如何定位消耗CPU最多的執行緒
阿新 • • 發佈：2018-11-28
之前有朋友反饋說發的內容希望有個梯度，逐步加深，前面發了幾篇關於jvm原始碼分析的文章，可能我覺得我已經把內容寫得淺顯易懂了，但是對於某些沒怎麼接觸的同學來說還是比較難理解，這個我以後慢慢改進吧，今天發篇輕鬆點的文章，可能大家在工作過程中也會可能碰到類似的問題，或許有經驗的同學看到這個題目就知道我要說什麼了，也有自己的定位方法。
話不多說了，先來看程式碼吧
public class Test{ public static void main(String args[]){ for(int i=0;i<10;i++){ new Thread(){ public void run(){ try{ Thread.sleep(100000); }catch(Exception e){} } }.start(); } Thread t=new Thread(){ public void run(){ int i=0; while(true){ i=(i++)/100; } } }; t.setName("Busiest Thread"); t.start(); } }
這個例子裡新建立了11個執行緒，其中10個執行緒沒幹什麼事，主要是sleep，另外有一個執行緒在迴圈裡一直跑著，可以想象這個執行緒是這個程序裡最耗cpu的執行緒了，那怎麼把這個執行緒給抓出來呢？
首先我們可以通過top -Hp <pid>來看這個程序裡所有執行緒的cpu消耗情況，得到類似下面的資料
$ top -Hp 18207 top - 19:11:43 up 573 days, 2:43, 2 users, load average: 3.03, 3.03, 3.02 Tasks: 44 total, 1 running, 43 sleeping, 0 stopped, 0 zombie Cpu(s): 18.8%us, 0.0%sy, 0.0%ni, 81.1%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 99191752k total, 98683576k used, 508176k free, 128248k buffers Swap: 1999864k total, 191064k used, 1808800k free, 17413760k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 18250 admin 20 0 26.1g 28m 10m R 99.9 0.0 0:19.50 java Test 18207 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18208 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.09 java Test 18209 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18210 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18211 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18212 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18213 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18214 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18215 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18216 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18217 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18218 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18219 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18220 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18221 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18222 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18223 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18224 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18225 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18226 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test 18227 admin 20 0 26.1g 28m 10m S 0.0 0.0 0:00.00 java Test
拿到這個結果之後，我們可以看到cpu最高的執行緒是pid為18250的執行緒，佔了99.9%：
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
18250 admin 20 0 26.1g 28m 10m R 99.9 0.0 0:19.50 java Test接著我們可以通過jstack <pid>的輸出來看各個執行緒棧:
$ jstack 18207
2016-03-30 19:12:23
Full thread dump OpenJDK 64-Bit Server VM (25.66-b60 mixed mode):
"Attach Listener" #30 daemon prio=9 os_prio=0 tid=0x00007fb90be13000 nid=0x47d7 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"DestroyJavaVM" #29 prio=5 os_prio=0 tid=0x00007fb96245b800 nid=0x4720 waiting on condition [0x0000000000000000]
java.lang.Thread.State: RUNNABLE
"Busiest Thread" #28 prio=5 os_prio=0 tid=0x00007fb91498d000 nid=0x474a runnable [0x00007fb9065fe000]
java.lang.Thread.State: RUNNABLE
at Test$2.run(Test.java:18)
"Thread-9" #27 prio=5 os_prio=0 tid=0x00007fb91498c800 nid=0x4749 waiting on condition [0x00007fb906bfe000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test$1.run(Test.java:9)
"Thread-8" #26 prio=5 os_prio=0 tid=0x00007fb91498b800 nid=0x4748 waiting on condition [0x00007fb906ffe000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test$1.run(Test.java:9)
"Thread-7" #25 prio=5 os_prio=0 tid=0x00007fb91498b000 nid=0x4747 waiting on condition [0x00007fb9073fe000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test$1.run(Test.java:9)
"Thread-6" #24 prio=5 os_prio=0 tid=0x00007fb91498a000 nid=0x4746 waiting on condition [0x00007fb9077fe000]
java.lang.Thread.State: TIMED_WAITING (sleeping)
at java.lang.Thread.sleep(Native Method)
at Test$1.run(Test.java:9)
...上面的執行緒棧我們注意到nid的值其實就是執行緒ID，它是十六進位制的，我們將消耗cpu最高的執行緒18250，轉成十六進位制0X474A，然後從上面的執行緒棧裡找到nid=0X474A的執行緒，其棧為：
"Busiest Thread" #28 prio=5 os_prio=0 tid=0x00007fb91498d000 nid=0x474a runnable [0x00007fb9065fe000]
java.lang.Thread.State: RUNNABLE
at Test$2.run(Test.java:18)即將最耗cpu的執行緒找出來了，是Busiest Thread