Java速度较慢,堆较大,没有过多的GC

2022-03-14 00:00:00 performance jvm java heap-memory

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(1 GB)和一个"大"初始堆(16 GB)。当我使用较小的初始堆(即

)运行它时
java -jar -Xms1g -Xmx16g program.jar

程序在12秒内终止(多次运行的平均值)。相反,当我使用较大的初始堆(即

)运行它时
java -jar -Xms16g -Xmx16g program.jar

程序在30秒后终止(多次运行的平均值)。

我从上的其他问题了解到,通常情况下,大堆可能会导致垃圾收集过多,从而降低程序运行速度:

  • Java very large heap sizes
  • Does the Sun JVM slow down when more memory is allocated via -Xmx?
  • Under what circumstances does Java performance degrade with more memory?
  • Java slower with big heap
但是,当我使用标志-verbose:gc运行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,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。

更令人困惑的是(至少对我而言),我有一个功能等价的程序,比如Program2.jar,它的输入输出行为与Program.jar相同。主要区别在于,至少在内存方面,Program2.jar使用的数据结构不如Program.jar高效(实际上我想确定的是,Program2.jar在时间方面是否也不如Program.jar高效)。但是,无论我是使用较小的初始堆还是较大的初始堆运行Program2.jar,它总是在大约22秒内终止(包括大约2-3秒的GC-ing)。

所以,这就是我的问题:(如何)大型堆可以降低程序的速度,而不考虑过度的GC?

(这个问题可能看起来类似于"Java slower with big heap"中Georg的问题,但他的问题原来与堆无关。在我的例子中,我觉得必须与堆有关,因为这是Program.jar的两次运行之间的唯一区别。)

以下是一些可能相关的详细信息。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(http://ark.intel.com/products/81713)(超线程和Turbo Boost禁用),并且有充足的内存(64 GB),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,该程序有多个线程(大约70个)。

编辑%1

响应Bruno Reis和kdgregory

我收集了一些额外的统计数据。这适用于初始堆较小的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

这是针对初始堆较大的Program.jar:

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

(由于以前未计时的初始化阶段,此处报告的挂钟时间与我的原始帖子中报告的挂钟时间不同。)

编辑%2

响应8472的初始answer和之后的comment

我收集了一些有关缓存的统计数据。这适用于初始堆较小(平均超过30次)的Program.jar:

  2719852136 cache-references                                              ( +-  1.56% ) [42.11%]
  1931377514 cache-misses              #   71.010 % of all cache refs      ( +-  0.07% ) [42.11%]
 56748034419 L1-dcache-loads                                               ( +-  1.34% ) [42.12%]
 16334611643 L1-dcache-load-misses     #   28.78% of all L1-dcache hits    ( +-  1.70% ) [42.12%]
 24886806040 L1-dcache-stores                                              ( +-  1.47% ) [42.12%]
  2438414068 L1-dcache-store-misses                                        ( +-  0.19% ) [42.13%]
           0 L1-dcache-prefetch-misses                                    [42.13%]
    23243029 L1-icache-load-misses                                         ( +-  0.66% ) [42.14%]
  2424355365 LLC-loads                                                     ( +-  1.73% ) [42.15%]
   278916135 LLC-stores                                                    ( +-  0.30% ) [42.16%]
   515064030 LLC-prefetches                                                ( +-  0.33% ) [10.54%]
 63395541507 dTLB-loads                                                    ( +-  0.17% ) [15.82%]
  7402222750 dTLB-load-misses          #   11.68% of all dTLB cache hits   ( +-  1.87% ) [21.08%]
 20945323550 dTLB-stores                                                   ( +-  0.69% ) [26.34%]
   294311496 dTLB-store-misses                                             ( +-  0.16% ) [31.60%]
    17012236 iTLB-loads                                                    ( +-  2.10% ) [36.86%]
      473238 iTLB-load-misses          #    2.78% of all iTLB cache hits   ( +-  2.88% ) [42.12%]
 29390940710 branch-loads                                                  ( +-  0.18% ) [42.11%]
    19502228 branch-load-misses                                            ( +-  0.57% ) [42.11%]

53.771209341 seconds time elapsed                                          ( +-  0.42% )

这是针对初始堆较大(平均超过30次)的Program.jar:

 10465831994 cache-references                                              ( +-  3.00% ) [42.10%]
  1921281060 cache-misses              #   18.358 % of all cache refs      ( +-  0.03% ) [42.10%]
 51072650956 L1-dcache-loads                                               ( +-  2.14% ) [42.10%]
 24282459597 L1-dcache-load-misses     #   47.54% of all L1-dcache hits    ( +-  0.16% ) [42.10%]
 21447495598 L1-dcache-stores                                              ( +-  2.46% ) [42.10%]
  2441970496 L1-dcache-store-misses                                        ( +-  0.13% ) [42.10%]
           0 L1-dcache-prefetch-misses                                    [42.11%]
    24953833 L1-icache-load-misses                                         ( +-  0.78% ) [42.12%]
 10234572163 LLC-loads                                                     ( +-  3.09% ) [42.13%]
   240843257 LLC-stores                                                    ( +-  0.17% ) [42.14%]
   462484975 LLC-prefetches                                                ( +-  0.22% ) [10.53%]
 62564723493 dTLB-loads                                                    ( +-  0.10% ) [15.80%]
 12686305321 dTLB-load-misses          #   20.28% of all dTLB cache hits   ( +-  0.01% ) [21.06%]
 19201170089 dTLB-stores                                                   ( +-  1.11% ) [26.33%]
   279236455 dTLB-store-misses                                             ( +-  0.10% ) [31.59%]
    16259758 iTLB-loads                                                    ( +-  4.65% ) [36.85%]
      466127 iTLB-load-misses          #    2.87% of all iTLB cache hits   ( +-  6.68% ) [42.11%]
 28098428012 branch-loads                                                  ( +-  0.13% ) [42.10%]
    18707911 branch-load-misses                                            ( +-  0.82% ) [42.10%]

73.576058909 seconds time elapsed                                          ( +-  0.54% )
比较绝对数字,较大的初始堆导致L1-dcache加载未命中增加约50%,dTLB加载未命中增加70%。我对dTLB加载未命中进行了粗略的计算,假设在我的2.6 GHz机器上有100个周期/未命中(来源:Wikipedia),对于较大的初始堆,延迟为484秒,而对于较小的初始堆,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数量?),但是这个数量级似乎是合理的。

收集完这些统计数据后,我还比较了大型和小型初始堆的-XX:+PrintFlagsFinal输出(基于这两种情况下的一次运行):

<     uintx InitialHeapSize                          := 17179869184     {product}           
---
>     uintx InitialHeapSize                          := 1073741824      {product}  

因此,似乎没有其他标志受-Xms影响。以下也是具有较小初始堆的Program.jar的-XX:+PrintGCDetails输出:

[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)

对于初始堆较大的Program.jar:

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)

解决方案

访问内存需要占用cpu时间。访问更多内存不仅会线性增加CPU时间,还可能会增加高速缓存压力,从而增加未命中率,从而超线性地增加CPU时间。

使用perf stat java -jar ...运行您的程序以查看缓存未命中的数量。请参见Perf tutorial



图片来源: "系统性能:企业与云平装书",布兰登·格雷格,ISBN978-0133390094


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,因此可以产生更紧凑的堆,从而对缓存更友好(不会有临时启动对象散布在堆中)。

要减少两个运行之间的差异,请尝试将两个运行的初始和最大年轻一代大小设置为相同的值,以便只有老一代大小不同。这应该--很可能--会带来同样的表现。

顺便说一句:您还可以尝试用巨大的页面启动JVM,可能(您需要衡量!)通过进一步减少TLB未命中,为您带来几个额外的性能百分比。

未来读者注意:限制新的生成大小不一定会使您的JVM更快,它只会触发GC,而GC恰好会使@Peng的特定工作负载更快。
启动后手动触发GC将具有相同的效果。

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