1 起源

  • 起源:《Java虚拟机 JVM故障诊断与性能优化》——葛一鸣
  • 章节:第三章

本文是第三章的一些笔记整顿。

2 GC日志:-Xlog:gc

要打印GC日志的话,能够加上-Xlog:gc参数(JDK8及以下请应用-XX:+PrintGC),开启GC打印后,每次GC就会打印如下的日志(OpenJDK11 -Xlog:gc):

[0.126s][info][gc] GC(0) Pause Young (Normal) (G1 Evacuation Pause) 25M->0M(502M) 1.902ms[0.205s][info][gc] GC(1) Pause Young (Normal) (G1 Evacuation Pause) 300M->0M(502M) 4.174ms[0.236s][info][gc] GC(2) Pause Young (Normal) (G1 Evacuation Pause) 300M->0M(502M) 2.067ms[0.268s][info][gc] GC(3) Pause Young (Normal) (G1 Evacuation Pause) 300M->0M(502M) 2.362ms

其中结尾的工夫示意产生GC的时刻,25M->0M(502M)示意:

  • GC前,堆使用量为25M
  • GC后,堆使用量为0M
  • 堆空间总和约为502M

开端的工夫示意本次GC的耗时。

另外如果须要更加具体的参数,能够应用-Xlog:gc*JDK8及以下请应用-XX:+PrintGCDetails),比方上面是一部分的GC日志(-Xlog:gc*):

[0.137s][info][gc,start     ] GC(0) Pause Young (Normal) (G1 Evacuation Pause)[0.138s][info][gc,task      ] GC(0) Using 10 workers of 10 for evacuation[0.147s][info][gc,phases    ] GC(0)   Pre Evacuate Collection Set: 0.0ms[0.147s][info][gc,phases    ] GC(0)   Evacuate Collection Set: 8.8ms[0.147s][info][gc,phases    ] GC(0)   Post Evacuate Collection Set: 0.2ms[0.147s][info][gc,phases    ] GC(0)   Other: 0.8ms[0.147s][info][gc,heap      ] GC(0) Eden regions: 25->0(300)[0.147s][info][gc,heap      ] GC(0) Survivor regions: 0->1(4)[0.147s][info][gc,heap      ] GC(0) Old regions: 0->0[0.147s][info][gc,heap      ] GC(0) Humongous regions: 0->0[0.147s][info][gc,metaspace ] GC(0) Metaspace: 6633K->6633K(1056768K)[0.147s][info][gc           ] GC(0) Pause Young (Normal) (G1 Evacuation Pause) 25M->0M(502M) 9.878ms[0.147s][info][gc,cpu       ] GC(0) User=0.05s Sys=0.00s Real=0.01s
  • 行首的工夫:事件产生的时刻
  • GC(0):这是第1次GC,接着会有GC(1)GC(2)
  • Pause Young(Normal):这次GC回收了新生代
  • Using 10 workers:应用10个工作线程
  • Pre Evacuate Collection Set/Evacuate Collection Set/Post Evacuate/Other:示意G1垃圾回收标记,革除算法不同阶段所破费的工夫
  • Eden/Survivor/Old/Humongous/Metaspace:别离示意eden区存活区老年区巨型对象区(就是很大很大的对象所在的区域)、元数据区GC前后的大小
  • 25M-0M(502M)GC前堆占用25MGC后为0M,可用堆空间为502M
  • User/Sys/Real:别离示意用户态CPU耗时零碎CPU耗时GC实在经验工夫

如果想查看更全面的堆信息,能够应用Visual VM,将在后续文章中叙述。

另外如果须要将日志长久化,能够应用-Xlog:gc:gc.log

3 零碎参数打印

参数-XX:+PrintVMOptinos能够打印运行时接管到的显式参数,而-XX:+PrintCommandLineFlags能够打印传递给JVM的隐式与显式参数:

另外一个参数是-XX:+PrintFlagsFinal,会打印所有零碎参数的值(数量很多):

4 堆参数

4.1 最大堆与初始堆参数

Java过程启动时,虚拟机就会调配一块初始堆空间,能够应用参数-Xms指定这块空间的初始化大小。一般来说虚构机会尽可能维持在初始堆空间范畴内运行,然而如果初始堆空间耗尽,虚构机会将堆空间进行扩大,扩大下限为最大堆空间,最大堆空间能够应用参数-Xmx指定。

来一段代码测试一下:

public class Main {    public static void main(String[] args){        printMemory();        byte [] bytes = new byte[1*1024*1024];        System.out.println("Allocate 1024 KB array");        printMemory();        bytes = new byte[4*1024*1024];        System.out.println("Allocate 4096 KB array");        printMemory();    }    public static void printMemory(){        System.out.println();        System.out.println("Max memory = " + Runtime.getRuntime().maxMemory() / 1024+ " KB");        System.out.println("Free memory = "+Runtime.getRuntime().freeMemory()/1024+ " KB");        System.out.println("Total memory = " + Runtime.getRuntime().totalMemory()/ 1024+ " KB");        System.out.println();    }}

参数:

-Xmx20m-Xms5m-XX:+PrintCommandLineFlags-Xlog:gc*-XX:+UseSerialGC

输入:

-XX:InitialHeapSize=5242880 -XX:MaxHeapSize=20971520 -XX:+PrintCommandLineFlags -XX:+PrintGCDetails -XX:ReservedCodeCacheSize=251658240 -XX:+SegmentedCodeCache -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseSerialGC [0.002s][info   ][gc] Using Serial[0.002s][info   ][gc,heap,coops] Heap address: 0x00000000fec00000, size: 20 MB, Compressed Oops mode: 32-bit[0.110s][info   ][gc,start     ] GC(0) Pause Young (Allocation Failure)[0.112s][info   ][gc,heap      ] GC(0) DefNew: 1664K->192K(1856K)[0.112s][info   ][gc,heap      ] GC(0) Tenured: 0K->598K(4096K)[0.112s][info   ][gc,metaspace ] GC(0) Metaspace: 6436K->6436K(1056768K)[0.112s][info   ][gc           ] GC(0) Pause Young (Allocation Failure) 1M->0M(5M) 2.069ms[0.112s][info   ][gc,cpu       ] GC(0) User=0.00s Sys=0.00s Real=0.01sMax memory = 19840 KBFree memory = 4797 KBTotal memory = 5952 KBAllocate 1024 KB arrayMax memory = 19840 KBFree memory = 3773 KBTotal memory = 5952 KB[0.128s][info   ][gc,start     ] GC(1) Pause Young (Allocation Failure)[0.129s][info   ][gc,start     ] GC(2) Pause Full (Allocation Failure)[0.129s][info   ][gc,phases,start] GC(2) Phase 1: Mark live objects[0.130s][info   ][gc,phases      ] GC(2) Phase 1: Mark live objects 1.366ms[0.130s][info   ][gc,phases,start] GC(2) Phase 2: Compute new object addresses[0.130s][info   ][gc,phases      ] GC(2) Phase 2: Compute new object addresses 0.235ms[0.130s][info   ][gc,phases,start] GC(2) Phase 3: Adjust pointers[0.131s][info   ][gc,phases      ] GC(2) Phase 3: Adjust pointers 0.624ms[0.131s][info   ][gc,phases,start] GC(2) Phase 4: Move objects[0.131s][info   ][gc,phases      ] GC(2) Phase 4: Move objects 0.042ms[0.131s][info   ][gc             ] GC(2) Pause Full (Allocation Failure) 1M->1M(5M) 2.335ms[0.131s][info   ][gc,heap        ] GC(1) DefNew: 1579K->0K(1856K)[0.131s][info   ][gc,heap        ] GC(1) Tenured: 598K->1899K(4096K)[0.131s][info   ][gc,metaspace   ] GC(1) Metaspace: 6624K->6624K(1056768K)[0.131s][info   ][gc             ] GC(1) Pause Young (Allocation Failure) 2M->1M(5M) 3.636ms[0.131s][info   ][gc,cpu         ] GC(1) User=0.00s Sys=0.01s Real=0.00sAllocate 4096 KB arrayMax memory = 19840 KBFree memory = 4087 KBTotal memory = 10116 KB[0.133s][info   ][gc,heap,exit   ] Heap[0.133s][info   ][gc,heap,exit   ]  def new generation   total 1920K, used 44K [0x00000000fec00000, 0x00000000fee10000, 0x00000000ff2a0000)[0.133s][info   ][gc,heap,exit   ]   eden space 1728K,   2% used [0x00000000fec00000, 0x00000000fec0b198, 0x00000000fedb0000)[0.133s][info   ][gc,heap,exit   ]   from space 192K,   0% used [0x00000000fedb0000, 0x00000000fedb0000, 0x00000000fede0000)[0.133s][info   ][gc,heap,exit   ]   to   space 192K,   0% used [0x00000000fede0000, 0x00000000fede0000, 0x00000000fee10000)[0.133s][info   ][gc,heap,exit   ]  tenured generation   total 8196K, used 5995K [0x00000000ff2a0000, 0x00000000ffaa1000, 0x0000000100000000)[0.133s][info   ][gc,heap,exit   ]    the space 8196K,  73% used [0x00000000ff2a0000, 0x00000000ff87aed0, 0x00000000ff87b000, 0x00000000ffaa1000)[0.133s][info   ][gc,heap,exit   ]  Metaspace       used 6640K, capacity 6723K, committed 7040K, reserved 1056768K[0.133s][info   ][gc,heap,exit   ]   class space    used 590K, capacity 623K, committed 640K, reserved 1048576K

最大内存由-XX:MaxHeapSize指定,该值为-Xmx的值,也就是20 * 1024 * 1024,而打印的最大可用内存为20316160,比设定的值少,这是因为调配给堆的内存空间与理论可用的内存空间并不是同一个概念,因为GC的须要,虚构机会对堆空间进行分区治理,不同的区会采纳不同的回收算法,一些算法会应用空间换工夫的策略,因而会存在损失,最终的后果是理论可用内存会节约大小等于from/to的空间,从输入能够晓得:

[0.139s][info][gc,heap,exit   ]   from space 192K,   0% used [0x00000000fedb0000, 0x00000000fedb0000, 0x00000000fede0000)

from的大小为192k,然而理论状况是最大可用内存19840k+from192k=20032k,并不是调配的内存20480k,这是因为虚构机会对from/to进行对齐,将最大可用内存加上对齐后的from/to即失去调配的内存大小。

另外,打印显示,初始运行闲暇内存4797k,调配一个1024k数组后,闲暇内存为3773k,正好合乎,接着调配4096k,因为内存不足,对堆空间进行扩大后再调配,扩大后的堆大小为10116k

在理论工作中,能够将-Xms-Xmx设置为雷同,这样能够缩小运行时的GC次数,进步性能。

4.2 新生代参数

-Xmn能够设置新生代的大小,设置一个较大的新生代会减小老年代的大小,这个参数堆GC有很大影响,个别设置为堆空间的1/3-1/4。参数-XX:SurvivorRatio,也就是幸存区比例,可用来设置eden区from/to区的比例,相当于:

-XX:SurvivorRatio=eden/from=eden/to

一个简略的例子如下:

public static void main(String[] args){    byte [] b = null;    for (int i = 0; i < 10; i++) {        b = new byte[1*1024*1024] ;    }}

参数:

-Xmx20m-Xms20m-Xmn1m-XX:SurvivorRatio=2-Xlog:gc*-XX:+UseSerialGC

输入:

[0.002s][info][gc] Using Serial[0.002s][info][gc,heap,coops] Heap address: 0x00000000fec00000, size: 20 MB, Compressed Oops mode: 32-bit[0.042s][info][gc,start     ] GC(0) Pause Young (Allocation Failure)[0.044s][info][gc,heap      ] GC(0) DefNew: 512K->256K(768K)[0.044s][info][gc,heap      ] GC(0) Tenured: 0K->172K(19456K)[0.044s][info][gc,metaspace ] GC(0) Metaspace: 3871K->3871K(1056768K)[0.044s][info][gc           ] GC(0) Pause Young (Allocation Failure) 0M->0M(19M) 1.617ms[0.044s][info][gc,cpu       ] GC(0) User=0.01s Sys=0.00s Real=0.00s[0.064s][info][gc,start     ] GC(1) Pause Young (Allocation Failure)[0.065s][info][gc,heap      ] GC(1) DefNew: 767K->76K(768K)[0.065s][info][gc,heap      ] GC(1) Tenured: 172K->425K(19456K)[0.065s][info][gc,metaspace ] GC(1) Metaspace: 4518K->4518K(1056768K)[0.065s][info][gc           ] GC(1) Pause Young (Allocation Failure) 0M->0M(19M) 0.870ms[0.065s][info][gc,cpu       ] GC(1) User=0.00s Sys=0.00s Real=0.00s[0.093s][info][gc,heap,exit ] Heap[0.093s][info][gc,heap,exit ]  def new generation   total 768K, used 562K [0x00000000fec00000, 0x00000000fed00000, 0x00000000fed00000)[0.093s][info][gc,heap,exit ]   eden space 512K,  94% used [0x00000000fec00000, 0x00000000fec79730, 0x00000000fec80000)[0.093s][info][gc,heap,exit ]   from space 256K,  29% used [0x00000000fec80000, 0x00000000fec93260, 0x00000000fecc0000)[0.093s][info][gc,heap,exit ]   to   space 256K,   0% used [0x00000000fecc0000, 0x00000000fecc0000, 0x00000000fed00000)[0.093s][info][gc,heap,exit ]  tenured generation   total 19456K, used 10665K [0x00000000fed00000, 0x0000000100000000, 0x0000000100000000)[0.093s][info][gc,heap,exit ]    the space 19456K,  54% used [0x00000000fed00000, 0x00000000ff76a630, 0x00000000ff76a800, 0x0000000100000000)[0.093s][info][gc,heap,exit ]  Metaspace       used 6190K, capacity 6251K, committed 6528K, reserved 1056768K[0.093s][info][gc,heap,exit ]   class space    used 535K, capacity 570K, committed 640K, reserved 1048576K

eden区from区的比值为2:1,因而eden区512K,总可用新生代大小为512K+256K=768K,新生代总大小为512K+256K+256K=1M,因为eden区无奈包容调配1MB数组,因而触发了新生代GC,所有数组调配在了老年代。

而如果应用-Xmn7m(其余参数放弃不变),输入如下:

[0.003s][info][gc] Using Serial[0.003s][info][gc,heap,coops] Heap address: 0x00000000fec00000, size: 20 MB, Compressed Oops mode: 32-bit[0.096s][info][gc,start     ] GC(0) Pause Young (Allocation Failure)[0.097s][info][gc,heap      ] GC(0) DefNew: 2684K->1752K(5376K)[0.097s][info][gc,heap      ] GC(0) Tenured: 0K->0K(13312K)[0.097s][info][gc,metaspace ] GC(0) Metaspace: 5929K->5929K(1056768K)[0.097s][info][gc           ] GC(0) Pause Young (Allocation Failure) 2M->1M(18M) 1.350ms[0.097s][info][gc,cpu       ] GC(0) User=0.00s Sys=0.00s Real=0.00s[0.098s][info][gc,start     ] GC(1) Pause Young (Allocation Failure)[0.099s][info][gc,heap      ] GC(1) DefNew: 4928K->1024K(5376K)[0.099s][info][gc,heap      ] GC(1) Tenured: 0K->727K(13312K)[0.099s][info][gc,metaspace ] GC(1) Metaspace: 5996K->5996K(1056768K)[0.099s][info][gc           ] GC(1) Pause Young (Allocation Failure) 4M->1M(18M) 1.142ms[0.099s][info][gc,cpu       ] GC(1) User=0.01s Sys=0.00s Real=0.00s[0.100s][info][gc,start     ] GC(2) Pause Young (Allocation Failure)[0.100s][info][gc,heap      ] GC(2) DefNew: 4180K->1024K(5376K)[0.100s][info][gc,heap      ] GC(2) Tenured: 727K->728K(13312K)[0.100s][info][gc,metaspace ] GC(2) Metaspace: 6008K->6008K(1056768K)[0.100s][info][gc           ] GC(2) Pause Young (Allocation Failure) 4M->1M(18M) 0.190ms[0.100s][info][gc,cpu       ] GC(2) User=0.00s Sys=0.00s Real=0.00s[0.100s][info][gc,heap,exit ] Heap[0.100s][info][gc,heap,exit ]  def new generation   total 5376K, used 4211K [0x00000000fec00000, 0x00000000ff300000, 0x00000000ff300000)[0.100s][info][gc,heap,exit ]   eden space 3584K,  88% used [0x00000000fec00000, 0x00000000fef1cc00, 0x00000000fef80000)[0.100s][info][gc,heap,exit ]   from space 1792K,  57% used [0x00000000ff140000, 0x00000000ff2402a0, 0x00000000ff300000)[0.100s][info][gc,heap,exit ]   to   space 1792K,   0% used [0x00000000fef80000, 0x00000000fef80000, 0x00000000ff140000)[0.100s][info][gc,heap,exit ]  tenured generation   total 13312K, used 728K [0x00000000ff300000, 0x0000000100000000, 0x0000000100000000)[0.100s][info][gc,heap,exit ]    the space 13312K,   5% used [0x00000000ff300000, 0x00000000ff3b61f8, 0x00000000ff3b6200, 0x0000000100000000)[0.100s][info][gc,heap,exit ]  Metaspace       used 6034K, capacity 6091K, committed 6272K, reserved 1056768K[0.100s][info][gc,heap,exit ]   class space    used 518K, capacity 538K, committed 640K, reserved 1048576K

此参数下,eden区有足够的空间,所有数组调配在eden区,然而不足以预留10M空间,因而产生了GC,每次申请空间也废除了上一次申请的空间,在新生代GC中无效回收了这些内存,最初的后果是所有内存调配都在新生代进行,只是在GC过程中局部新生代对象降职到了老年代。

再次增大新生代,应用-Xmn15m -XX:SurvivorRatio=8(其余参数不变),输入如下:

[0.003s][info][gc] Using Serial[0.003s][info][gc,heap,coops] Heap address: 0x00000000fec00000, size: 20 MB, Compressed Oops mode: 32-bitstart[0.097s][info][gc,start     ] GC(0) Pause Young (Allocation Failure)[0.099s][info][gc,heap      ] GC(0) DefNew: 11416K->1471K(13696K)[0.099s][info][gc,heap      ] GC(0) Tenured: 0K->294K(5312K)[0.099s][info][gc,metaspace ] GC(0) Metaspace: 6103K->6103K(1056768K)[0.099s][info][gc           ] GC(0) Pause Young (Allocation Failure) 11M->1M(18M) 2.322ms[0.099s][info][gc,cpu       ] GC(0) User=0.00s Sys=0.00s Real=0.01send[0.099s][info][gc,heap,exit ] Heap[0.099s][info][gc,heap,exit ]  def new generation   total 13696K, used 2934K [0x00000000fec00000, 0x00000000ffad0000, 0x00000000ffad0000)[0.099s][info][gc,heap,exit ]   eden space 12224K,  11% used [0x00000000fec00000, 0x00000000fed6d908, 0x00000000ff7f0000)[0.099s][info][gc,heap,exit ]   from space 1472K,  99% used [0x00000000ff960000, 0x00000000ffacfff8, 0x00000000ffad0000)[0.099s][info][gc,heap,exit ]   to   space 1472K,   0% used [0x00000000ff7f0000, 0x00000000ff7f0000, 0x00000000ff960000)[0.099s][info][gc,heap,exit ]  tenured generation   total 5312K, used 294K [0x00000000ffad0000, 0x0000000100000000, 0x0000000100000000)[0.099s][info][gc,heap,exit ]    the space 5312K,   5% used [0x00000000ffad0000, 0x00000000ffb19960, 0x00000000ffb19a00, 0x0000000100000000)[0.099s][info][gc,heap,exit ]  Metaspace       used 6164K, capacity 6251K, committed 6528K, reserved 1056768K[0.099s][info][gc,heap,exit ]   class space    used 532K, capacity 570K, committed 640K, reserved 1048576K

能够看到新生代应用15M空间,eden区占了12288K,齐全满足了10MB须要,并没有产生GC(日志的GC只是在for循环完结后产生的,一次性回收了10M)。

理论工作中,应依据零碎的特点,做正当的设置,根本策略是:

  • 尽可能将对象预留在新生代
  • 缩小老年代GC次数

另外,能够应用-XX:NewRatio=老年代/新生代指定新生代和老年代的比例,比方应用参数:

-Xmx20m-Xms20m-XX:NewRatio=2-Xlog:gc*-XX:+UseSerialGC

输入:

[0.005s][info][gc] Using Serial[0.005s][info][gc,heap,coops] Heap address: 0x00000000fec00000, size: 20 MB, Compressed Oops mode: 32-bit[0.096s][info][gc,start     ] GC(0) Pause Young (Allocation Failure)[0.097s][info][gc,heap      ] GC(0) DefNew: 4852K->639K(6144K)[0.097s][info][gc,heap      ] GC(0) Tenured: 0K->1112K(13696K)[0.097s][info][gc,metaspace ] GC(0) Metaspace: 5905K->5905K(1056768K)[0.097s][info][gc           ] GC(0) Pause Young (Allocation Failure) 4M->1M(19M) 1.413ms[0.097s][info][gc,cpu       ] GC(0) User=0.00s Sys=0.00s Real=0.00s[0.098s][info][gc,start     ] GC(1) Pause Young (Allocation Failure)[0.099s][info][gc,heap      ] GC(1) DefNew: 5920K->0K(6144K)[0.099s][info][gc,heap      ] GC(1) Tenured: 1112K->2776K(13696K)[0.099s][info][gc,metaspace ] GC(1) Metaspace: 5970K->5970K(1056768K)[0.099s][info][gc           ] GC(1) Pause Young (Allocation Failure) 6M->2M(19M) 1.129ms[0.099s][info][gc,cpu       ] GC(1) User=0.00s Sys=0.01s Real=0.00s[0.100s][info][gc,heap,exit ] Heap[0.100s][info][gc,heap,exit ]  def new generation   total 6144K, used 2238K [0x00000000fec00000, 0x00000000ff2a0000, 0x00000000ff2a0000)[0.100s][info][gc,heap,exit ]   eden space 5504K,  40% used [0x00000000fec00000, 0x00000000fee2f690, 0x00000000ff160000)[0.100s][info][gc,heap,exit ]   from space 640K,   0% used [0x00000000ff160000, 0x00000000ff160398, 0x00000000ff200000)[0.100s][info][gc,heap,exit ]   to   space 640K,   0% used [0x00000000ff200000, 0x00000000ff200000, 0x00000000ff2a0000)[0.100s][info][gc,heap,exit ]  tenured generation   total 13696K, used 2776K [0x00000000ff2a0000, 0x0000000100000000, 0x0000000100000000)[0.100s][info][gc,heap,exit ]    the space 13696K,  20% used [0x00000000ff2a0000, 0x00000000ff556250, 0x00000000ff556400, 0x0000000100000000)[0.100s][info][gc,heap,exit ]  Metaspace       used 5998K, capacity 6091K, committed 6272K, reserved 1056768K[0.100s][info][gc,heap,exit ]   class space    used 517K, capacity 538K, committed 640K, reserved 1048576K

堆大小为20M,新生代和老年代的比为1:2,因而新生代大小约为7M,老年代为13M,调配1M时,因为from/to空间有余,导致两个1MB的数组进入了老年代。

4.3 堆溢出解决

如果在Java程序运行过程中,堆空间有余,会抛出内存溢出谬误,也就是常见的OOM。想要剖析起因,能够应用参数-XX:+HeapDumpOnOutOfMemoryError,能够在内存溢出时导出整个堆的信息,配合应用的还有-XX:HeapDumpPath,指定导出堆的寄存门路,例子如下:

public static void main(String[] args){    List<byte[]> list = new ArrayList<>();    for (int i = 0; i < 25; i++) {        list.add(new byte[1*1024*1024]);    }}

参数:

-Xmx20m-Xms5m-XX:+HeapDumpOnOutOfMemoryError-XX:HeapDumpPath=out.dump

例子调配了25M的内存,然而堆只有20M,会抛出OOM,并且文件保留到out.dump中,留神该文件是二进制文件,须要应用业余的工具(如MAT等)查看。

5 非堆参数

5.1 办法区

JDK8开始,永恒区被移除,应用了新的元数据区来寄存类的元数据,默认状况下,元数据区受零碎可用内存的限度,然而依然能够应用-XX:MaxMetaspaceSize指定永恒区的最大可用值。

5.2 栈

栈是每个线程公有的空间,能够应用-Xss指定线程的栈大小,具体在笔者之前的文章中。

5.3 间接内存

间接内存跳过了Java堆,能够使得程序间接拜访原生堆空间,在肯定水平上放慢了内存空间的访问速度。最大可用间接内存能够应用-XX:MaxDirectMemorySize设置,如果不设置,默认为最大堆空间,即-Xmx的值,当间接内存使用量达到最大值时,会触发GC,如果GC后不能无效开释足够的空间,间接内存仍然会引起零碎的OOM

上面测试一下间接内存与堆的速度:

public class Main {    public static final int count = 1000000;    public static void directAccess(){        long start = System.currentTimeMillis();        ByteBuffer b = ByteBuffer.allocateDirect(500);        for(int i=0;i<count;++i){            for (int j = 0; j < 99; j++) {                b.putInt(j) ;            }            b.flip();            for (int j = 0; j < 99; j++) {                b.getInt();            }            b.clear();        }        long end = System.currentTimeMillis();        System.out.println("Direct access: "+(end-start)+" ms");    }    public static void bufferAccess(){        long start = System.currentTimeMillis();        ByteBuffer b = ByteBuffer.allocate(500);        for(int i=0;i<count;++i){            for (int j = 0; j < 99; j++) {                b.putInt(j) ;            }            b.flip();            for (int j = 0; j < 99; j++) {                b.getInt();            }            b.clear();        }        long end = System.currentTimeMillis();        System.out.println("Buffer access: "+(end-start)+" ms");    }    public static void main(String[] args){        directAccess();        bufferAccess();        directAccess();        bufferAccess();    }}

输入(不带任何参数):

Direct access: 167 msBuffer access: 70 msDirect access: 176 msBuffer access: 67 ms

间接内存的访问速度要快于堆内存,然而有一个毛病就是申请的时候速度慢:

public static void directAllocate(){    long start = System.currentTimeMillis();    for (int i = 0; i < count; i++) {        ByteBuffer byteBuffer = ByteBuffer.allocateDirect(1000);    }    long end = System.currentTimeMillis();    System.out.println("Direct allocate: "+(end-start)+" ms");}public static void bufferAllocate(){    long start = System.currentTimeMillis();    for (int i = 0; i < count; i++) {        ByteBuffer byteBuffer = ByteBuffer.allocate(1000);    }    long end = System.currentTimeMillis();    System.out.println("Buffer allocate: "+(end-start)+" ms");}public static void main(String[] args){    directAllocate();    bufferAllocate();    directAllocate();    bufferAllocate();}

输入:

Direct allocate: 867 msBuffer allocate: 287 msDirect allocate: 676 msBuffer allocate: 208 ms

简略来说,间接内存适宜申请次数较少、拜访较频繁的场合,如果须要频繁申请内存空间,并不适宜应用间接内存。