1 查看某进程的线程数量
top -H -p {pid}
top之后，界面类似如下，Threads: 309 total，即是线程数
Threads: 309 total, 1 running, 308 sleeping, 0 stopped, 0 zombie
%Cpu(s): 4.6 us, 4.2 sy, 0.0 ni, 90.9 id, 0.1 wa, 0.0 hi, 0.2 si, 0.0 st
KiB Mem : 32778364 total, 700616 free, 27804168 used, 4273580 buff/cache
KiB Swap: 0 total, 0 free, 0 used. 4344696 avail Mem
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
200 root 20 0 14.0g 5.0g 8680 R 4.7 16.1 138:43.37 java
115 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:26.11 java
121 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:26.44 java
124 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:21.59 java
127 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:26.56 java
145 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:26.50 java
154 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:26.50 java
157 root 20 0 14.0g 5.0g 8680 S 0.7 16.1 15:29.32 java
103 root 20 0 14.0g 5.0g 8680 S 0.3 16.1 15:26.32 java
2 java线程数
jstack {pid} | grep "tid" | wc -l
pid为进程id
3 查看gc情况
jstat -gcutil {pid} 5000
5000表示5秒更新一次数据
百分比
[root@144 ~]# jstat -gcutil 20923 5000
S0 S1 E O M CCS YGC YGCT FGC FGCT GCT
0.00 28.05 90.94 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 91.22 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 91.22 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 91.41 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 91.42 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 91.69 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 91.82 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 92.07 48.34 95.98 93.86 73 1.274 5 0.648 1.922
0.00 28.05 92.11 48.34 95.98 93.86 73 1.274 5 0.648 1.922
jstat -gc {pid} 5000
比较详细的数据
[root@144 ~]# jstat -gc 20923 5000
S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT GCT
3072.0 3072.0 1588.9 0.0 156672.0 146168.1 124416.0 61038.2 72384.0 69598.7 7936.0 7464.6 94 1.410 5 0.648 2.058
3072.0 3072.0 1588.9 0.0 156672.0 146414.2 124416.0 61038.2 72384.0 69598.7 7936.0 7464.6 94 1.410 5 0.648 2.058
3072.0 3072.0 1588.9 0.0 156672.0 146753.3 124416.0 61038.2 72384.0 69598.7 7936.0 7464.6 94 1.410 5 0.648 2.058
3072.0 3072.0 1588.9 0.0 156672.0 146843.5 124416.0 61038.2 72384.0 69598.7 7936.0 7464.6 94 1.410 5 0.648 2.058
另外
查看哪几个进程内存占用最高：top -c，输入大写M，以内存使用率从高到低排序
假设占用内存最高的Java进程PID为16818
确认是不是内存本身分配过小：jmap -heap 16818
找到最耗内存的对象：jmap -histo 16818 （带上:live则表示先进行一次FGC再统计，如jmap -histo:live 16818）
导出内存转储快照：jmap -dump:live,format=b,file=heap.bin 16818 （使用Eclipse mat分析）
统计进程打开的句柄数：ls /proc/16818/fd |wc -l
统计进程打开的线程数：ls /proc/16818/task |wc -l
参考资料：
https://www.analysys.cn/article/detail/20019016
https://blog.csdn.net/u012167045/article/details/57426640
https://blog.51cto.com/kusorz/1962548
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