AIX Version 4.3 Differences Guide

applications that call load or loadbind many times since breakpoints and other
changes will not have to be reinserted.
4.6 Direct I/O
Direct I/O is a way of opening JFS files that allows for disk reads and writes using
less CPU cycles than normal I/O. The main CPU savings come from avoiding the
memory-to-memory copy done by the JFS. As the difference between memory
and CPU cycle times increases, the savings achieved by avoiding this copy
becomes greater.
Direct I/O offers a big performance gain for applications that do large block I/O
(32 KB or greater) to JFS files and a smaller increase in performance for small
block I/O. It does not improve raw I/O performance.
With normal I/O, the I/O request is first sent to the device driver. To service the
request the device driver uses Direct Memory Access (DMA) to copy the data to
or from pages in a file persistent segment. The data is then copied between the
persistent segment and userspace through calls to vmcopyin() or vmcopyout().
Thus, a files persistent segment acts as a file cache.
With direct I/O, the data is not cached, but rather, I/O is done directly to a user
supplied buffer through cross-memory technology. In other words, DMA is done
directly from the disk to user space and conversely through the device strategy
routine of the JFS file system.
Optimization was also made to the DMA setup routines. This improves large
block I/O to JFS files and to raw logical volumes. However, the benefits of the
DMA changes are much less than the benefits of direct I/O.
Take Note
It is important to note that since direct I/O reads are done synchronously and
there is no read-ahead benefit, if they are not used correctly, they can also take
much longer. The only read-ahead-like semantics that direct I/O can benefit
from will be read-ahead performed by the disk device driver (normally 32 KB).
For this reason, it is very important for a direct I/O reader to specify large read
requests to match the performance of normal cached I/O readers. To match the
performance of normal cached I/O readers, a direct I/O reader should issue
read requests of at least 128 KB.
Direct I/O is considered advisory. This means that if a file is opened for direct I/O,
and another application decides to open that same file for normal I/O, the file will
be opened using normal cached I/O until direct I/O can be resumed (the normal
I/O application closes the file).
Files can also be opened for a deferred update using the O_DEFER flag. If a file
is opened with O_DEFER, and a subsequent open for direct I/O is issued, all
writes will use normal cached I/O. Similarly, if another application opens the file
with O_DEFER while it is already opened for direct I/O, all I/O to the file will be
cached.
Application Development and Pthreads 77