Java 技巧合集

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Java 技巧合集

获取一直FullGC下的java进程HeapDump的小技巧

就是小技巧，操作步骤需要查询，随手记录

  • 找到java进程，gdb attach上去， 例如 gdb -p 12345
  • 找到这个HeapDumpBeforeFullGC的地址（这个flag如果为true，会在FullGC之前做HeapDump，默认是false）
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(gdb) p &HeapDumpBeforeFullGC
$2 = (<data variable, no debug info> *) 0x7f7d50fc660f <HeapDumpBeforeFullGC>
  • Copy 地址：0x7f7d50fc660f
  • 然后把他设置为true，这样下次FGC之前就会生成一份dump文件
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(gdb) set *0x7f7d50fc660f = 1
(gdb) quit
  • 最后，等一会，等下次FullGC触发，你就有HeapDump了！
    (如果没有指定heapdump的名字，默认是 java_pidxxx.hprof)

(PS. jstat -gcutil pid 可以查看gc的概况)

(操作完成后记得gdb上去再设置回去，不然可能一直fullgc，导致把磁盘打满).

其它

在jvm还有响应的时候可以： jinfo -flag +HeapDumpBeforeFullGC pid 设置HeapDumpBeforeFullGC 为true（- 为false，+-都不要为只打印值）

kill -3 产生coredump 存放在 kernel.core_pattern=/root/core （/etc/sysctl.conf , 先 ulimit -c unlimited；或者 gcore id 获取coredump)

得到core文件后，采用 gdb -c 执行文件 core文件 进入调试模式，对于java，有以下2个技巧：

进入gdb调试模式后，输入如下命令： info threads，观察异常的线程，定位到异常的线程后，则可以输入如下命令：thread 线程编号，则会打印出当前java代码的工作流程。

而对于这个core，亦可以用jstack jmap打印出堆信息，线程信息，具体命令：

jmap -heap 执行文件 core文件 jstack -F -l 执行文件 core文件

容器中的进程的话需要到宿主机操作，并且将容器中的 jdk文件夹复制到宿主机对应的位置。

ps auxff |grep 容器id -A10 找到JVM在宿主机上的进程id

coredump

Coredump叫做核心转储，它是进程运行时在突然崩溃的那一刻的一个内存快照。操作系统在程序发生异常而异常在进程内部又没有被捕获的情况下，会把进程此刻内存、寄存器状态、运行堆栈等信息转储保存在一个文件里。

kill -3 产生coredump 存放在 kernel.core_pattern=/root/core （/etc/sysctl.conf , 先 ulimit -c unlimited；）

或者 gcore id 获取coredump

coredump 所在位置：

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$cat /proc/sys/kernel/core_pattern
/home/admin/

coredump 分析

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//打开 coredump
$gdb /opt/taobao/java/bin/java core.24086
[New LWP 27184]
[New LWP 27186]
[New LWP 24086]
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
Core was generated by `/opt/tt/java_coroutine/bin/java'.
#0  0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
Missing separate debuginfos, use: debuginfo-install jdk-8.9.14-20200203164153.alios7.x86_64
(gdb) info threads  //查看所有thread
  Id   Target Id         Frame
  583  Thread 0x7f2fa56177c0 (LWP 24086) 0x00007f2fa4fab017 in pthread_join () from /lib64/libpthread.so.0
  582  Thread 0x7f2f695f3700 (LWP 27186) 0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  581  Thread 0x7f2f6cbfb700 (LWP 27184) 0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  580  Thread 0x7f2f691ef700 (LWP 27176) 0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  579  Thread 0x7f2f698f6700 (LWP 27174) 0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
  

(gdb) thread apply all bt  //查看所有线程堆栈
Thread 583 (Thread 0x7f2fa56177c0 (LWP 24086)):
#0  0x00007f2fa4fab017 in pthread_join () from /lib64/libpthread.so.0
#1  0x00007f2fa4b85085 in ContinueInNewThread0 (continuation=continuation@entry=0x7f2fa4b7fd70 <JavaMain>, stack_size=1048576, args=args@entry=0x7ffe529432d0)
    at /ssd1/jenkins_home/workspace/ajdk.8.build.master/jdk/src/solaris/bin/java_md_solinux.c:1044
#2  0x00007f2fa4b81877 in ContinueInNewThread (ifn=ifn@entry=0x7ffe529433d0, threadStackSize=<optimized out>, argc=<optimized out>, argv=0x7f2fa3c163a8, mode=mode@entry=1,
    what=what@entry=0x7ffe5294be17 "com.taobao.tddl.server.TddlLauncher", ret=0) at /ssd1/jenkins_home/workspace/ajdk.8.build.master/jdk/src/share/bin/java.c:2033
#3  0x00007f2fa4b8513b in JVMInit (ifn=ifn@entry=0x7ffe529433d0, threadStackSize=<optimized out>, argc=<optimized out>, argv=<optimized out>, mode=mode@entry=1,
    what=what@entry=0x7ffe5294be17 "com.taobao.tddl.server.TddlLauncher", ret=ret@entry=0) at /ssd1/jenkins_home/workspace/ajdk.8.build.master/jdk/src/solaris/bin/java_md_solinux.c:1091
#4  0x00007f2fa4b8254d in JLI_Launch (argc=0, argv=0x7f2fa3c163a8, jargc=<optimized out>, jargv=<optimized out>, appclassc=1, appclassv=0x0, fullversion=0x400885 "1.8.0_232-b604",
    dotversion=0x400881 "1.8", pname=0x40087c "java", lname=0x40087c "java", javaargs=0 '\000', cpwildcard=1 '\001', javaw=0 '\000', ergo=0)
    at /ssd1/jenkins_home/workspace/ajdk.8.build.master/jdk/src/share/bin/java.c:304
#5  0x0000000000400635 in main ()

Thread 582 (Thread 0x7f2f695f3700 (LWP 27186)):
#0  0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007f2fa342d863 in Parker::park(bool, long) () from /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so
#2  0x00007f2fa35ba3c3 in Unsafe_Park () from /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so
#3  0x00007f2f9343b44a in ?? ()
#4  0x000000008082e778 in ?? ()
#5  0x0000000000000003 in ?? ()
#6  0x00007f2f88e32758 in ?? ()
#7  0x00007f2f6f532800 in ?? ()

(gdb) thread apply 582 bt //查看582这个线程堆栈，LWP 27186(0x6a32)对应jstack 线程10进程id

Thread 582 (Thread 0x7f2f695f3700 (LWP 27186)):
#0  0x00007f2fa4fada35 in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1  0x00007f2fa342d863 in Parker::park(bool, long) () from /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so
#2  0x00007f2fa35ba3c3 in Unsafe_Park () from /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so
#3  0x00007f2f9343b44a in ?? ()
#35 0x00000000f26fc738 in ?? ()
#36 0x00007f2fa51cec5b in arena_run_split_remove (arena=0x7f2f6ab09c34, chunk=0x80, run_ind=0, flag_dirty=0, flag_decommitted=<optimized out>, need_pages=0) at src/arena.c:398
#37 0x00007f2f695f2980 in ?? ()
#38 0x0000000000000001 in ?? ()
#39 0x00007f2f88e32758 in ?? ()
#40 0x00007f2f695f2920 in ?? ()
#41 0x00007f2fa32f46b8 in CallInfo::set_common(KlassHandle, KlassHandle, methodHandle, methodHandle, CallInfo::CallKind, int, Thread*) ()
   from /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so
#42 0x00007f2f7d800000 in ?? ()

以上堆栈涉及到Java代码部分都是看不到函数，需要进一步把Java 符号替换进去

coredump 转 jmap hprof

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jmap -dump:format=b,file=24086.hprof /opt/taobao/java/bin/java core.24086

以上命令输入是 core.24086 这个 coredump，输出是一个 jmap 的dump 24086.hprof

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$jmap -J-d64 /opt/taobao/java/bin/java core.24086

Attaching to core core.24086 from executable /opt/taobao/java/bin/java, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.232-b604
0x0000000000400000      8K      /opt/taobao/java/bin/java
0x00007f2fa51be000      6679K   /opt/taobao/install/ajdk-8_9_14-b604/bin/../lib/amd64/libjemalloc.so.2
0x00007f2fa4fa2000      138K    /lib64/libpthread.so.0
0x00007f2fa4d8c000      88K     /lib64/libz.so.1
0x00007f2fa4b7d000      280K    /opt/taobao/install/ajdk-8_9_14-b604/bin/../lib/amd64/jli/libjli.so
0x00007f2fa4979000      18K     /lib64/libdl.so.2
0x00007f2fa45ab000      2105K   /lib64/libc.so.6
0x00007f2fa43a3000      42K     /lib64/librt.so.1
0x00007f2fa40a1000      1110K   /lib64/libm.so.6
0x00007f2fa5406000      159K    /lib64/ld-linux-x86-64.so.2
0x00007f2fa2af1000      17898K  /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so
0x00007f2fa25f1000      64K     /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libverify.so
0x00007f2fa23c2000      228K    /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libjava.so
0x00007f2fa21af000      60K     /lib64/libnss_files.so.2
0x00007f2fa1fa5000      47K     /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libzip.so
0x00007f2f80ded000      96K     /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libnio.so
0x00007f2f80bd4000      119K    /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libnet.so
0x00007f2f7e1f6000      50K     /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libmanagement.so
0x00007f2f75dc8000      209K    /home/admin/drds-server/lib/native/libsigar-amd64-linux.so
0x00007f2f6d8ad000      293K    /opt/taobao/install/ajdk-8_9_14-b604/jre/lib/amd64/libsunec.so
0x00007f2f6d697000      86K     /lib64/libgcc_s.so.1
0x00007f2f6bdf9000      30K     /lib64/libnss_dns.so.2
0x00007f2f6bbdf000      107K    /lib64/libresolv.so.2

coredump 生成 java stack

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jstack -J-d64 /opt/taobao/java/bin/java core.24086 

Attaching to core core.24086 from executable /opt/taobao/java/bin/java, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.232-b604
Deadlock Detection:

No deadlocks found.

Thread 27186: (state = BLOCKED)
 - sun.misc.Unsafe.park0(boolean, long) @bci=0 (Compiled frame; information may be imprecise)
 - sun.misc.Unsafe.park(boolean, long) @bci=63, line=1038 (Compiled frame)
 - java.util.concurrent.locks.LockSupport.park(java.lang.Object) @bci=14, line=176 (Compiled frame)
 - java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await() @bci=42, line=2047 (Compiled frame)
 - java.util.concurrent.LinkedBlockingQueue.take() @bci=29, line=446 (Compiled frame)
 - java.util.concurrent.ThreadPoolExecutor.getTask() @bci=149, line=1074 (Compiled frame)
 - java.util.concurrent.ThreadPoolExecutor.runWorker(java.util.concurrent.ThreadPoolExecutor$Worker) @bci=26, line=1134 (Compiled frame)
 - java.util.concurrent.ThreadPoolExecutor$Worker.run() @bci=5, line=624 (Compiled frame)
 - java.lang.Thread.run() @bci=11, line=858 (Compiled frame)

gdb coredump with java symbol

需要安装JVM debug info包，同时要求gdb版本在7.10以上

设置：

home 目录下创建 .gdbinit 然后放入如下内容，libjvm.so-gdb.py 就是 dbg.py 脚本，gdb启动的时候会自动加载这个脚本 

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$cat ~/.gdbinit
add-auto-load-safe-path /opt/install/jdk-8_9_14-b604/jre/lib/amd64/server/libjvm.so-gdb.py

使用：

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gdb -iex "set auto-load safe-path /" /opt/install/java/bin/java ./core.24086

G1 GC为什么快

https://ata.alibaba-inc.com/articles/199497

G1比CMS GC 暂停短、更稳定，但是最终吞吐大概率是CMS要好，这是因为G1编译后代码更大

-XX:InlineSmallCode=3000告诉编译器, 汇编3000字节以内的函数需要被inline, 这个值默认是2000

另外CMS用的是Dirty Card，而G1 为了降低GC时间在Remeber Set（类似Dirty Card）的维护上花了更多的代价

Dirty Card维护代价：

  • 会影响code size
    Code size影响了inline机会
    Code size增大则instruction cache miss几率变大 (几十倍的执行时间差距)
  • 本身执行mark dirty动作耗时, 这是一个写内存+GC/mutator线程同步的操作, 可以很复杂, 也可以很简单

比如G1为了降低暂停时间，就要尽量控制Remeber Set的更新，所以还需要判断write动作是否真的有必要更新Remeber Set(类似old.ref = null这种写操作是不需要更新Remeber Set的)

简单说CMS的每次 Dirty Card维护只需要3条汇编，而G1的Remember Set维护需要十多条、几十条汇编