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java - Max Live Size 和 Max Live Count 没有意义

问题描述

在“内存”选项卡中查看时，我有一个具有 8.96 GiB Max Live Size 的类，而 Max Live Count 为 401,000,000。

这是一个 Scala Cons类，它是类型为 LinkedList 的元素Byte。按照这个逻辑，401M 字节是 401 MiB（可能是某个常数的倍数），所以我不明白为什么实时大小是 8.96 GiB，这要大几个数量级。

在此处输入图像描述

有人可以帮我理解这一点吗？

标签： javascalaprofilingjmcjava-mission-control

解决方案

根据Java Object Layout工具实例大小::或

scala.collection.immutable.$colon$colon

是 24 个字节

➜ java -cp scala-library-2.13.3.jar:jol-cli.jar org.openjdk.jol.Main internals 'scala.collection.immutable.$colon$colon'
# WARNING: Unable to attach Serviceability Agent. You can try again with escalated privileges. Two options: a) use -Djol.tryWithSudo=true to try with sudo; b) echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope
# Running 64-bit HotSpot VM.
# Using compressed oop with 3-bit shift.
# Using compressed klass with 3-bit shift.
# WARNING | Compressed references base/shifts are guessed by the experiment!
# WARNING | Therefore, computed addresses are just guesses, and ARE NOT RELIABLE.
# WARNING | Make sure to attach Serviceability Agent to get the reliable addresses.
# Objects are 8 bytes aligned.
# Field sizes by type: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]
# Array element sizes: 4, 1, 1, 2, 2, 4, 4, 8, 8 [bytes]

Instantiated the sample instance via public scala.collection.immutable.$colon$colon(java.lang.Object,scala.collection.immutable.List)

scala.collection.immutable.$colon$colon object internals:
 OFFSET  SIZE                              TYPE DESCRIPTION                               VALUE
      0     4                                   (object header)                           01 00 00 00 (00000001 00000000 00000000 00000000) (1)
      4     4                                   (object header)                           00 00 00 00 (00000000 00000000 00000000 00000000) (0)
      8     4                                   (object header)                           77 0c 02 f8 (01110111 00001100 00000010 11111000) (-134083465)
     12     4                  java.lang.Object $colon$colon.head                         null
     16     4   scala.collection.immutable.List $colon$colon.next                         null
     20     4                                   (loss due to the next object alignment)
Instance size: 24 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

因此

401,000,000 * 24 byte = 9.62 Gigabtye = 8.96 Gigibyte

这是有道理的，因为除了标准对象头之外，::至少还存储了对头和尾的引用

case class::[+A](head: A, next: List[A])

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