen

Nice. Max heap (3) is shown in MaxHeapSize. Used heap (1) is sum of "used" in Eden Space, From Space, To Space, and PS Old Generation. But how to get heap size (2)? – Tonguing

Also, unfortunately jmap in OpenJDK doesn't have -heap param... Any alternatives? – Tonguing

Which OpenJDK version are you using? I can see the option here (openjdk version "1.8.0_131"). – Wellchosen

openjdk8-8.111.14-r0 Alpine Linux package, javac -version shows javac 1.8.0_111-internal – Tonguing

I did a check. Updateing Alpine Linux to 3.5/3.6 doesn't bring you up to 131, as it's simply not yet available. see blogs.oracle.com/developers/… – Wellchosen

Answer 1

See https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstat.html for general reference.

Current heap size would be the sum of all the fields that end with "C" - S0C, S1C, EC, OC (except for metaspace which is the fields that start with "M")

Used heap would be the sum of all the fields that end with "U" - S0U, S1U, EU, OU (again, except metaspace).

Note that the "C" values (current) are greater than or equal to "U" values (actually used).

To get maximum values run jstat with the -gccapacity flag and add up all the fields that end with "MX" (NGCMX, OGCMX, ... except for MCMX which is metaspace).

Shotwell

What about CCSC and CCSU columns? – Tonguing

Can you provide a source that heap consists of these spaces? – Tonguing

CCSC/CCSU are the Compressed Class Space and apparently not part of the heap. Perhaps the following link may help: java-latte.blogspot.co.il/2014/03/metaspace-in-java-8.html – Shotwell

is there any tool taht can show this data easily? – Deerskin