Poison

MaxRAMPercentage

使用 -XX:MaxRAMPercentage 能够根据物理内存计算出最大堆大小,在容器服务等场景下相比 -Xmx 更易用。在 JDK 8 中计算相关源码位于:jdk8u/arguments.cpp at jdk8u352-b05 · openjdk/jdk8u · GitHub:

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void Arguments::set_heap_size() {
if (!FLAG_IS_DEFAULT(DefaultMaxRAMFraction)) {
// Deprecated flag
FLAG_SET_CMDLINE(uintx, MaxRAMFraction, DefaultMaxRAMFraction);
}

julong phys_mem =
FLAG_IS_DEFAULT(MaxRAM) ? MIN2(os::physical_memory(), (julong)MaxRAM)
: (julong)MaxRAM;

// Experimental support for CGroup memory limits
if (UseCGroupMemoryLimitForHeap) {
// This is a rough indicator that a CGroup limit may be in force
// for this process
const char* lim_file = "/sys/fs/cgroup/memory/memory.limit_in_bytes";
FILE *fp = fopen(lim_file, "r");
if (fp != NULL) {
julong cgroup_max = 0;
int ret = fscanf(fp, JULONG_FORMAT, &cgroup_max);
if (ret == 1 && cgroup_max > 0) {
// If unlimited, cgroup_max will be a very large, but unspecified
// value, so use initial phys_mem as a limit
if (PrintGCDetails && Verbose) {
// Cannot use gclog_or_tty yet.
tty->print_cr("Setting phys_mem to the min of cgroup limit ("
JULONG_FORMAT "MB) and initial phys_mem ("
JULONG_FORMAT "MB)", cgroup_max/M, phys_mem/M);
}
phys_mem = MIN2(cgroup_max, phys_mem);
} else {
warning("Unable to read/parse cgroup memory limit from %s: %s",
lim_file, errno != 0 ? strerror(errno) : "unknown error");
}
fclose(fp);
} else {
warning("Unable to open cgroup memory limit file %s (%s)", lim_file, strerror(errno));
}
}

// Convert Fraction to Precentage values
if (FLAG_IS_DEFAULT(MaxRAMPercentage) &&
!FLAG_IS_DEFAULT(MaxRAMFraction))
MaxRAMPercentage = 100.0 / MaxRAMFraction;

if (FLAG_IS_DEFAULT(MinRAMPercentage) &&
!FLAG_IS_DEFAULT(MinRAMFraction))
MinRAMPercentage = 100.0 / MinRAMFraction;

if (FLAG_IS_DEFAULT(InitialRAMPercentage) &&
!FLAG_IS_DEFAULT(InitialRAMFraction))
InitialRAMPercentage = 100.0 / InitialRAMFraction;

// If the maximum heap size has not been set with -Xmx,
// then set it as fraction of the size of physical memory,
// respecting the maximum and minimum sizes of the heap.
if (FLAG_IS_DEFAULT(MaxHeapSize)) {
julong reasonable_max = (julong)((phys_mem * MaxRAMPercentage) / 100);
const julong reasonable_min = (julong)((phys_mem * MinRAMPercentage) / 100);
if (reasonable_min < MaxHeapSize) {
// Small physical memory, so use a minimum fraction of it for the heap
reasonable_max = reasonable_min;
} else {
// Not-small physical memory, so require a heap at least
// as large as MaxHeapSize
reasonable_max = MAX2(reasonable_max, (julong)MaxHeapSize);
}

if (!FLAG_IS_DEFAULT(ErgoHeapSizeLimit) && ErgoHeapSizeLimit != 0) {
// Limit the heap size to ErgoHeapSizeLimit
reasonable_max = MIN2(reasonable_max, (julong)ErgoHeapSizeLimit);
}
if (UseCompressedOops) {
// Limit the heap size to the maximum possible when using compressed oops
julong max_coop_heap = (julong)max_heap_for_compressed_oops();
if (HeapBaseMinAddress + MaxHeapSize < max_coop_heap) {
// Heap should be above HeapBaseMinAddress to get zero based compressed oops
// but it should be not less than default MaxHeapSize.
max_coop_heap -= HeapBaseMinAddress;
}
reasonable_max = MIN2(reasonable_max, max_coop_heap);
}
reasonable_max = limit_by_allocatable_memory(reasonable_max);

if (!FLAG_IS_DEFAULT(InitialHeapSize)) {
// An initial heap size was specified on the command line,
// so be sure that the maximum size is consistent. Done
// after call to limit_by_allocatable_memory because that
// method might reduce the allocation size.
reasonable_max = MAX2(reasonable_max, (julong)InitialHeapSize);
}

if (PrintGCDetails && Verbose) {
// Cannot use gclog_or_tty yet.
tty->print_cr(" Maximum heap size " SIZE_FORMAT, (size_t) reasonable_max);
}
FLAG_SET_ERGO(uintx, MaxHeapSize, (uintx)reasonable_max);
}

// If the minimum or initial heap_size have not been set or requested to be set
// ergonomically, set them accordingly.
if (InitialHeapSize == 0 || min_heap_size() == 0) {
julong reasonable_minimum = (julong)(OldSize + NewSize);

reasonable_minimum = MIN2(reasonable_minimum, (julong)MaxHeapSize);

reasonable_minimum = limit_by_allocatable_memory(reasonable_minimum);

if (InitialHeapSize == 0) {
julong reasonable_initial = (julong)((phys_mem * InitialRAMPercentage) / 100);

reasonable_initial = MAX3(reasonable_initial, reasonable_minimum, (julong)min_heap_size());
reasonable_initial = MIN2(reasonable_initial, (julong)MaxHeapSize);

reasonable_initial = limit_by_allocatable_memory(reasonable_initial);

if (PrintGCDetails && Verbose) {
// Cannot use gclog_or_tty yet.
tty->print_cr(" Initial heap size " SIZE_FORMAT, (uintx)reasonable_initial);
}
FLAG_SET_ERGO(uintx, InitialHeapSize, (uintx)reasonable_initial);
}
// If the minimum heap size has not been set (via -Xms),
// synchronize with InitialHeapSize to avoid errors with the default value.
if (min_heap_size() == 0) {
set_min_heap_size(MIN2((uintx)reasonable_minimum, InitialHeapSize));
if (PrintGCDetails && Verbose) {
// Cannot use gclog_or_tty yet.
tty->print_cr(" Minimum heap size " SIZE_FORMAT, min_heap_size());
}
}
}
}

需要注意的是 JDK 8 中的实现是基于 cgroup v1 的,如果内核使用的是 cgroup v2,则容器内存探测不会生效,当前使用的 cgroup 版本可以参考 Runtime metrics | Docker Documentation。关于 JDK 8 中基于 cgroup v2 的容器感知,可以参考 JDK-8230305 Cgroups v2: Container awareness - Java Bug System,可知,该 bug 在 JDK 15 中得到修复,并反向移植至了 JDK 11,但是 JDK 8 中并未进行反向移植,所以当使用 JDK 8 且使用了 -XX:MaxRAMPercentage 选项时,需要保证操作系统使用的 cgroup 版本为 v1,否则计算出的最大堆内存是基于物理机的内存,而不是对容器限制的内存。

我们可以用一台 Ubuntu 22.04 LTS 的机器来进行验证,首先确认物理机操作系统,执行 lsb_release -a 命令输出如下:

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No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 22.04 LTS
Release: 22.04
Codename: jammy

执行 unama -a 确认内核版本如下:

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Linux VM-0-16-ubuntu 5.15.0-40-generic #43-Ubuntu SMP Wed Jun 15 12:54:21 UTC 2022 x86_64 x86_64 x86_64 GNU/Linux

执行 free -h 确认物理机内存如下:

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               total        used        free      shared  buff/cache   available
Mem: 3.3Gi 295Mi 671Mi 2.0Mi 2.4Gi 2.8Gi
Swap: 0B 0B 0B

可知物理机内存为 3.3G,我们再执行 ll /sys/fs/cgroup/cgroup.controllers 可确认该文件存在,表明当前操作系统使用的 cgroup 版本为 v2。

我们首先执行 sudo docker run -m 1GB openjdk:11 java -XX:MaxRAMPercentage=80.0 -XshowSettings:vm -version 验证 JDK 11 中对 cgroup v2 的支持,输出如下:

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VM settings:
Max. Heap Size (Estimated): 792.69M
Using VM: OpenJDK 64-Bit Server VM

openjdk version "11.0.16" 2022-07-19
OpenJDK Runtime Environment 18.9 (build 11.0.16+8)
OpenJDK 64-Bit Server VM 18.9 (build 11.0.16+8, mixed mode, sharing)

可知在容器中使用 JDK 11 时 JVM 实例的最大内存为我们设置的 1GB 的 80% 左右,说明探测生效。

我们再执行 sudo docker run -m 1GB openjdk:8 java -XX:MaxRAMPercentage=80.0 -XshowSettings:vm -version 验证 JDK 8 中对 cgroup v2 的支持,输出如下:

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VM settings:
Max. Heap Size (Estimated): 2.58G
Ergonomics Machine Class: client
Using VM: OpenJDK 64-Bit Server VM

openjdk version "1.8.0_342"
OpenJDK Runtime Environment (build 1.8.0_342-b07)
OpenJDK 64-Bit Server VM (build 25.342-b07, mixed mode)

可知在容器中使用 JDK 8 时 JVM 实例的最大内存为 2.58G,并非我们指定的容器内存上限 1G 的 80% 左右,而是物理内存 3.3G 的 80% 左右,说明探测并未生效,此时读取到了物理机的内存。所以,在 JDK 8 下使用 -XX:MaxRAMPercentage 选项时需要注意操作系统使用的 cgroup 版本一定为 v1,否则最大堆内存限制不能按预期工作。

Reference

JVM Parameters InitialRAMPercentage, MinRAMPercentage, and MaxRAMPercentage | Baeldung
JDK-8146115 Improve docker container detection and resource configuration usage - Java Bug System
8146115: Improve docker container detection and resource configuratio… · openjdk/jdk8u@392e56e · GitHub
/sys/fs/cgroup/memory/memory.limit_in_bytes is missing in the container in version > 4.2.0 · Issue #6118 · docker/for-mac · GitHub