共享內存和信號量的解釋

Shared memory allows an application to allocate a chunk of memory which can be viewed by other processes. Oracle uses shared memory to create the System Global Area (SGA) which all Oracle processes must be able to access.

Semaphores act as flags for shared memory. Semaphores are either set on or off. When an Oracle process accesses the SGA (in shared memory) it will check for a semaphore for that portion of memory. If it finds a semaphore set on for that portion of memory that process will sleep and check again later. If there is no semaphore set on for that portion of memory it will set one on and proceed with its operation. When it is done it will switch that semaphore back to off.

 

從oracle8.1.5起，oracle在unix/linux平臺提供了一個叫做sysresv的小工具來查看oracle佔用的共享內存段和信號量等系統資源的一些關鍵信息。

[oracle@localhost ~]$ sysresv
sysresv: error while loading shared libraries: libclntsh.so.11.1: cannot open shared object file: No such file or directory
出現該錯誤是由於沒有設置LD_LIBRARY_PATH環境變量，導致無法找到需要的庫文件

[oracle@localhost ~]$ LD_LIBRARY_PATH=$ORACLE_HOME/lib
[oracle@localhost ~]$ export LD_LIBRARY_PATH
[oracle@localhost ~]$ sysresv

IPC Resources for ORACLE_SID "ning" :
Shared Memory:
ID              KEY
458763          0x78b6869c
Semaphores:
ID              KEY
98304           0x5ed8a0f0
Oracle Instance alive for sid "ning"

根據上述信息，可以通過ipcs查看sid爲ning的instance具體的共享內存段和信號量信息：

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status
0x78b6869c 458763     oracle    660        243269632  23

------ Semaphore Arrays --------
key        semid      owner      perms      nsems
0x5ed8a0f0 98304      oracle    660        154

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages
sysresv命令比較簡單，只有幾個不同的option，默認只顯示$ORACLE_SID指定的instance的信息：

usage   : sysresv [-if] [-d <on/off>] [-l sid1 <sid2> ...]
          -i : Prompt before removing ipc resources for each sid
          -f : Remove ipc resources silently, oevrrides -i option
          -d <on/off> : List ipc resources for each sid if on
          -l sid1 <sid2> .. : apply sysresv to each sid
Default : sysresv -d on -l $ORACLE_SID
Note    : ipc resources will be attempted to be deleted for a
          sid only if there is no currently running instance
          with that sid.