Interpreting jstat results

Question

I am new to jstat tool. Therefore I did a sample as below.

./jstat -gcutil -t 4001 5000
Timestamp         S0     S1     E      O      P     YGC     YGCT    FGC           


        

Answer 1

See the documentation:

https://docs.oracle.com/javase/8/docs/technotes/tools/unix/jstat.html

Basically one row is one point in time. The columns show data about the JVM memory areas (Survivor, Eden, ...), understanding them is impossible without knowing how the JVM works.

For example in the article JVM garbage collection in young generation there is some explanation.

Here is the excerpt how JVM object generation works:

Eden is a place where new objects created. When the Eden is full, a small GC is run: if an object has no reference to it, it will be deleted, otherwise it will survive, and move to the Survivor space (only one of the survivor spaces in use at a time, all objects from the other space is copied there).

If an object survives a certain number of back-and-forth copying, it is moved to Old space. If the Old space is full, a Full GC is run, which affects all objects in the JVM, so it is much heavier operation.

Also, there is the Permanent space, where the "metadata" (class descriptors, field, method, ... descriptors) are stored.

Answer 2

Use this simple online jstat visualizer tool to plot jstat GC statistics.

Answer 3

gcutil gives stats in terms of percentage utilization

-gcutil Option
Summary of Garbage Collection Statistics 
Column  Description
S0      Survivor space 0 utilization as a percentage of the space's current capacity.
S1      Survivor space 1 utilization as a percentage of the space's current capacity.
E       Eden space utilization as a percentage of the space's current capacity.
O       Old space utilization as a percentage of the space's current capacity.
P       Permanent space utilization as a percentage of the space's current capacity.
YGC     Number of young generation GC events.
YGCT    Young generation garbage collection time.
FGC     Number of full GC events.
FGCT    Full garbage collection time.
GCT     Total garbage collection time.

gc gives statistics in terms of alloted space and utilized space.

-gc Option
Garbage-collected heap statistics 
Column  Description
S0C     Current survivor space 0 capacity (KB).
S1C     Current survivor space 1 capacity (KB).
S0U     Survivor space 0 utilization (KB).
S1U     Survivor space 1 utilization (KB).
EC      Current eden space capacity (KB).
EU      Eden space utilization (KB).
OC      Current old space capacity (KB).
OU      Old space utilization (KB).
PC      Current permanent space capacity (KB).
PU      Permanent space utilization (KB).
YGC     Number of young generation GC Events.
YGCT    Young generation garbage collection time.
FGC     Number of full GC events.
FGCT    Full garbage collection time.
GCT     Total garbage collection time.

Source : Docs