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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.

Table of Contents
1. Distribution Policy
2. Introduction
3. Avoiding Bounce Buffers
    3.1. Memory and Addressing in the Linux 2.4 Kernel
    3.2. The Problem with Bounce Buffers
    3.3. Locating the Patch
    3.4. Modifying Your Device Driver to Avoid Bounce Buffers
4. Raw I/O Variable-Size Optimization Patch
    4.1. Locating the Patch
    4.2. Modifying Your Driver for the Raw I/O Variable-Size Optimization
5. I/O Request Lock Patch
    5.1. Locating the Patch
    5.2. Modifying Your Driver for the I/O Request Lock Patch
6. Additional Resources

1. Distribution Policy

The I/O Performance-HOWTO is copyrighted � 2002, by IBM Corporation

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 []

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 [mailto:] Sharon Snider.

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://]

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 [
Kernel-HOWTO.html] explains
the process of re-compiling the Linux kernel.

The following kernel configuration options are required to enable the bounce
buffer patch:

Development Code - To enable the configurator to display the High I/O Support
option, select the Code maturity level options category and specify "y" to
Prompt for development and/or incomplete code/drivers.

High Memory Support - To enable support for physical memory that is greater
than 1 GB, select the Processor type and features category, and select a
value from the High Memory Support option.

High Memory I/O Support - To enable DMA I/O to physical addresses greater
than 1 GB, select the Processor type and features category, and enter "y" to
the HIGHMEM I/O support option. This configuration option is a new option
introduced by the bounce buffer patch.

3.3.2. Enabled Device Drivers

The bounce buffer patch provides the kernel infrastructure, as well as the
SCSI and IDE mid-level driver modifications to support DMA I/O to high
memory. Updates for several device drivers to make use of the added support
are also included with the patch.

If the bounce buffer patch is applied and you configure the kernel to support
high memory I/O, many IDE configurations and the device drivers listed below
perform DMA I/O without the use of bounce buffers:


3.4. Modifying Your Device Driver to Avoid Bounce Buffers

If your device drivers are not listed above in the Enabled Device Drivers
section, and the device is capable of high-memory DMA I/O, you can modify
your device driver to make use of the bounce buffer patch as follows. More
information on rebuilding a Linux device driver is available at [http://]

 1. A.) For SCSI Adapter Drivers: set the highmem_io bit in the
    Scsi_Host_Template structure.
    B.) For IDE Adapter Drivers: set the highmembit in the ide_hwif_t
 2. Call pci_set_dma_mask(struct pci_dev *pdev, dma_addr_t mask) to specify
    the address bits that the device can successfully use on DMA operations.
    If DMA I/O can be supported with the specified mask, pci_set_dma_mask()
    will set pdev->dma_mask and return 0. For SCSI or IDE, the mask value
    will also be passed by the mid-level drivers to blk_queue_bounce_limit
    (request_queue_t *q, u64 dma_addr) so that bounce buffers are not created
    for memory directly addressable by the device. Drivers other than SCSI or
    IDE must call blk_queue_bounce_limit() directly.
 3. Use pci_map_page(dev, page, offset, size, direction), instead of
    pci_map_single(dev, address, size, direction) to map a memory region so
    that it is accessible by the peripheral device. pci_map_page() supports
    both high and low memory.
    The address parameter for pci_map_single() correlates to the page and
    offset parameters for pci_map_page(). Use the virt_to_page() macro to
    convert an address to a page and offset. The virt_to_page() macro is
    defined by including pci.h. For example:
    void *address;                                                           
    struct page *page;                                                       
    unsigned long offset;                                                    
    page = virt_to_page(address);                                            
    offset = (unsigned long) address & ~PAGE_MASK;                           
    Call pci_unmap_page() after the DMA I/O transfer is complete to remove
    the mapping established by pci_map_page().
    Note pci_map_single() is implemented using virt_to_bus(). virt_to_bus()  
         handles low memory addresses only. Drivers supporting high memory   
         should no longer call virt_to_bus() or bus_to_virt().               
 4. If your driver calls pci_map_sg() to map a scatter-gather DMA operation,
    your driver should set the page and offset fields instead of the address
    field of the scatterlist structure. Refer to step 3 for converting an
    address to a page and offset.
    Note If your driver is already using the PCI DMA API, continue to use    
         pci_map_page() or pci_map_sg() as appropriate. However, do not use  
         the address field of the scatterlist structure.                     

4. Raw I/O Variable-Size Optimization Patch

This section provides information on the raw I/O variable-size optimization
patch for the Linux 2.4 kernel written by Badari Pulavarty. This patch is
also known as the RAW VARY or PAGESIZE_io patch.

The raw I/O variable-size patch changes the block size used for raw I/O from
hardsect_size (normally 512 bytes) to 4 kilobytes (K). The patch improves I/O
throughput and CPU utilization by reducing the number of buffer heads needed
for raw I/O operations.

4.1. Locating the Patch

You can download the patch from one of the following locations:

��*�Andrea Arcangeli has made the patch available at [
    pub/linux/kernel/people/andrea/kernels/v2.4/2.4.18pre7aa2/] http://
    /. The name of the file is 10_rawio-vary-io-1.
��*�Alan Cox has included the patch in the 2.4.18pre9-ac2 kernel patch. The
    patch is available at [
��*�The patch is available from SourceForge at [
    projects/lse/io] The latest
    version is PAGESIZE_io-2.4.17.patch.

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

In previous versions of this patch, changes were enabled for all drivers.
However, the 2.4.17 and later versions of the patch enable the changes only
for the Adaptec, Qlogic ISP1020, and IBM ServerRAID drivers. All other
drivers for version 2.4.17 and later must be modified to make use of the
patch by setting the can_do_varyio bit in the Scsi_Host_Template structure.

Note Drivers that have the raw I/O patch enabled must support buffer heads of
     variable sizes (b_size) in a single I/O request because hardsect_size is
     used until the data buffer is aligned on a 4 K boundary.                
     Additional information is available on rebuilding Linux device drivers  
     at [] 

5. I/O Request Lock Patch

This section provides information on the I/O request lock patch, also known
as the scsi concurrent queuing patch (sior1), written by Johnathan Lahr.

The I/O request lock patch improves SCSI I/O performance on Linux 2.4
multi-processor systems by providing concurrent I/O request queuing. There
are significant I/O performance and CPU utilization improvements possible by
enabling multi-processors to concurrently drive multiple block devices.

Before the patch is applied block I/O requests are queued one at a time
holding the global spin lock, io_request_lock. Once the patch is applied,
SCSI requests are queued while holding the lock specific to the queue
associated with the request. Requests that are made to different devices are
queued concurrently, and requests that are made to the same device are queued

5.1. Locating the Patch

You can download the I/O request patch from Sourceforge at [http://] The
latest version is sior1-v1.2416. Patches that enable concurrent queuing for
specific drivers are also available at SourceForge. The patch for the Emulex
SCSI/FC is lpfc_sior1-v0.249 and the patch for Adaptec SCSI is 

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

The I/O request lock patch installs concurrent queuing capability into the
SCSI midlayer. Concurrent queuing is activated for each SCSI adapter device
driver. To activate the driver, the concurrent_queue field in the
Scsi_Host_Template structure must be set when the driver is registered.

Note Drivers that activate concurrent queuing must ensure that any access of 
     the request_queue by the driver is protected by the                     
     Additional information is available on rebuilding device drivers at     

6. Additional Resources

The following list of Web sites provides additional information on modifying
device drivers and configuring the Linux kernel.

��*�Information on Dynamic DMA mapping is available at [
��*�Kernel-HOWTO is available from the Linux Documentation Project at [http:/
��*�Linux Device Drivers, 2nd Edition published by O'Reilly is available
    online at [] http://

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