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12.5. Verifying Asynchronous I/O Usage
To verify whether
$ORACLE_HOME/bin/oracle was linked with asynchronous I/O, you can use the Linux commands ldd and nm.
In the following example,
$ORACLE_HOME/bin/oracle was relinked with asynchronous I/O:
ldd $ORACLE_HOME/bin/oracle | grep libaio nm $ORACLE_HOME/bin/oracle | grep io_getevent
$ ldd $ORACLE_HOME/bin/oracle | grep libaio
libaio.so.1 => /usr/lib/libaio.so.1 (0x0093d000)
$ nm $ORACLE_HOME/bin/oracle | grep io_getevent
w io_getevents@@LIBAIO_0.1
$
In the following example,
$ORACLE_HOME/bin/oracle has NOT been relinked with asynchronous I/O:
ldd $ORACLE_HOME/bin/oracle | grep libaio nm $ORACLE_HOME/bin/oracle | grep io_getevent
$ ldd $ORACLE_HOME/bin/oracle | grep libaio
$ nm $ORACLE_HOME/bin/oracle | grep io_getevent
w io_getevents
$
If
$ORACLE_HOME/bin/oracle is relinked with asynchronous I/O it does not necessarily mean that Oracle is really using it. You also have to ensure that Oracle is configured to use asynchronous I/O calls, see Enabling Asynchronous I/O Support.
To verify whether Oracle is making asynchronous I/O calls, you can take a look at the
/proc/slabinfo file assuming there are no other applications performing asynchronous I/O calls on the system. This file shows kernel slab cache information in real time.
On a Red Hat Enterprise Linux 3 system where Oracle does not make asynchronous I/O calls, the output looks like this:
egrep "kioctx|kiocb" /proc/slabinfo
$ egrep "kioctx|kiocb" /proc/slabinfo
kioctx 0 0 128 0 0 1 : 1008 252
kiocb 0 0 128 0 0 1 : 1008 252
$
Once Oracle makes asynchronous I/O calls, the output on a Red Hat Enterprise Linux 3 system will look like this:
egrep "kioctx|kiocb" /proc/slabinfo
$ egrep "kioctx|kiocb" /proc/slabinfo
kioctx 690 690 128 23 23 1 : 1008 252
kiocb 58446 65160 128 1971 2172 1 : 1008 252
$
The numbers in red (number of active objects) show whether Oracle makes asynchronous I/O calls. The output will look a little bit different in Red Hat Enterprise Linux 4 and 5. However, the numbers in red will show same behavior in Red Hat Enterprise Linux 3 and Red Hat Enterprise Linux 4 and 5. The first column displays the cache names kioctx and kiocb. The second column shows the number of active objects currently in use. And the third column shows how many objects are available in total, used and unused.
To see kernel slab cache information in real time, you can also use the slabtop command:
Slab caches are a special memory pool in the kernel for adding and removing objects, such as data structures or data buffers, of the same size. Its a cache for commonly used objects where the kernel does not have to re-allocate and initialize the object each time it is being reused, and free the object each time it is being destroyed. The slab allocator scheme basically prevents memory fragmentation and it prevents the kernel from spending too much time allocating, initializing, and freeing the same objects.
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