请教jcmd GC.heap_info输出中'heap total'的具体含义

想请教：在以下jcmd GC.heap_info的输出中，heap total 3063808K具体代表什么含义？

root@ip-10-210-3-44:/app# jcmd 34 GC.heap_info
34:
 garbage-first heap   total 3063808K, used 1095736K [0x0000000380000000, 0x0000000800000000)
  region size 16384K, 14 young (229376K), 6 survivors (98304K)
 Metaspace       used 396721K, committed 401984K, reserved 1400832K
  class space    used 48373K, committed 51328K, reserved 1048576K

补充信息 #

jps -lvm输出

root@ip-10-210-3-44:/app# jps -lvm
34 /app/spring-boot-application.jar -XX:+UseContainerSupport -XX:MaxRAMPercentage=75 -XX:NativeMemoryTracking=summary -XX:+UseG1GC -Xlog:gc --add-modules=jdk.incubator.foreign --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.math=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.base/java.util.concurrent=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.text=ALL-UNNAMED --add-opens=java.sql/java.sql=ALL-UNNAMED --add-opens=java.base/java.time=ALL-UNNAMED -Djava.security.egd=file:/dev/./urandom -Dcontainer.id=
30533 jdk.jcmd/sun.tools.jps.Jps -lvm -Dapplication.home=/opt/openjdk-17 -Xms8m -Djdk.module.main=jdk.jcmd

• 容器总内存：24 GB

Java版本信息

openjdk version "17.0.5" 2022-10-18
OpenJDK Runtime Environment Temurin-17.0.5+8 (build 17.0.5+8)
OpenJDK 64-Bit Server VM Temurin-17.0.5+8 (build 17.0.5+8, mixed mode, sharing)

jmap堆信息输出

root@ip-10-210-3-44:/app# jhsdb jmap --heap --pid 34
Attaching to process ID 34, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 17.0.5+8

using thread-local object allocation.
Garbage-First (G1) GC with 4 thread(s)

Heap Configuration:
   MinHeapFreeRatio         = 40
   MaxHeapFreeRatio         = 70
   MaxHeapSize              = 19327352832 (18432.0MB)
   NewSize                  = 1363144 (1.2999954223632812MB)
   MaxNewSize               = 11593056256 (11056.0MB)
   OldSize                  = 5452592 (5.1999969482421875MB)
   NewRatio                 = 2
   SurvivorRatio            = 8
   MetaspaceSize            = 22020096 (21.0MB)
   CompressedClassSpaceSize = 1073741824 (1024.0MB)
   MaxMetaspaceSize         = 17592186044415 MB
   G1HeapRegionSize         = 16777216 (16.0MB)

Heap Usage:
G1 Heap:
   regions  = 1152
   capacity = 19327352832 (18432.0MB)
   used     = 1382900640 (1318.8368225097656MB)
   free     = 17944452192 (17113.163177490234MB)
   7.1551476915677386% used
G1 Young Generation:
Eden Space:
   regions  = 28
   capacity = 1677721600 (1600.0MB)
   used     = 469762048 (448.0MB)
   free     = 1207959552 (1152.0MB)
   28.0% used
Survivor Space:
   regions  = 1
   capacity = 16777216 (16.0MB)
   used     = 16777216 (16.0MB)
   free     = 0 (0.0MB)
   100.0% used
G1 Old Generation:
   regions  = 57
   capacity = 1442840576 (1376.0MB)
   used     = 896361376 (854.8368225097656MB)
   free     = 546479200 (521.1631774902344MB)
   62.124769077744595% used

heap total 3063808K（约2992MB）指的是当前JVM已向操作系统申请、并可用于G1堆的内存总量，也就是已提交的堆内存，和堆的最大可分配内存（MaxHeapSize）不是一回事。

结合你的配置和输出可以明确：

1. 容器总内存24GB，JVM参数-XX:MaxRAMPercentage=75设置堆最大为容器内存的75%，即18GB（18432MB），这和jmap输出的MaxHeapSize = 19327352832 (18432.0MB)完全一致。
2. G1是动态扩容的垃圾回收器：JVM不会一开始就占用全部最大堆内存，而是根据应用的实际内存需求，逐步向操作系统申请内存。total 3063808K就是当前实际申请到的堆内存，后续如果内存需求增加，这个值会逐步提升，直到达到MaxHeapSize的上限。
3. 对比jmap里的capacity = 19327352832 (18432.0MB)（堆的最大容量），就能看出两者的差异——当前应用内存需求不大，所以JVM只申请了约3GB堆内存，剩余的堆空间会在需要时再向操作系统申请。

另外，输出里的used 1095736K是当前堆中实际存放对象的内存大小，已提交内存（total）减去已使用内存（used）就是当前堆里的空闲内存。

内容的提问来源于stack exchange，提问作者tuk