View pid

The ips command finds the pid of your current system name. The pid is very important. Many subsequent commands are queried based on the pid.
Note: the tomcat is bootstrap

View thread snapshot

Example of jstack -l pid :
jstack -l 12340 >a.text
Get thread snapshot information and redirect to the current directory a.text
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Note: We will talk about thread snapshot analysis later

Heap snapshot

jmap -dump:format=b,file=dump.txt pid
jmap -dump:format=b,file=dump.txt 12340

Snapshots need to use tools

jvisualvm

Jdk comes with a graphical interface, which can automatically generate thread dumps, heap dumps, and visualize memory allocation tools
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jconsole

Similar to jvisualvm, both are graphical interfaces
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Automatically print heap dump when memory overflows

-XX:+HeapDumpOnOutOfMemoryError
-XX:HeapDumpPath=D:\temp
-Xms20M
-Xmx20M

When the memory overflows, head dump the file, and then analyze the heap memory usage according to the tool.
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jmap -head pid

Print heap memory usage

Print gc log

-XX:+PrintGCDateStamps
-XX:+PrintGCDetails
-Xloggc:D:/temp/gclogs

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MinorGC

2020-09-06T15:45:38.653+0800: 0.330: [GC (Allocation Failure) [PSYoungGen: 5632K->504K(6144K)] 5632K->1496K(19968K), 0.0019669 secs] [Times: user=0.00 sys=0.00, real=0.00 secs]

Analysis :
2020-09-06T15:45:38 : time
38.653+0800: 0.330 : jvm start gc time
GC (Allocation Failure) : the identification of MinorGC and FullGC, this time represents MinorGC
PSYoungGen : refers to Parallel Scavenge) is Eden +FromSpace
5632K->504K(6144K) : the usage of the young generation (size of the young generation)
5632K->1496K(19968K) : the usage of the entire heap before and after collection, the total heap size is
0.0019669 secs : the ParNew collector marks and copies the young generation alive The time spent by the object

FullGC

2020-09-06T15:45:39.978+0800: 1.656: [Full GC (Metadata GC Threshold) [PSYoungGen: 441K->0K(5120K)] [ParOldGen: 10846K->6883K(13824K)] 11287K->6883K(18944K), [Metaspace: 20298K->20298K(1067008K)], 0.0258899 secs] [Times: user=0.16 sys=0.00, real=0.03 secs]

demo

mat analyzes memory, thread status, etc.

Main interface information
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Histogram

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You can see which type of memory in which objects occupy more. At this time, we make sure that the object of this class of student occupies most of the memory. We continue to analyze

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Right click: Merge shortest Path to GC Roots–>exclude all phantom/weak/soft etc.reference to view the call chain and find the thread @ 0xff3fe388