【148】如何解决Linux上Java的-Xmx参数失效的问题
环境
操作系统：Linux CentOS 7
Java版本：jdk1.8.0_191
问题复现
我写了个 start.sh 文件来启动Java程序，sh文件里面带有Java启动参数。start.sh 文件内容如下：
nohup /opt/jdk1.8.0_191/bin/java -jar your_jar_file.jar -server -Xms512m -Xmx512m -Xmn170m > /data0/out/outfile.out --spring.profiles.active=test --server.port=80 &
echo "start"
然后我线用 ps -ef|grep your_jar_file 查询到Java程序的进程ID，再用jmap -heap 命令查看JVM参数
/$ jmap -heap 24653
Attaching to process ID 24653, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
using thread-local object allocation.
Parallel GC with 4 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 4164943872 (3972.0MB)
NewSize = 87031808 (83.0MB)
MaxNewSize = 1388314624 (1324.0MB)
OldSize = 175112192 (167.0MB)
NewRatio = 2
SurvivorRatio = 8
MetaspaceSize = 21807104 (20.796875MB)
CompressedClassSpaceSize = 1073741824 (1024.0MB)
MaxMetaspaceSize = 17592186044415 MB
G1HeapRegionSize = 0 (0.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 846200832 (807.0MB)
used = 712246456 (679.2511520385742MB)
free = 133954376 (127.74884796142578MB)
84.16990731580846% used
From Space:
capacity = 22020096 (21.0MB)
used = 21963048 (20.945594787597656MB)
free = 57048 (0.05440521240234375MB)
99.74092755998883% used
To Space:
capacity = 25690112 (24.5MB)
used = 0 (0.0MB)
free = 25690112 (24.5MB)
0.0% used
PS Old Generation
capacity = 299892736 (286.0MB)
used = 90683136 (86.482177734375MB)
free = 209209600 (199.517822265625MB)
30.238523683347903% used
35761 interned Strings occupying 3503720 bytes.
注意 MaxHeapSize 这一行，是 3972 M，根本不是我设置Xmx的512M，MaxNewSize 是1324 M，也根本不是Xmn设置的170M。
我们也可以用 jcmd 看一下JVM参数：
/root$ jcmd 24653 VM.flags
24653:
-XX:CICompilerCount=3 -XX:InitialHeapSize=262144000 -XX:MaxHeapSize=4164943872 -XX:MaxNewSize=1388314624 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=87031808 -XX:OldSize=175112192 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC
参数单位是字节（byte），可以看出 -XX:MaxHeapSize 和 -XX:NewSize 都不是我们想设置的值。
修改方案
后来我发现应该是参数位置不对，-Xmx -Xms -Xmn 参数应该放在jar文件名之前，变成如下样子：
start.sh
nohup /opt/jdk1.8.0_191/bin/java -jar your_jar_file.jar -server -Xms512m -Xmx512m -Xmn170m > /data0/out/outfile.out --spring.profiles.active=test --server.port=80 &
echo "start"
重启程序，然后用 jmap 看一下设置:
/$ jmap -heap 7132
Attaching to process ID 7132, please wait...
Debugger attached successfully.
Server compiler detected.
JVM version is 25.191-b12
using thread-local object allocation.
Parallel GC with 4 thread(s)
Heap Configuration:
MinHeapFreeRatio = 0
MaxHeapFreeRatio = 100
MaxHeapSize = 536870912 (512.0MB)
NewSize = 178257920 (170.0MB)
MaxNewSize = 178257920 (170.0MB)
OldSize = 358612992 (342.0MB)
NewRatio = 2
SurvivorRatio = 8
MetaspaceSize = 21807104 (20.796875MB)
CompressedClassSpaceSize = 1073741824 (1024.0MB)
MaxMetaspaceSize = 17592186044415 MB
G1HeapRegionSize = 0 (0.0MB)
Heap Usage:
PS Young Generation
Eden Space:
capacity = 112721920 (107.5MB)
used = 21211560 (20.228919982910156MB)
free = 91510360 (87.27108001708984MB)
18.817599984102472% used
From Space:
capacity = 33030144 (31.5MB)
used = 14072064 (13.420166015625MB)
free = 18958080 (18.079833984375MB)
42.60370163690476% used
To Space:
capacity = 32505856 (31.0MB)
used = 0 (0.0MB)
free = 32505856 (31.0MB)
0.0% used
PS Old Generation
capacity = 358612992 (342.0MB)
used = 172781264 (164.7770538330078MB)
free = 185831728 (177.2229461669922MB)
48.18042509737071% used
55733 interned Strings occupying 6060800 bytes.
可以看到 MaxHeapSize、MaxNewSize 都已经改成 512M、170M了。
用 jcmd VM.flags 查看：
/$ jcmd 7132 VM.flags
7132:
-XX:CICompilerCount=3 -XX:InitialHeapSize=536870912 -XX:MaxHeapSize=536870912 -XX:MaxNewSize=178257920 -XX:MinHeapDeltaBytes=524288 -XX:NewSize=178257920 -XX:OldSize=358612992 -XX:+UseCompressedClassPointers -XX:+UseCompressedOops -XX:+UseParallelGC
可以看出来 -XX:MaxHeapSize 和 -XX:MaxNewSize 都已经改过来了。