TCP SOCKET (KGAS)处理过程

1.缘起

最近处理一个长时间运行的job，记录一下处理过程

2.定位执行Job的session

SQL>SELECT * FROM DBA_JOBS_RUNNING;

  SID      JOB      FAILURES     LAST_DATE          LAST_SEC    THIS_DATE       THIS_SEC    INSTANCE
------- ---------- ---------- ------------------ ------------- ------------- ------------- ---------- 
  1054    127807       0        20-SEP-23          02:00:04      21-SEP-23       02:00:02       0
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3.查看等待事件

可以看到这个job当前等待在TCP Socket (KGAS) ,等待时间已经长达131239秒

SQL> select sid,serial#,sql_id,program,username,event,p1,p2,p3,state,seconds_in_wait from v$session where sid=1054;

       SID    SERIAL# SQL_ID        PROGRAM                        USERNAME             EVENT                        P1         P2         P3 STATE                SECONDS_IN_WAIT
---------- ---------- ------------- ------------------------------ -------------------- -------------------- ---------- ---------- ---------- -------------------- ---------------
      1054      41311 arg0arysf30mj oracle@gsi2database (J007)     SKB2ADMIN            TCP Socket (KGAS)             6          0          0 WAITING                       131239
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4. Oracle官方说明

这是oracle官方对此事件的说明：

TCP Socket (KGAS)
A session is waiting for an external host to provide requested data over a network 
socket. The time that this wait event tracks does not indicate a problem, and even a 
long wait time is not a reason to contact Oracle Support. It naturally takes time for
data to flow between hosts over a network, and for the remote aspect of an application
to process any request made to it. An application that communicates with a remote 
host must wait until the data it will read has arrived.
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5.追踪问题

很明显，问题出在数据库之外。 上述跟踪输出中的图p1=6,表示会话正在等待远程主机发回的数据。 切换到操作系统工具。 看看是哪台远程主机，这时我们需要数据库会话的spid。

SQL> SELECT SPID FROM V$PROCESS WHERE ADDR IN (
SELECT PADDR FROM V$SESSION WHERE SID IN (SELECT SID FROM DBA_JOBS_RUNNING));
SPID
------------
17380
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strace是一个有用的工具，追踪一下这个spid

[root@gsi2database ~]# strace -p 17380
Process 17380 attached - interrupt to quit
...
getrusage(RUSAGE_SELF, {ru_utime={6, 451019}, ru_stime={0, 166974}, ...}) = 0
poll([{fd=24, events=POLLIN|POLLRDNORM}, {fd=25, events=POLLIN|POLLRDNORM}], 2, 30000) = 0 (Timeout)
...
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Strace提供了进程正在等待什么的线索，检查一下文件描述符 25（上面输出中的fd=25)

[oracle@gsi2database udump]$ ll /proc/17380/fd|grep 25
lrwx------ 1 oracle oinstall 64 Sep 22 13:49 25 -> socket:[35135988]
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如同我们之前看到的等待事件TCP Socket (KGAS) ，这是一个套接字。 下一步是确定与套接字建立了什么连接。

[oracle@gsi2database udump]$ /usr/sbin/lsof |head -n 1|/usr/sbin/lsof|grep 35135988
oracle    17380    oracle   25u     IPv4           35135988         0t0      TCP 172.19.194.2:42205->172.19.160.52:smtp (ESTABLISHED)
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6.根因

明显这是一个smtp 服务器问题导致数据库会话在尝试发送通知时挂起。 SMTP服务器修复后恢复正常