Configuring and Optimizing Your I/O Scheduler (
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In this conclusion to a seven-part series on Linux I/O file system calls, you'll learn how to select, configure, and optimize your I/O scheduler. This article is excerpted from chapter four of the book Linux System Programming: Talking Directly to the Kernel and C Library, written by Robert Love (O'Reilly, 2007; ISBN: 0596009585). Copyright © 2007 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
Selecting and Configuring Your I/O Scheduler
The default I/O scheduler is selectable at boot time via the iosched kernel command-line parameter. Valid options are as, cfq, deadline, and noop. The I/O scheduler is also runtime-selectable on a per-device basis via /sys/block/device/queue/scheduler, where device is the block device in question. Reading this file returns the current I/O scheduler; writing one of the valid options to this file sets the I/O scheduler. For example, to set the device hda to the CFQ I/O Scheduler, one would do the following:
# echo cfq > /sys/block/hda/queue/scheduler
The directory /sys/block/
device/queue/iosche
d
contains files that allow the adminis
trator to retrieve and set tunable values related to the I/O scheduler. The exact options depend on the current I/O scheduler. Changing any of these settings requires root privileges.
A good programmer writes programs that are agnostic to the underlying I/O subsystem. Nonetheless, knowledge of this subsystem can surely help one write optimal code.
Optimizing I/O Performance
Because disk I/O is so slow relative to the performance of other components in the system, yet I/O is such an important aspect of modern computing, maximizing I/O performance is crucial.
Minimizing I/O operations (by coalescing many smaller operations into fewer larger operations), performing block-size-aligned I/O, or using user buffering (see Chapter 3), and taking advantage of advanced I/O techniques, such as vectored I/O, positional I/O (see Chapter 2), and asynchronous I/O, are important steps to always consider when system programming.
The most demanding mission-critical and I/O-intense applications, however, can employ additional tricks to maximize performance. Although the Linux kernel, as discussed previously, utilizes advanced I/O schedulers to minimize dreaded disk seeks, user-space applications can work toward the same end, in a similar fashion, to further improve performance.
