IO-Perf-HOWTO(1).pdf

I/O Performance HOWTO

Sharon Snider

v1.1, 05/2002

Revision History

Revision v1.1

2002−05−01

Revised by: sds

Updated technical information and links.

Revision v1.0

2002−04−01

Revised by: sds

Wrote and converted to DocBook XML.

This HOWTO covers information on available patches for the 2.4 kernel that can improve the I/O

performance of your Linux" operating system.

I/O Performance HOWTO

Table of Contents

1. Distribution Policy. .........................................................................................................................................1

2. Introduction. ....................................................................................................................................................2

3. Avoiding Bounce Buffers. ...............................................................................................................................3

3.1. Memory and Addressing in the Linux 2.4 Kernel. ...........................................................................3

3.2. The Problem with Bounce Buffers. ..................................................................................................3

3.3. Locating the Patch.. ...........................................................................................................................3

3.3.1. Configuring the Linux Kernel to Avoid Bounce Buffers. ................................................3

3.3.2. Enabled Device Drivers. ...................................................................................................4

3.4. Modifying Your Device Driver to Avoid Bounce Buffers. ..............................................................4

4. Raw I/O Variable−Size Optimization Patch.. ...............................................................................................6

4.1. Locating the Patch.. ...........................................................................................................................6

4.2. Modifying Your Driver for the Raw I/O Variable−Size Optimization Patch. ..................................6

5. I/O Request Lock Patch. .................................................................................................................................7

5.1. Locating the Patch.. ...........................................................................................................................7

5.2. Modifying Your Driver for the I/O Request Lock Patch. .................................................................7

6. Additional Resources. .....................................................................................................................................8

1. Distribution Policy

Permission is granted to copy, distribute, and/or modify this document under the terms of the GNU Free

Documentation License, Version 1.1 or any later version published by the Free Software Foundation with no

Invariant Sections, no Front−Cover text, and no Back−Cover text. A copy of the license can be found at

http://www.gnu.org/licenses/fdl.txt .

1. Distribution Policy

2. Introduction

This HOWTO provides information on improving the input/output (I/O) performance of the Linux operating

system for the 2.4 kernel. Additional patches will be added as they become available.

Please send any comments, or contributions via e−mail to Sharon Snider .

2. Introduction

3. Avoiding Bounce Buffers

This section provides information on applying and using the bounce buffer patch on the Linux 2.4 kernel. The

bounce buffer patch, written by Jens Axboe, enables device drivers that support direct memory access (DMA)

I/O to high−address physical memory to avoid bounce buffers.

This document provides a brief overview on memory and addressing in the Linux kernel, followed by

information on why and how to make use of the bounce buffer patch.

3.1. Memory and Addressing in the Linux 2.4 Kernel

The Linux 2.4 kernel includes configuration options for specifying the amount of physical memory in the

target computer. By default, the configuration is limited to the amount of memory that can be directly

mapped into the kernel's virtual address space starting at PAGE_OFFSET. On i386 systems the default

mapping scheme limits kernel−mode addressability to the first gigabyte (GB) of physical memory, also

known as low memory. Conversely, high memory is normally the memory above 1 GB. High memory is not

directly accessible or permanently mapped by the kernel. Support for high memory is an option that is

enabled during configuration of the Linux kernel .

3.2. The Problem with Bounce Buffers

When DMA I/O is performed to or from high memory, an area is allocated in low memory known as a

bounce buffer. When data travels between a device and high memory, it is first copied through the bounce

buffer.

Systems with a large amount of high memory and intense I/O activity can create a large number of bounce

buffers that can cause memory shortage problems. In addition, the excessive number of bounce buffer data

copies can lead to performance degradation.

Peripheral component interface (PCI) devices normally address up to 4 GB of physical memory. When a

bounce buffer is used for high memory that is below 4 GB, time and memory are wasted because the

peripheral has the ability to address that memory directly. Using the bounce buffer patch can decrease, and

possibly eliminate, the use of bounce buffers.

3.3. Locating the Patch

The latest version of the bounce buffer patch is block−highmem−all−18b.bz2 , and it is available from

Andrea Arcangeli's −aa series kernels at http://kernel.org/pub/linux/kernel/people/andrea/kernels/v2.4/ .

3.3.1. Configuring the Linux Kernel to Avoid Bounce Buffers

This section includes information on configuring the Linux kernel to avoid bounce buffers. The Linux

Kernel−HOWTO at http://www.linuxdoc.org/HOWTO/Kernel−HOWTO.html explains the process of

3. Avoiding Bounce Buffers

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