Introduction to jstat
jstat (JVM Statistics Monitoring Tool): A command-line tool for monitoring various running status information of virtual machines. It can display running data such as class loading, memory, garbage collection, JIT compilation, etc. in the local or remote virtual machine process.
The option parameters are as follows:
| Types of | content | illustrate |
|---|---|---|
| Class loading related | -class | Display the relevant information of ClassLoader: the number of loaded and unloaded classes, the total space, the time consumed by class loading, etc. |
| Garbage Collection Related | -gc | Displays heap information related to GC. Including information such as the capacity of the Eden area, the two Survivor areas, the old generation, the permanent generation, etc., the used space, and the total GC time. |
| -gccapacity- | The display content is basically the same as -gc, but the output mainly focuses on the maximum and minimum space used by each area of the Java heap | |
| -gcutil | The display is basically the same as -gc, but the output focuses on the used space as a percentage of the total space | |
| -gccause | Same as -gcutil, but additionally outputs the cause of the last or currently occurring GC | |
| -gcnew | Display the new generation GC status | |
| -gcnewcapacity | The display content is basically the same as -gcnew, and the output mainly focuses on the maximum and minimum space used | |
| -gcold | Display old age GC status | |
| -gcoldcapacity | The display content is basically the same as -gcold, and the output mainly focuses on the maximum and minimum space used | |
| -gcpermcapacity | Display the maximum and minimum space used by the permanent generation | |
| JIT related | -compiler | Displays information such as methods compiled by the JIT compiler and time-consuming |
| -printcompilation | Output methods that have been JIT compiled |
jstat use
1) Statistical class loading information
[root@bogon ~]# jps -l
781 sun.tools.jps.Jps
25887 org.apache.catalina.startup.Bootstrap
[root@bogon ~]# jstat -class 25887
Loaded Bytes Unloaded Bytes Time
3377 6492.3 37 53.6 30.34
| content | illustrate |
|---|---|
| Loaded | The number of classes loaded |
| Bytes | The size of the space occupied by the loaded class |
| Unloaded | Number of classes not loaded |
| Bytes | Unloaded class footprint size |
| Time | Class loading takes time |
Print 5 times at 1s interval
output program execution time
jstat -class -t 25887 1000 5
Print the header every 3 times
jstat -class -h 3 -t 25887 1000
2) View JIT compilation statistics
[root@bogon ~]# jps -l
8264 sun.tools.jps.Jps
7706 ParamTest.jar
25887 org.apache.catalina.startup.Bootstrap
[root@bogon ~]# jstat -compiler 7706
Compiled Failed Invalid Time FailedType FailedMethod
10 0 0 0.00 0
| content | illustrate |
|---|---|
| Compiled | Number of classes compiled by JIT |
| Failed | Number of JIT compilation failures |
| Invalid | Unavailable quantity |
| Time | time |
| FailedType | failure type |
| FailedMethod | fail method |
jstat -printcompilation 7706
3) Garbage collection statistics
A brief introduction to garbage statistics
[root@bogon ~]# jstat -gc 25887
S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT
2880.0 2880.0 0.0 548.0 23552.0 8410.8 58488.0 41320.5 21632.0 20846.0 2432.0 2221.7 215 1.416 3 0.247 1.663
| content | illustrate |
|---|---|
| S0C | Size of the first Survivor area (KB) |
| S1C | Size of the second Survivor area (KB) |
| S0U | The used size of the first Survivor area (KB) |
| S1U | The used size of the second Survivor area (KB) |
| EC | Size of Eden area (KB) |
| I | Use size of Eden area (KB) |
| OC | Size of the old area (KB) |
| WHERE | The used size of the old area (KB) |
| MC | Method area size (KB) |
| MU | Use size of method area (KB) |
| CCSC | Compressed class space size (KB) |
| CCSU | Compressed class space usage size (KB) |
| YGC | Young generation garbage collections |
| YGCT | Young generation garbage collection takes time |
| FGC | Number of old age garbage collections |
| FGCT | Old age garbage collection takes time |
| GCT | Total garbage collection time |
oracle官方文档：https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gc-ergonomics.html
default heap size
When the heap memory size is not specified, the default heap memory is 1/64 of the physical memory, and the maximum heap memory is 1/4 or 1G of the physical machine memory.
Note about the meta space: the meta space will automatically expand and is not limited by default.
[root@bogon ~]# jstat -gc 6619
S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT
1024.0 1024.0 0.0 0.0 8192.0 672.0 20480.0 0.0 4480.0 774.5 384.0 75.9 0 0.000 0 0.000 0.000
[root@bogon ~]# jinfo -flag InitialHeapSize 6619
-XX:InitialHeapSize=31457280
Percentage shows GC usage information
How to judge possible OOM?
We can compare the startup time of the Java process and the total GC time (GCT column), or the time between two measurements and the delta of the total GC time, to get the GC time as a proportion of the running time.
If the ratio exceeds 20%, it means that the current heap pressure is high;
if the ratio exceeds 90%, it means that there is almost no free space in the heap, and an OOM exception may be thrown at any time.
How to judge possible memory leaks?
Step 1: In a long-running Java program, we can run the jstat command to obtain multiple rows of performance data continuously, and take the minimum value of the OU column (that is, the occupied old memory) in these rows of data.
Step 2: Then, we repeat the above operation at long intervals to obtain multiple sets of OU minimum values. If these values show an upward trend, it means that the old-age memory usage of the Java program is increasing, which means that the number of objects that cannot be reclaimed is increasing, so there is a high possibility of a memory leak.