释放双眼，带上耳机，听听看~！
查看jvm进程id
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28353 Launcher
313301 Jps
48581 Main
57528 RemoteMavenServer
68363 App
74301 Main
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查看操作系统内存占用
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2USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
3yeqiang 8363 2.3 2.8 4121404 473348 pts/0 Sl 10:10 1:41 /usr/local/jdk1.8.0_161/bin/java -Xmx400m -javaagent:/home/yeqiang/program/idea-IC-171.4694.70/lib/idea_rt.jar=43137:/home/yeqiang/program/idea-IC-171.4694.70/bin -Dfile.encoding=UTF-8
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内存使用状态快速求和方法
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2203272
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堆内存（S0U+S1U+EU+OU）+元数据内存(MU)+已编译class占用内存(CSSU)
共203272KB 大约200MB
jstat 查看堆，元数据，编译class内存占用情况：
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2 S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT
3 35328.0 36352.0 0.0 1504.0 63488.0 56313.1 273408.0 39001.0 121176.0 118360.5 10840.0 10150.4 31 0.327 8 0.520 0.848
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6 [yeqiang@localhost ~]$ man jstat
7 -gc option
8 Garbage-collected heap statistics.
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10 S0C: Current survivor space 0 capacity (kB).
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12 S1C: Current survivor space 1 capacity (kB).
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14 S0U: Survivor space 0 utilization (kB).
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16 S1U: Survivor space 1 utilization (kB).
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18 EC: Current eden space capacity (kB).
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20 EU: Eden space utilization (kB).
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22 OC: Current old space capacity (kB).
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24 OU: Old space utilization (kB).
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26 MC: Metaspace capacity (kB).
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28 MU: Metacspace utilization (kB).
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30 CCSC: Compressed class space capacity (kB).
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32 CCSU: Compressed class space used (kB).
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34 YGC: Number of young generation garbage collection events.
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36 YGCT: Young generation garbage collection time.
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38 FGC: Number of full GC events.
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40 FGCT: Full garbage collection time.
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42 GCT: Total garbage collection time.
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jvm 内存相关内存参数
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2 Sets the initial and maximum size (in bytes) of the heap for the young generation (nursery). Append the letter k or K to indicate kilobytes, m or M to indicate megabytes, g or G to indicate
3 gigabytes.
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5 The young generation region of the heap is used for new objects. GC is performed in this region more often than in other regions. If the size for the young generation is too small, then a lot of
6 minor garbage collections will be performed. If the size is too large, then only full garbage collections will be performed, which can take a long time to complete. Oracle recommends that you
7 keep the size for the young generation between a half and a quarter of the overall heap size.
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9-Xmssize
10 Sets the initial size (in bytes) of the heap. This value must be a
11 multiple of 1024 and greater than 1 MB. Append the letter k or K to
12 indicate kilobytes, m or M to indicate megabytes, g or G to
13 indicate gigabytes.
14-Xmxsize
15 Specifies the maximum size (in bytes) of the memory allocation pool
16 in bytes. This value must be a multiple of 1024 and greater than 2
17 MB. Append the letter k or K to indicate kilobytes, m or M to
18 indicate megabytes, g or G to indicate gigabytes. The default value
19 is chosen at runtime based on system configuration. For server
20 deployments, -Xms and -Xmx are often set to the same value. See the
21 section "Ergonomics" in Java SE HotSpot Virtual Machine Garbage
22 Collection Tuning Guide at
23 http://docs.oracle.com/javase/8/docs/technotes/guides/vm/gctuning/index.html.
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26-XX:+PrintGC
27 Enables printing of messages at every GC. By default, this option is disabled.
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29-XX:+PrintGCDetails
30 Enables printing of detailed messages at every GC. By default, this option is disabled.
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32-XX:NativeMemoryTracking=mode
33 Specifies the mode for tracking JVM native memory usage. Possible mode arguments for this option include the following:
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35 off
36 Do not track JVM native memory usage. This is the default behavior if you do not specify the -XX:NativeMemoryTracking option.
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38 summary
39 Only track memory usage by JVM subsystems, such as Java heap, class, code, and thread.
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41 detail
42 In addition to tracking memory usage by JVM subsystems, track memory usage by individual CallSite, individual virtual memory region and its committed regions.
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44-XX:NativeMemoryTracking=detail -XX:+PrintGC -XX:+PrintGCDetails -Xmn100m -Xms200m -Xmx200m
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jcmd
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217108:
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4Native Memory Tracking:
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6Total: reserved=2589996KB, committed=1291028KB
7- Java Heap (reserved=204800KB, committed=204800KB)
8 (mmap: reserved=204800KB, committed=204800KB)
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10- Class (reserved=1062080KB, committed=10176KB)
11 (classes #532)
12 (malloc=5312KB #1296)
13 (mmap: reserved=1056768KB, committed=4864KB)
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15- Thread (reserved=1048963KB, committed=1048963KB)
16 (thread #1017)
17 (stack: reserved=1044408KB, committed=1044408KB)
18 (malloc=3363KB #5086)
19 (arena=1192KB #2033)
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21- Code (reserved=249712KB, committed=2648KB)
22 (malloc=112KB #451)
23 (mmap: reserved=249600KB, committed=2536KB)
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25- GC (reserved=13191KB, committed=13191KB)
26 (malloc=5771KB #119)
27 (mmap: reserved=7420KB, committed=7420KB)
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29- Compiler (reserved=133KB, committed=133KB)
30 (malloc=2KB #29)
31 (arena=131KB #3)
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33- Internal (reserved=8809KB, committed=8809KB)
34 (malloc=8777KB #13536)
35 (mmap: reserved=32KB, committed=32KB)
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37- Symbol (reserved=1512KB, committed=1512KB)
38 (malloc=961KB #178)
39 (arena=552KB #1)
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41- Native Memory Tracking (reserved=590KB, committed=590KB)
42 (malloc=194KB #2585)
43 (tracking overhead=396KB)
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45- Arena Chunk (reserved=206KB, committed=206KB)
46 (malloc=206KB)
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