java 15: 查看jdk默认的gc和正在使用的gc

一，查看jdk默认使用的gc

[lhdop@blog ~]$ java -XX:+PrintCommandLineFlags -version
-XX:ConcGCThreads=1 -XX:G1ConcRefinementThreads=4 -XX:GCDrainStackTargetSize=64 -XX:InitialHeapSize=251580608 -XX:MarkStackSize=4194304 -XX:MaxHeapSize=4025289728 -XX:MinHeapSize=6815736 -XX:+PrintCommandLineFlags -XX:ReservedCodeCacheSize=251658240 -XX:+SegmentedCodeCache -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseG1GC
java version "15" 2020-09-15
Java(TM) SE Runtime Environment (build 15+36-1562)
Java HotSpot(TM) 64-Bit Server VM (build 15+36-1562, mixed mode, sharing)

可以看到默认的参数使用了 G1GC

 

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说明：作者:刘宏缔 邮箱: 371125307@qq.com

 

二，查看当前jdk支持的有哪些gc

[lhdop@blog tools]$ java -XX:+PrintFlagsFinal -version | grep 'bool Use' | grep 'GC ' | grep -v java
java version "15" 2020-09-15
Java(TM) SE Runtime Environment (build 15+36-1562)
Java HotSpot(TM) 64-Bit Server VM (build 15+36-1562, mixed mode, sharing)
     bool UseAdaptiveSizePolicyWithSystemGC        = false                                     {product} {default}
     bool UseG1GC                                  = true                                      {product} {ergonomic}
     bool UseMaximumCompactionOnSystemGC           = true                                      {product} {default}
     bool UseParallelGC                            = false                                     {product} {default}
     bool UseSerialGC                              = false                                     {product} {default}
     bool UseShenandoahGC                          = false                                     {product} {default}
     bool UseZGC                                   = false                                     {product} {default}

说明：可以看到jdk15已支持zgc,但并未将它设置为默认

 

三，查看指定进程当前正在使用的gc

[lhdop@blog ~]$ jhsdb jmap --heap --pid 22397
Attaching to process ID 22397, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 15+36-1562
 
using thread-local object allocation.
Garbage-First (G1) GC with 4 thread(s)
 
Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 4026531840 (3840.0MB)
   NewSize                  = 1363144 (1.2999954223632812MB)
   MaxNewSize               = 2415919104 (2304.0MB)
   OldSize                  = 5452592 (5.1999969482421875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 21807104 (20.796875MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 2097152 (2.0MB)
 
Heap Usage:
G1 Heap:
   regions  = 1920
   capacity = 4026531840 (3840.0MB)
   used     = 99822592 (95.1982421875MB)
   free     = 3926709248 (3744.8017578125MB)
   2.479120890299479% used
G1 Young Generation:
Eden Space:
   regions  = 29
   capacity = 69206016 (66.0MB)
   used     = 60817408 (58.0MB)
   free     = 8388608 (8.0MB)
   87.87878787878788% used
Survivor Space:
   regions  = 3
   capacity = 6291456 (6.0MB)
   used     = 6291456 (6.0MB)
   free     = 0 (0.0MB)
   100.0% used
G1 Old Generation:
   regions  = 18
   capacity = 58720256 (56.0MB)
   used     = 32713728 (31.1982421875MB)
   free     = 26006528 (24.8017578125MB)
   55.711146763392854% used

>>Garbage-First (G1) GC with 4 thread(s)

 可以看到使用的是G1这个gc

 

四，查看java的版本:

[lhdop@blog tools]$ java --version
java 15 2020-09-15
Java(TM) SE Runtime Environment (build 15+36-1562)
Java HotSpot(TM) 64-Bit Server VM (build 15+36-1562, mixed mode, sharing)