共享内存和信号量的解释

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.

 

从oracle8.1.5起，oracle在unix/linux平台提供了一个叫作sysresv的小工具来查看oracle占用的共享内存段和信号量等系统资源的一些关键信息。

[oracle@localhost ~]$ sysresv
sysresv: error while loading shared libraries: libclntsh.so.11.1: cannot open shared object file: No such file or directory
出现该错误是因为没有设置LD_LIBRARY_PATH环境变量，致使没法找到须要的库文件

[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"

根据上述信息，能够经过ipcs查看sid为ning的instance具体的共享内存段和信号量信息：

------ 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命令比较简单，只有几个不一样的option，默认只显示$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.