最近，公司線上tomcat經常無緣無辜宕機，總結了一下定位問題的方法，僅供參考：
報錯資訊：

Maximum number of threads (200) created for connector with address null and port 9443
# There is insufficient memory for the Java Runtime Environment to continue.
# Cannot create GC thread. Out of system resources.

一、檢視當前使用者執行緒和檔案控制代碼數是否超出限制

（1）顯示當前使用者程序限制：ulimit -a 
顯示結果：

core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 0
file size               (blocks, -f) unlimited
pending signals                 (-i) 256612
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 102400
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 1024
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited
 

（2）修改所有 linux 使用者的環境變數檔案：

vi /etc/profile
ulimit -u 10000
ulimit -n 4096

儲存後執行#source /etc/profile 使其生效

二、檢視當前埠號程序資訊和GC使用情況

（1）顯示埠的PID：lsof -i:埠
示例：lsof -i:7074

COMMAND  PID   USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
java    3195  ligang  34u  IPv4   37416693  0t0    TCP *:7074 (LISTEN)

（2）gc資訊統計：jstat -gcutil PID
示例：jstat -gcutil 3195

 S0     S1     E      O      P     YGC     YGCT    FGC    FGCT     GCT   
12.63   0.00  52.03  78.63  99.13   4148   24.274   200   40.246   64.520
 

（3）輸出執行緒數：ps -mp PID -o THREAD,tid,time | wc -l
示例：ps -mp 3195 -o THREAD,tid,time | wc -l
43

三、檢視程序記憶體使用情況及定位到對應程式

（1）記憶體使用情況：top -p PID
示例：top 3195

top - 15:29:27 up 25 days, 20:05,  2 users,  load average: 0.01, 0.05, 0.01
Tasks:   1 total,   0 running,   1 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.0%us,  0.1%sy,  0.0%ni, 99.8%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   8058868k total,  6821684k used,  1237184k free,   181936k buffers
Swap:  2097144k total,   492300k used,  1604844k free,  1897320k cached

PID  USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                           
3195 ligang    20   0 4862m 196m  10m S  0.0  2.5   7:57.48 java

（2）找到該程序後，如何定位具體執行緒或程式碼呢，首先顯示執行緒列表,並按照CPU佔用高的執行緒排序：
ps -mp PID -o THREAD,tid,time | sort -rn | head -10
示例：ps -mp PID -o THREAD,tid,time | sort -rn | head -10

USER     %CPU PRI SCNT WCHAN  USER SYSTEM   TID     TIME
ligang    0.6   -    - -         -      -     - 00:07:58
ligang    0.2  19    - futex_    -      -  3270 00:02:49
ligang    0.0  19    - inet_c    -      -  3277 00:00:00
ligang    0.0  19    - inet_c    -      -  3273 00:00:00
ligang    0.0  19    - inet_c    -      -  3271 00:00:00
ligang    0.0  19    - inet_c    -      -  3203 00:00:05
ligang    0.0  19    - futex_    -      -  7644 00:00:00
ligang    0.0  19    - futex_    -      -  3420 00:00:00
ligang    0.0  19    - futex_    -      -  3288 00:00:06

（3）將需要的執行緒ID轉換為16進位制格式：printf "%x\n" TID
示例：printf "%x\n" 3270
cc6
（4）最後列印執行緒的堆疊資訊：jstack PID |grep cc6 -A 30
示例：
jstack 2633 |grep e18 -A 30

結果就可以看到哪段程式碼導致的問題...