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VJMap

VJMap is jmap with per GC generation (Eden, Survivor, OldGen) object stats printing. It is built with advanced techniques to disclose situations like memory leaks and fast-growing tenured objects.

1. Introduction

You may print stats about objects with the stock jmap via jmap -histo PID but only by viewing the heap as a whole. However, additional information like OldGen object counting and survivor object age counting can play a vital role in troubleshooting. VJMap is built to make such information available.

Initially inspired by tbjmap, JDK8 compatibility was added as well as query on aged survivor objects.

[Note]: Does not work with G1. Use it with CMS and ParallelGC only.

2. Getting Started

download from Maven Central - 27k

[Important]: VJMap DOES cause stop-of-the-world of the target app. Make sure the target app is isolated from user access before you start using VJMap in production.

Run VJMap under the same user who started the target process. If access errors are still met, try again with root user.

VJMap may take quite some time to finish. Use kill <PID_OF_VJMap> to allow for a graceful exit. If kill -9 <PID_OF_VJMap> is mistakenly issued to the VJMap process, the target app will end up in blocked state, in which case you will have to execute kill -18 <PID_OF_TARGET_APP> TWICE to awaken the target app.

2.1 Commands

// Prints object stats of all the heap, ordered by their respective size in total.
./vjmap.sh -all PID > /tmp/histo.log

// Prints oldgen object stats, ordered by size in OldGen. Only CMS is supported for this option. 
./vjmap.sh -old PID > /tmp/histo-old.log


// Prints survivor objects over the age of 3.
./vjmap.sh -sur PID > /tmp/histo-sur.log


// Prints survivor objects over the age of 10, as desinated by the argument -sur:minage=10
// When the promotion threshold -XX:MaxTenuringThreshold is lifted, objects with a high age value will be bound 
for the CMS oldgen
./vjmap.sh -sur:minage=10 PID > /tmp/histo-sur.log

PID is the process ID of target java application

2.2 Display Larger Objects, Leaving Smaller Ones Out

// Shows objects with sizes over 1KB over the whole heap
./vjmap.sh -all:minsize=1024 PID > /tmp/histo.log

// shows objects with sizes over 1KB in OldGen specifically 
./vjmap.sh -old:minsize=1024 PID > /tmp/histo-old.log

// shows objects with sizes over 1KB in survivor space 
./vjmap.sh -sur:minsize=1024 PID > /tmp/histo-sur.log

2.3 Order by Classname and Filter by Size for Periodic Comparisons

./vjmap.sh -all:minsize=1024,byname PID > /tmp/histo.log

./vjmap.sh -old:minsize=1024,byname PID > /tmp/histo-old.log

./vjmap.sh -sur:minsize=1024,byname PID > /tmp/histo-sur.log

3.Outputs

3.1 Count Survivor Objects over the Age of 3.

Survivor Object Histogram:

 #num  #count     #bytes #Class description
-----------------------------------------------------------------------------------
   1:      37         1k io.netty.buffer.PoolThreadCache$MemoryRegionCache$Entry
   2:       2         64 java.util.concurrent.locks.AbstractQueuedSynchronizer$Node
Total: 39/    1k over age 2

Heap traversal took 1.3 seconds.

4. Eclipse MAT

如果只依靠对象统计信息，不足以定位问题，需要使用完整HeapDump，计算对象关联关系来进一步分析时，可以在MAT中使用OQL过滤出老生代的对象。

假设，OldGen地址范围是"0xfbd4c000" ～ "0xfce94050"

SELECT * FROM INSTANCEOF java.lang.Object t WHERE (toHex(t.@objectAddress) >= "0xfbd4c000" AND toHex(t.@objectAddress) <= "0xfce94050")

用如下方式可获得老生代地址：

第一种方式是在启动参数增加 -XX:+PrintHeapAtGC

第二种方式是使用vjmap的命令，在-old, -sur, -address 中，都会打印出区间的地址。

./vjmap.sh -address PID

输出如下：

  eden [0x0000000119000000,0x0000000119c4a258,0x0000000121880000) space capacity = 143130624, 9.003395387977907 used
  from [0x0000000121880000,0x0000000121880000,0x0000000122990000) space capacity = 17891328, 0.0 used
  to   [0x0000000122990000,0x0000000122990000,0x0000000123aa0000) space capacity = 17891328, 0.0 used
concurrent mark-sweep generation
free-list-space[ 0x0000000123aa0000 , 0x0000000139000000 ) space capacity = 357957632 used(4%)= 17024696 free= 340932936

5. Enhancements over TBJMap

  • Added JDK8 Support.
  • Added Display: survivor objects over the specified age.
  • Performance Boost: by accessing Survivor and OldGen directly instead of by accessing the whole heap with Heap Visitor callbacks.
  • New config Arg: order objects by size and leave out small ones.
  • New Config Arg: order objects by name for periodic comparison.
  • Reading Friendliness: output by the unit of (k, m, g) and fix alignment, order objects in OldGen by size in OldGen view by default.