IBM Power7 Optimization And Tuning Manual page 87

YIELDLOOPTIME=n
The YIELDLOOPTIME variable controls the number of times that the system yields the logical
processor when it tries to acquire a busy mutex or spin lock before it goes to sleep on the
lock. The logical processor is yielded to another kernel thread, assuming that there is
another executable thread with sufficient priority. This variable is effective in complex
applications, where multiple locks are in use. The number of times to yield the logical
processor before blocking on a busy lock is n. The default is 0 and n must be a
positive value.
For more information about this topic, see 4.4, "Related publications" on page 94.
pollset
AIX 5L V5.3 introduced the pollset APIs. Pollsets are an AIX replacement for UNIX select()
and poll(). Pollset, select(), and poll() all allow an application to efficiently query the
status of file descriptors. This action is typically done to allow a single application to multiplex
I/O across many file descriptors. Pollset APIs can be more efficient when the number of file
descriptors that are queried becomes large.
Efficient I/O event polling through the pollset interface on AIX contains a pollset summary and
outlines the most advantageous use of Java. To see this topic, go to:
http://www.ibm.com/developerworks/aix/library/au-pollset/index.html
For more information about this topic, see 4.4, "Related publications" on page 94.
File system performance benefits
AIX Enhanced Journaled File System (JFS2) is the default file system for 64-bit kernel
environments. Applications can capitalize on the features of JFS2 for better performance.
Direct I/O
The AIX read-ahead and write-behind JFS2 feature might not be suitable for applications that
perform large sized I/O operations, as the cache hit ratio is low. In those cases, an application
developer must evaluate Direct I/O for I/O intensive applications.
Programs that are good candidates for direct I/O are typically CPU-limited and perform much
disk I/O. Technical applications that have large sequential I/Os are good candidates.
Applications that benefit from striping are also good candidates.
The direct I/O access method bypasses the file cache and transfers data directly from disk
into the user space buffer, as opposed to using the normal cache policy of placing pages in
kernel memory.
At the user level, file systems can be mounted using the dio option on the mount command.
At the programming level, applications enable direct I/O access to a file by passing the
O_DIRECT flag to the open subroutine. This flag is defined in the fcntl.h file. Applications must
be compiled with _ALL_SOURCE enabled to see the definition of O_DIRECT.
For more information, see Working with File I/O, available at:
http://pic.dhe.ibm.com/infocenter/aix/v7r1/index.jsp?topic=/com.ibm.aix.genprogc/d
oc/genprogc/fileio.htm
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Chapter 4. AIX