Java速度较慢，堆较大，没有过多的gc调用 原始问题

我正在运行一个Java程序，比如program.jar，它有一个“小”初始堆（1gb）和一个“大”初始堆（16gb）。当我使用小的初始堆运行它时，即

java -jar -Xms1g -Xmx16g program.jar

java -jar -Xms16g -Xmx16g program.jar

程序在12秒后终止（多次运行的平均值）。相反，当我使用较大的初始堆运行它时，即

java -jar -Xms1g -Xmx16g program.jar

java -jar -Xms16g -Xmx16g program.jar

程序在30秒后终止（多次运行的平均值）

我从其他问题中了解到，通常情况下，大堆可能会导致过度垃圾收集，从而减慢程序：

> free
             total       used       free     shared    buffers     cached
Mem:      65588960   31751316   33837644         20     154616   23995164
-/+ buffers/cache:    7601536   57987424
Swap:      1023996      11484    1012512

---

但是，当我运行带有标志

-verbose:gc

的program.jar时，对于较大的初始堆，不会报告任何gc活动。对于较小的初始堆，在我开始测量时间之前，在程序的初始化阶段只有一些gc活动。因此，过度收集垃圾似乎并不能澄清我的观察结果

为了让它更混乱（至少对我来说），我有一个功能相当的程序，比如program2.jar，它具有与program.jar相同的输入输出行为。主要区别在于program2.jar使用的数据结构比program.jar效率低，至少在内存方面是如此（实际上，我正试图确定program2.jar是否在时间方面效率较低）。但是，无论我使用小的初始堆还是大的初始堆运行program2.jar，它总是在大约22秒后终止（包括大约2-3秒的gc调用）

所以，这是我的问题：（如何）大堆会降低程序的速度，而不考虑过多的垃圾收集？

（这个问题可能与Georg在“”中的问题相似，但他的问题与堆无关。在我的例子中，我觉得它一定与堆有关，因为这是两次运行program.jar的唯一区别。）

以下是一些可能相关的细节。我使用的是Java 7，OpenJDK：

> java -version
java version "1.7.0_75"
OpenJDK Runtime Environment (rhel-2.5.4.0.el6_6-x86_64 u75-b13)
OpenJDK 64-Bit Server VM (build 24.75-b04, mixed mode)

我的机器在两个插槽中有两个E5-2690V3处理器（）（禁用了超线程和Turbo Boost），并且有足够的内存（64gb），其中大约一半在运行程序之前是空闲的：

> free
             total       used       free     shared    buffers     cached
Mem:      65588960   31751316   33837644         20     154616   23995164
-/+ buffers/cache:    7601536   57987424
Swap:      1023996      11484    1012512

最后，程序有多个线程（大约70个）

编辑1 回应和

我收集了一些额外的统计数据。这适用于具有小初始堆的program.jar：

Command being timed: "java -Xms1g -Xmx16g -verbose:gc -jar program.jar"
User time (seconds): 339.11
System time (seconds): 29.86
Percent of CPU this job got: 701%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:52.61
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 12192224
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 1
Minor (reclaiming a frame) page faults: 771372
Voluntary context switches: 7446
Involuntary context switches: 27788
Swaps: 0
File system inputs: 10216
File system outputs: 120
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

Command being timed: "java -Xms16g -Xmx16g -verbose:gc -jar program.jar"
User time (seconds): 769.13
System time (seconds): 28.04
Percent of CPU this job got: 1101%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:12.34
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 10974528
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 16
Minor (reclaiming a frame) page faults: 687727
Voluntary context switches: 6769
Involuntary context switches: 68465
Swaps: 0
File system inputs: 2032
File system outputs: 160
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

[GC [PSYoungGen: 239882K->33488K(306176K)] 764170K->983760K(1271808K), 0.0840630 secs] [Times: user=0.70 sys=0.66, real=0.09 secs]
[Full GC [PSYoungGen: 33488K->0K(306176K)] [ParOldGen: 950272K->753959K(1508352K)] 983760K->753959K(1814528K) [PSPermGen: 2994K->2993K(21504K)], 0.0560900 secs] [Times: user=0.20 sys=0.03, real=0.05 secs]
[GC [PSYoungGen: 234744K->33056K(306176K)] 988704K->983623K(1814528K), 0.0416120 secs] [Times: user=0.69 sys=0.03, real=0.04 secs]
[GC [PSYoungGen: 264198K->33056K(306176K)] 1214765K->1212999K(1814528K), 0.0489600 secs] [Times: user=0.61 sys=0.23, real=0.05 secs]
[Full GC [PSYoungGen: 33056K->0K(306176K)] [ParOldGen: 1179943K->1212700K(2118656K)] 1212999K->1212700K(2424832K) [PSPermGen: 2993K->2993K(21504K)], 0.1589640 secs] [Times: user=2.27 sys=0.10, real=0.16 secs]
[GC [PSYoungGen: 230538K->33056K(431616K)] 1443238K->1442364K(2550272K), 0.0523620 secs] [Times: user=0.69 sys=0.23, real=0.05 secs]
[GC [PSYoungGen: 427431K->33152K(557568K)] 1836740K->1835676K(2676224K), 0.0774750 secs] [Times: user=0.64 sys=0.72, real=0.08 secs]
[Full GC [PSYoungGen: 33152K->0K(557568K)] [ParOldGen: 1802524K->1835328K(2897920K)] 1835676K->1835328K(3455488K) [PSPermGen: 2993K->2993K(21504K)], 0.2019870 secs] [Times: user=2.74 sys=0.13, real=0.20 secs]
[GC [PSYoungGen: 492503K->33280K(647168K)] 2327831K->2327360K(3545088K), 0.0870810 secs] [Times: user=0.61 sys=0.92, real=0.09 secs]
[Full GC [PSYoungGen: 33280K->0K(647168K)] [ParOldGen: 2294080K->2326876K(3603968K)] 2327360K->2326876K(4251136K) [PSPermGen: 2993K->2993K(21504K)], 0.0512730 secs] [Times: user=0.09 sys=0.12, real=0.05 secs]

Heap
 PSYoungGen  total 647168K, used 340719K [0x00000006aaa80000, 0x00000006dd000000, 0x0000000800000000)
  eden space 613376K, 55% used [0x00000006aaa80000,0x00000006bf73bc10,0x00000006d0180000)
  from space 33792K, 0% used [0x00000006d2280000,0x00000006d2280000,0x00000006d4380000)
  to   space 33792K, 0% used [0x00000006d0180000,0x00000006d0180000,0x00000006d2280000)
 ParOldGen   total 3603968K, used 2326876K [0x0000000400000000, 0x00000004dbf80000, 0x00000006aaa80000)
  object space 3603968K, 64% used [0x0000000400000000,0x000000048e0572d8,0x00000004dbf80000)
 PSPermGen   total 21504K, used 3488K [0x00000003f5a00000, 0x00000003f6f00000, 0x0000000400000000)
  object space 21504K, 16% used [0x00000003f5a00000,0x00000003f5d68070,0x00000003f6f00000)

Heap
 PSYoungGen  total 4893696K, used 2840362K [0x00000006aaa80000, 0x0000000800000000, 0x0000000800000000)
  eden space 4194816K, 67% used [0x00000006aaa80000,0x000000075804a920,0x00000007aab00000)
  from space 698880K, 0% used [0x00000007d5580000,0x00000007d5580000,0x0000000800000000)
  to   space 698880K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007d5580000)
 ParOldGen   total 11185152K, used 0K [0x00000003fff80000, 0x00000006aaa80000, 0x00000006aaa80000)
  object space 11185152K, 0% used [0x00000003fff80000,0x00000003fff80000,0x00000006aaa80000)
 PSPermGen   total 21504K, used 3489K [0x00000003f5980000, 0x00000003f6e80000, 0x00000003fff80000)
  object space 21504K, 16% used [0x00000003f5980000,0x00000003f5ce8400,0x00000003f6e80000)

这适用于初始堆较大的program.jar：

Command being timed: "java -Xms1g -Xmx16g -verbose:gc -jar program.jar"
User time (seconds): 339.11
System time (seconds): 29.86
Percent of CPU this job got: 701%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:52.61
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 12192224
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 1
Minor (reclaiming a frame) page faults: 771372
Voluntary context switches: 7446
Involuntary context switches: 27788
Swaps: 0
File system inputs: 10216
File system outputs: 120
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

Command being timed: "java -Xms16g -Xmx16g -verbose:gc -jar program.jar"
User time (seconds): 769.13
System time (seconds): 28.04
Percent of CPU this job got: 1101%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:12.34
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 10974528
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 16
Minor (reclaiming a frame) page faults: 687727
Voluntary context switches: 6769
Involuntary context switches: 68465
Swaps: 0
File system inputs: 2032
File system outputs: 160
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

[GC [PSYoungGen: 239882K->33488K(306176K)] 764170K->983760K(1271808K), 0.0840630 secs] [Times: user=0.70 sys=0.66, real=0.09 secs]
[Full GC [PSYoungGen: 33488K->0K(306176K)] [ParOldGen: 950272K->753959K(1508352K)] 983760K->753959K(1814528K) [PSPermGen: 2994K->2993K(21504K)], 0.0560900 secs] [Times: user=0.20 sys=0.03, real=0.05 secs]
[GC [PSYoungGen: 234744K->33056K(306176K)] 988704K->983623K(1814528K), 0.0416120 secs] [Times: user=0.69 sys=0.03, real=0.04 secs]
[GC [PSYoungGen: 264198K->33056K(306176K)] 1214765K->1212999K(1814528K), 0.0489600 secs] [Times: user=0.61 sys=0.23, real=0.05 secs]
[Full GC [PSYoungGen: 33056K->0K(306176K)] [ParOldGen: 1179943K->1212700K(2118656K)] 1212999K->1212700K(2424832K) [PSPermGen: 2993K->2993K(21504K)], 0.1589640 secs] [Times: user=2.27 sys=0.10, real=0.16 secs]
[GC [PSYoungGen: 230538K->33056K(431616K)] 1443238K->1442364K(2550272K), 0.0523620 secs] [Times: user=0.69 sys=0.23, real=0.05 secs]
[GC [PSYoungGen: 427431K->33152K(557568K)] 1836740K->1835676K(2676224K), 0.0774750 secs] [Times: user=0.64 sys=0.72, real=0.08 secs]
[Full GC [PSYoungGen: 33152K->0K(557568K)] [ParOldGen: 1802524K->1835328K(2897920K)] 1835676K->1835328K(3455488K) [PSPermGen: 2993K->2993K(21504K)], 0.2019870 secs] [Times: user=2.74 sys=0.13, real=0.20 secs]
[GC [PSYoungGen: 492503K->33280K(647168K)] 2327831K->2327360K(3545088K), 0.0870810 secs] [Times: user=0.61 sys=0.92, real=0.09 secs]
[Full GC [PSYoungGen: 33280K->0K(647168K)] [ParOldGen: 2294080K->2326876K(3603968K)] 2327360K->2326876K(4251136K) [PSPermGen: 2993K->2993K(21504K)], 0.0512730 secs] [Times: user=0.09 sys=0.12, real=0.05 secs]

Heap
 PSYoungGen  total 647168K, used 340719K [0x00000006aaa80000, 0x00000006dd000000, 0x0000000800000000)
  eden space 613376K, 55% used [0x00000006aaa80000,0x00000006bf73bc10,0x00000006d0180000)
  from space 33792K, 0% used [0x00000006d2280000,0x00000006d2280000,0x00000006d4380000)
  to   space 33792K, 0% used [0x00000006d0180000,0x00000006d0180000,0x00000006d2280000)
 ParOldGen   total 3603968K, used 2326876K [0x0000000400000000, 0x00000004dbf80000, 0x00000006aaa80000)
  object space 3603968K, 64% used [0x0000000400000000,0x000000048e0572d8,0x00000004dbf80000)
 PSPermGen   total 21504K, used 3488K [0x00000003f5a00000, 0x00000003f6f00000, 0x0000000400000000)
  object space 21504K, 16% used [0x00000003f5a00000,0x00000003f5d68070,0x00000003f6f00000)

Heap
 PSYoungGen  total 4893696K, used 2840362K [0x00000006aaa80000, 0x0000000800000000, 0x0000000800000000)
  eden space 4194816K, 67% used [0x00000006aaa80000,0x000000075804a920,0x00000007aab00000)
  from space 698880K, 0% used [0x00000007d5580000,0x00000007d5580000,0x0000000800000000)
  to   space 698880K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007d5580000)
 ParOldGen   total 11185152K, used 0K [0x00000003fff80000, 0x00000006aaa80000, 0x00000006aaa80000)
  object space 11185152K, 0% used [0x00000003fff80000,0x00000003fff80000,0x00000006aaa80000)
 PSPermGen   total 21504K, used 3489K [0x00000003f5980000, 0x00000003f6e80000, 0x00000003fff80000)
  object space 21504K, 16% used [0x00000003f5980000,0x00000003f5ce8400,0x00000003f6e80000)

（此处报告的挂钟时间与我在原始帖子中报告的时间不同，因为之前有一个未计时的初始化阶段。）

编辑2 响应8472的首字母和后面的字母

我收集了一些关于缓存的统计数据。这适用于初始堆较小的program.jar（平均运行30次）：

这适用于初始堆较大的program.jar（平均运行30次）：

比较绝对数，较大的初始堆会导致L1 dcache负载未命中增加约50%，dTLB负载未命中增加70%。我对dTLB加载未命中进行了一次封套后计算，假设在我的2.6 ghz机器上有100个循环/未命中（源代码：），这使得大初始堆的延迟为484秒，而小初始堆的延迟为284秒。我不知道如何将这个数字转换回每个内核的延迟（可能不只是除以内核的数量？），但数量级似乎是合理的

在收集了这些统计数据之后，我还对大型和小型初始堆的

-XX:+PrintFlagsFinal

输出进行了区分（基于这两种情况中每一种的一次运行）：

对于初始堆较大的program.jar：

Command being timed: "java -Xms1g -Xmx16g -verbose:gc -jar program.jar"
User time (seconds): 339.11
System time (seconds): 29.86
Percent of CPU this job got: 701%
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:52.61
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 12192224
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 1
Minor (reclaiming a frame) page faults: 771372
Voluntary context switches: 7446
Involuntary context switches: 27788
Swaps: 0
File system inputs: 10216
File system outputs: 120
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

Command being timed: "java -Xms16g -Xmx16g -verbose:gc -jar program.jar"
User time (seconds): 769.13
System time (seconds): 28.04
Percent of CPU this job got: 1101%
Elapsed (wall clock) time (h:mm:ss or m:ss): 1:12.34
Average shared text size (kbytes): 0
Average unshared data size (kbytes): 0
Average stack size (kbytes): 0
Average total size (kbytes): 0
Maximum resident set size (kbytes): 10974528
Average resident set size (kbytes): 0
Major (requiring I/O) page faults: 16
Minor (reclaiming a frame) page faults: 687727
Voluntary context switches: 6769
Involuntary context switches: 68465
Swaps: 0
File system inputs: 2032
File system outputs: 160
Socket messages sent: 0
Socket messages received: 0
Signals delivered: 0
Page size (bytes): 4096
Exit status: 0

[GC [PSYoungGen: 239882K->33488K(306176K)] 764170K->983760K(1271808K), 0.0840630 secs] [Times: user=0.70 sys=0.66, real=0.09 secs]
[Full GC [PSYoungGen: 33488K->0K(306176K)] [ParOldGen: 950272K->753959K(1508352K)] 983760K->753959K(1814528K) [PSPermGen: 2994K->2993K(21504K)], 0.0560900 secs] [Times: user=0.20 sys=0.03, real=0.05 secs]
[GC [PSYoungGen: 234744K->33056K(306176K)] 988704K->983623K(1814528K), 0.0416120 secs] [Times: user=0.69 sys=0.03, real=0.04 secs]
[GC [PSYoungGen: 264198K->33056K(306176K)] 1214765K->1212999K(1814528K), 0.0489600 secs] [Times: user=0.61 sys=0.23, real=0.05 secs]
[Full GC [PSYoungGen: 33056K->0K(306176K)] [ParOldGen: 1179943K->1212700K(2118656K)] 1212999K->1212700K(2424832K) [PSPermGen: 2993K->2993K(21504K)], 0.1589640 secs] [Times: user=2.27 sys=0.10, real=0.16 secs]
[GC [PSYoungGen: 230538K->33056K(431616K)] 1443238K->1442364K(2550272K), 0.0523620 secs] [Times: user=0.69 sys=0.23, real=0.05 secs]
[GC [PSYoungGen: 427431K->33152K(557568K)] 1836740K->1835676K(2676224K), 0.0774750 secs] [Times: user=0.64 sys=0.72, real=0.08 secs]
[Full GC [PSYoungGen: 33152K->0K(557568K)] [ParOldGen: 1802524K->1835328K(2897920K)] 1835676K->1835328K(3455488K) [PSPermGen: 2993K->2993K(21504K)], 0.2019870 secs] [Times: user=2.74 sys=0.13, real=0.20 secs]
[GC [PSYoungGen: 492503K->33280K(647168K)] 2327831K->2327360K(3545088K), 0.0870810 secs] [Times: user=0.61 sys=0.92, real=0.09 secs]
[Full GC [PSYoungGen: 33280K->0K(647168K)] [ParOldGen: 2294080K->2326876K(3603968K)] 2327360K->2326876K(4251136K) [PSPermGen: 2993K->2993K(21504K)], 0.0512730 secs] [Times: user=0.09 sys=0.12, real=0.05 secs]

Heap
 PSYoungGen  total 647168K, used 340719K [0x00000006aaa80000, 0x00000006dd000000, 0x0000000800000000)
  eden space 613376K, 55% used [0x00000006aaa80000,0x00000006bf73bc10,0x00000006d0180000)
  from space 33792K, 0% used [0x00000006d2280000,0x00000006d2280000,0x00000006d4380000)
  to   space 33792K, 0% used [0x00000006d0180000,0x00000006d0180000,0x00000006d2280000)
 ParOldGen   total 3603968K, used 2326876K [0x0000000400000000, 0x00000004dbf80000, 0x00000006aaa80000)
  object space 3603968K, 64% used [0x0000000400000000,0x000000048e0572d8,0x00000004dbf80000)
 PSPermGen   total 21504K, used 3488K [0x00000003f5a00000, 0x00000003f6f00000, 0x0000000400000000)
  object space 21504K, 16% used [0x00000003f5a00000,0x00000003f5d68070,0x00000003f6f00000)

Heap
 PSYoungGen  total 4893696K, used 2840362K [0x00000006aaa80000, 0x0000000800000000, 0x0000000800000000)
  eden space 4194816K, 67% used [0x00000006aaa80000,0x000000075804a920,0x00000007aab00000)
  from space 698880K, 0% used [0x00000007d5580000,0x00000007d5580000,0x0000000800000000)
  to   space 698880K, 0% used [0x00000007aab00000,0x00000007aab00000,0x00000007d5580000)
 ParOldGen   total 11185152K, used 0K [0x00000003fff80000, 0x00000006aaa80000, 0x00000006aaa80000)
  object space 11185152K, 0% used [0x00000003fff80000,0x00000003fff80000,0x00000006aaa80000)
 PSPermGen   total 21504K, used 3489K [0x00000003f5980000, 0x00000003f6e80000, 0x00000003fff80000)
  object space 21504K, 16% used [0x00000003f5980000,0x00000003f5ce8400,0x00000003f6e80000)

---

访问内存需要CPU时间。访问更多内存不仅会线性增加CPU时间，而且可能会增加缓存压力，从而增加未命中率，从而超级线性地增加CPU时间

使用

perf stat java-jar…

运行程序以查看缓存未命中的数量。看

图像来源： “系统性能：企业与云平装本”，Brendan Gregg，ISBN:978-0133390094

---

由于初始堆大小也会影响eden空间大小，并且较小的eden空间会触发GC，因此这可能会导致更紧凑的堆，从而对缓存更友好（堆中没有临时启动对象）

要减少两次运行之间的差异数量，请尝试将两次运行的初始和最大年轻代大小设置为相同的值，以便只有旧代大小不同。这可能会导致同样的表现

顺便说一句：您也可以尝试用巨大的页面启动JVM，它可能（您需要测量！）通过进一步减少TLB未命中而为您带来几%的额外性能

未来读者注意：限制新一代的大小并不一定会使JVM更快，它只会触发一个GC，使@Peng的特定工作负载更快。

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启动后手动触发GC也会产生同样的效果。

您是否有任何关于两次运行期间（1GB vs 16GB Xms）获得的分页/插入量的信息？“没有报告任何GC活动”似乎可疑；它表示程序的所有数据都保留在堆的“eden”部分。任何可能需要16GB堆的东西似乎都至少涉及一些小集合。除了@BrunoReis建议的页面错误之外，我还将研究交换统计数据和驻留集大小（RSS）。我可以想象一种可能的情况，线程本地分配块被分散开来，导致内存子系统需要更多的工作，但是（1）70线程似乎太低，这不是一个真正的问题，以及（2）我无法想象这会导致3倍的速度减慢，除非你正在遭受分页的痛苦。这就足够了还是你还需要更多？我关于没有gc活动的声明