默认垃圾收集器 Hotspot JVM 11/17 的标准

Question

我发现 source 描述使用的默认 gc 会根据可用资源而变化。似乎 jvm 在硬件和 os.

上使用 g1gc 或串行 gc dependnig

The serial collector is selected by default on certain hardware and operating system configurations

有人可以指出更详细的来源，说明具体标准是什么以及如何在 dockerized/kubernetes 环境中应用。换句话说：

可以将k8s中pod的资源请求设置为eg。 1500 mCpu 使 jvm 使用串行 gc 并更改为 2 Cpu 将默认 gc 更改为 g1gc？何时使用哪个 gc 的限制是否会根据 jvm 版本（11 对 17）而变化？

Answer 1

在JDK 11 和17 Serial 收集器只有一个CPU 可用时使用。否则选择G1

如果您限制容器可用的 CPUS 数量，JVM 选择 Serial 而不是默认值 G1

JDK11

1 CPU

docker run --cpus=1 --rm -it eclipse-temurin:11 java -Xlog:gc* -version
[0.004s][info][gc] Using **Serial**

它使用Serial

不止一个CPU

docker run --cpus=2 --rm -it eclipse-temurin:11 java -Xlog:gc* -version
[0.008s][info][gc     ] Using G1

它使用G1

JDK17

1 CPU

docker run --cpus=1 --rm -it eclipse-temurin:17 java -Xlog:gc* -version
[0.004s][info][gc] Using Serial

它使用Serial

不止一个CPU

docker run --cpus=2 --rm -it eclipse-temurin:17 java -Xlog:gc* -version
[0.007s][info][gc] Using G1

它使用G1

Answer 2

在当前的 OpenJDK 中，“服务器 class 机器”默认选择 G1 GC，否则为串行 GC。 “服务器 class 机器”定义为具有 2 个或更多非 HT CPU 和 2 个或更多 GiB RAM 的系统。

具体算法见src/hotspot/share/runtime/os.cpp:

// This is the working definition of a server class machine:
// >= 2 physical CPU's and >=2GB of memory, with some fuzz
// because the graphics memory (?) sometimes masks physical memory.
// If you want to change the definition of a server class machine
// on some OS or platform, e.g., >=4GB on Windows platforms,
// then you'll have to parameterize this method based on that state,
// as was done for logical processors here, or replicate and
// specialize this method for each platform.  (Or fix os to have
// some inheritance structure and use subclassing.  Sigh.)
// If you want some platform to always or never behave as a server
// class machine, change the setting of AlwaysActAsServerClassMachine
// and NeverActAsServerClassMachine in globals*.hpp.
bool os::is_server_class_machine() {
  // First check for the early returns
  if (NeverActAsServerClassMachine) {
    return false;
  }
  if (AlwaysActAsServerClassMachine) {
    return true;
  }
  // Then actually look at the machine
  bool         result            = false;
  const unsigned int    server_processors = 2;
  const julong server_memory     = 2UL * G;
  // We seem not to get our full complement of memory.
  //     We allow some part (1/8?) of the memory to be "missing",
  //     based on the sizes of DIMMs, and maybe graphics cards.
  const julong missing_memory   = 256UL * M;

  /* Is this a server class machine? */
  if ((os::active_processor_count() >= (int)server_processors) &&
      (os::physical_memory() >= (server_memory - missing_memory))) {
    const unsigned int logical_processors =
      VM_Version::logical_processors_per_package();
    if (logical_processors > 1) {
      const unsigned int physical_packages =
        os::active_processor_count() / logical_processors;
      if (physical_packages >= server_processors) {
        result = true;
      }
    } else {
      result = true;
    }
  }
  return result;
}

默认垃圾收集器 Hotspot JVM 11/17 的标准

Criteria for default garbage collector Hotspot JVM 11/17

java

garbage-collection

jvm

java-11

java-17

JDK11

1 CPU

不止一个CPU

JDK17

1 CPU

不止一个CPU