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UPDATED: Wonderful World of Linux 2.4 – Final Candidate #3 – 4/10/00 (aka the “Sleeping in the Flowers” Update)

By Joe
Pranevich

In the beginning, there was Linus and his 386. For
reasons far too complicated to be discussed here, he decided not to
use the commonly available operating system of the time and instead
decided to write his own. Several years and many thousands of lines
of code later, Linux 2.2 was released. Linux 2.2 was a milestone in
and of itself and I wrote an article about it which I am quite
happy with. Unfortunately for me however (and fortunately for the
rest of the world), Linus (and company) continued to hack away at
the Linux OS and the 2.4 release of the Linux kernel is nearing
completion. Submitted for your approval, this document describes
some of the new features in Linux 2.4. (This document is based on
Linux 2.3.99-pre3, the most recent development kernel to date. When
the development series reaches maturity, it will become Linux
2.4.0.) This is a “final” candidate, if there is anything of
importance that I’m missing (in your opinion), let me know and I’ll
add it in if I can.

Unlike the pre-release announcements of some other operating
systems, the features described in this document already exist in
the Linux 2.3 (developers) kernel (although all features may not be
entirely stable in the development tree and some of them may be
flagged as “experimental” for the real 2.4 release.). As Linux is
true to the Open Source philosophy, features are added by largely
independent developers as they discover a particular need or a bug
in the existing code. These submissions are checked out by Linus
and by his unofficial lieutenants for quality; this filtering
process is what keeps Linux cohesive even with its potentially huge
developer base.

Also in the spirit of the Open Source movement, products are
released “when they are ready” and generally do not have announced
release dates. Currently, Linux 2.3 is in a final pre-release cycle
which shouldn’t change much beyond bug fixes. We may be able to
expect a real Linux 2.4 in a couple months and maybe several months
after that before they will be prepackaged by distributions for
general use. (It remains to be seen how many of the new features
distributions will choose to support from the beginning. Support
for DevFS and LVM, in particular, will probably be a big issue for
future distributions.)

This draft of the WWOL2.4 is subtitled the “Sleeping in the
Flowers Update” edition as it’s just a small update over the
previous draft. This draft is a FINAL CANDIDATE. That means that
you won’t see many changes between now and the actual release.
Feedback is especially important at this point.

In this document, I have attempted to bring attention to areas
where Linux 2.4 is not compatible with Linux 2.2. Please remember
however that binary modules will most definitely not work as many
internal structures have changed. The formats of some of the files
in the /proc filesystem have changed. The names of nearly all files
in the /dev directory have been changed, but compatibility names
are provided. Most applications should not even notice the changes.
(Low level applications, such as the ppp daemon, or other programs
which rely on an intimate connection with the kernel will most
likely not be 100% compatible between major kernel revisions.) If
you are the type to update your distribution manually, please be
sure to read the CHANGES file and update any necessary packages
before submitting bug reports.

[email protected]
(Home)
[email protected]
(Work)

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The Many Flavors of Linux

In terms of sheer lines of code, the Linux kernel is
predominantly drivers. In fact, the size of the Linux core has not
increased much over the last several revisions. Some of these
drivers are architecture independent such as the IDE driver. That
is, these drivers have been written to work on multiple platforms.
Other drivers are dependent on a particular architecture. For
example, the ADB (Apple Desktop Bus) mouse driver isn’t really
applicable on the i386 port and so isn’t supported. Linux kernel
developers strive to make drivers as general as possible, so as to
allow a driver to be reused with relatively little effort on a
different platform if a device becomes available. The Linux OS is a
multi-headed beast with facets similar to but different from each
other, depending on the platform you are using.

Having said this, this document will mostly stick to Intel
hardware as that is the hardware that I use most often at home.
While I won’t go into the specifics of each individual port because
I lack the time and the knowledge, it should be mentioned that
Linux 2.4 adds support for three new architectures: ia64 (Itanium),
S/390, and SuperH. I have no experience with these platforms so am
unsure as to their level of hardware support, etc. (Or even what
classes of hardware they represent.) It is likely that they will
mature over the years just as the other Linux ports have. Exact
feature support is different from port to port and certain
limitations on hardware, memory, etc. will differ depending on the
underlying architecture.

In terms of Intel-like hardware, Linux 2.4 is very similar in
support to Linux 2.2. All Intel chips since the 386 are still
supported, up to the Pentium III. Compatible chips such as those
produced by AMD and Cyrix are also supported. Additionally, Linux
2.4 will include support for other hardware often present on newer
chips including non-Intel varieties of the MTRRs (Memory Type Range
Registers) (aka MCRs) which will improve performance on some kinds
of high bandwidth devices. While Linux 2.2 included support for the
IO-APIC (Advanced Programmable Interrupt Controller) on
multi-processor systems, Linux 2.4 will support these on
uni-processor systems and also support machines with multiple
IO-APICs. The support for multiple IO-APICs will allow Linux 2.4 to
scale much better than previous incarnations of Linux on high-end
hardware.

Linux 2.4 and ia64

While not yet delivered to the starving masses, Intel’s 64-bit
replacement to the x86 line is coming down the pipeline. While no
real hardware is available, patches that include support for this
chipset and its successors have been included in the mainstream
kernel release. This porting process was no doubt simplified by
Linux’s existing support for 64-bit processors (including Compaq’s
Alpha chips and the Sparc64) which were already merged into the
main Linux tree.

Linux 2.4 and Crusoe

As Linus works at Transmeta, the Crusoe chips will definitely
support Linux when the first Crusoe-based products come to market.
In fact, Linux on the Crusoe has already been conclusively
demonstrated using the Quake test. (Arguably, the most important
test for any new processor.) As Crusoe emulates i386, there won’t
be an explicit port for the processor family but features
(especially power management and maybe a new “virtual” keyboard
layer) are expected to be in the release kernel by the time Linux
2.4 ships.

Linux 2.4 and Pre-386 Intel Chips

Surprisingly, I do get a number of questions about pre-386
Linux. The answer, at least right now, is that there is no such
animal. A sister project, ELKS (Embedable Linux Kernel Subset) is
working to make a Linux-like operating system run on these
machines, including protected mode support for chips that support
it. This project is separate from Linux-proper however and is
outside the scope of this document.

A separate port of the Linux kernel, called uLinux, also exists
and is working to provide Linux on embedded and older processors,
including processors without MMUs. The work presently is based
around the Linux 2.0 kernel and has largely not been integrated
into the master tree.

Linux Internals

Linux 2.2 was a major improvement over Linux 2.0 and the Linux
1.x series. It supported many new filesystems, a new system of file
caching, and it was much more scalable. (If you want a list of
features new to Linux 2.2, you can read my article about it.) Linux
2.4 builds on these things and more to be the best darned Linux
kernel yet in a variety of situations.

The Linux kernel is an assortment of modular components and
subsystems including device drivers, protocols, and other component
types. These are glued to the core of the Linux kernel by APIs,
programming interfaces, that provide a standard method by which the
Linux kernel can be expanded. Most of this document will focus on
these components of the Linux OS as these are the components that
seem to do the most work. These are the components that drive your
disks, read your files, and do all of the obvious and physical
things. Linux 2.4 is however much more than just these components.
These assorted drivers and APIs all revolve around a common center
of the Linux kernel. This center includes such fundamental features
as the scheduler, the memory manager, the virtual filesystem, and
the resource allocator.

Linux 2.4 is the first release of the Linux kernel which will
include a full-featured resource management subsystem. Previous
incarnations of Linux included some vestiges of support, but it was
considered kludgy and did not provide the functionality needed for
the “Plug and Play” world. Unlike many of the other internal
changes, many users will be able to directly experience this change
as it impacts the way resources are allocated and reported in the
kernel. As part of this change, the PCI card database deprecated in
Linux 2.2 has been un-deprecated so that all resources can have an
associated device name, rather than just an associated driver.

The new release of the Linux kernel also fixes some problems
with the way the VFS (virtual filesystem) layer and the file caches
were handled. In older versions of Linux, file caching was
dependent on a dual-buffer system which simplified some issues, but
caused many headaches for kernel developers who had to make sure
that it was not possible for these buffers to be out of synch.
Additionally, the presence of the redundant buffer increased memory
use and slowed down the system as the kernel would have to do extra
work to keep things in synch. Linux 2.4 solves these problems by
moving to a simpler single-buffer system.

A number of changes in Linux 2.4 can be described as “enterprise
level.” That is, they may not be immediately useful to many desktop
users by work to strengthen Linux as a while. For the most part,
the addition of these features does not degrade Linux in more
“normal” environments. First, Linux 2.4 can handle many more
simultaneous processes by being more scalable on multiprocessor
systems and also by providing a configurable process limit. Second,
the scheduler has been revised somewhat to be more efficient on
systems with a larger number of concurrent processes. Third, the
revised Linux kernel can now handle an amazing number of users and
groups– about 4.2 billion. (And that’s a lot of users!) In
addition, support for more powerful hardware is provided in the new
kernel which now supports 4 gigabytes of RAM on Intel hardware, up
to 16 ethernet cards, 10 IDE controlletrs, multiple IO-APICs, and
other pointless abuses of good hardware. The 2 gigabyte file size
restriction has also been lifted. With these changes and others,
the Linux kernel development team is proving that Linux can be an
option in many new environments.

The way Linux handles shared memory has also been changed in
Linux 2.4 to be more standards compliant. One side effect of this
set of changes is that Linux 2.4 will require a special “shared
memory” filesystem to be mounted in order for shared memory
segments to work. This should be handled by your distribution when
they become ready for Linux 2.4.

Linux 2.4 also includes a much larger assortment of device
drivers and supported hardware than any other Linux revision and
any particular device you care to name has a decent shot at working
under Linux 2.4. (Of course, you should consult the documentation
before you go out and buy any new hardware, just in case. New
hardware especially may not be supported yet.)

One frequently asked question about Linux 2.4 is how much memory
it will require. Many operating systems seem to require more and
more memory and resources as they mature, but Linux 2.4 will
largely buck that trend by actually requiring less memory in
certain situations. Of course, Linux 2.4 includes much more
functionality than does Linux 2.2 and many of these features do
take up space so your mileage may vary. (Remember that most kernel
components can be disabled at compile-time, unlike many other
operating systems.)

Hardware Support – /dev/*

Before we can talk about Linux 2.4’s hardware support, I have to
bring attention to one of Linux 2.4’s latest and most controversial
features: DevFS: the device filesystem. DevFS is a (currently
optional) feature that fundamentally rewrites the way Linux users
handle interactions with devices. This will be primarily presented
to the user in two very obvious ways. First, nearly all device
names have been changed. For example, “/dev/hda” may have been your
harddisk, but it would now be located at “/dev/ide0/…” (I’m not
sure exactly what the new naming convention will be.) This modified
scheme increases the available namespace for devices and allows for
USB and other “modern” device systems to be more easily integrated
into the UNIX/Linux device model. Secondly, device names will now
be added to the /dev/ directory as drivers are loaded into the
kernel rather than having all possible device names pre-existing in
the directoy. Old names will still be available for compatibility
using a userspace program “devfsd.”

While this may impact distribution maintainers who want to make
sure that all applications are modified to use the new names, end
users should not be overly affected by these changes. This change
will, if nothing else, come as a surprise to many users. While some
users may be turned off by the more verbose naming of devices, it
is easy to see how limiting the older names could be. (What, for
instance, would happen if you had more than 26 harddisks?)

Buses – ISA, PCI, USB, MCA, etc.

Processors are just one small part of the nifty world that
exists inside your computer. Equally important is the computer’s
bus architecture, the component(s) of the system that is often
responsible for internal and external devices. Some bus
architectures, such as the original ISA, are more irresponsible
towards their hardware than anything else– they don’t provide any
resource management functionality, just a place to put in cards.
Others, such as PCI, support much more advanced levels of
configuration and allow for devices to be relocated and other
things. As all primary internal Intel-hardware busses were
supported by Linux 2.2 ((E)ISA, VLB, PCI, MCA), there are no really
amazing announcements to be made in this area. Linux 2.4 does
however improve on each of these busses by stringing them into
Linux’s new resource subsystem.

There are two major improvement that was made in this area,
however. Linux 2.4 includes, for the first time in the kernel,
support for ISA Plug-and-Play devices. Previously, Linux could
support these devices through a user-mode utility and some elbow
grease on the part of the user or distribution. Linux 2.4 however
will allow these devices to be used during the boot process (for
example, booting from an ISAPnP IDE controller) and the
configuration of these devices happen auto-magically. Linux 2.4
also includes I2O support in the kernel. I2O (Intelligent
Input/Output) is a superset of PCI that attempts to allow OS
independent drivers to be written for many devices. Between these
two changes, many users will find that Linux 2.4 supports many more
pieces of PC-class hardware.

However, Linux 2.4 does increase support for external devices.
PCMCIA (aka PC Card) support has been added into Linux 2.4. This
support was available in many distributions from an outside source
and most distributions included it by default. Linux 2.4 supports
better integration with the PCMCIA driver set and should ease the
installation and configuration process for these devices for many
users. Like previous versions of this driver however, Linux 2.4
will still require an external daemon and components to get the
most out of PCMCIA devices.

Perhaps the most exciting news on this front is the Universal
Serial Bus (USB), an external bus that is coming into prominence
for devices such as keyboards, mice, sound systems, scanners, and
printers. USB is a popular option on many new pieces of hardware,
including non-Intel hardware. Linux’s support for these devices is
still in early stages but a large percentage of common USB hardware
(including keyboards, mice, speakers, etc.) is already supported in
the kernel.

More recently, Firewire support has been added into the Linux
kernel. Firewire is a popular option for many high-bandwidth
devices. Not many drivers (or devices) exist for this hardware
architecture yet, but this support is likely to improve over time,
as the architecture matures.

Block Devices – LVM, Disk Drives, etc.

In its simplest form, a block device is a device which can be
expressed as an array of bytes that can be accessed
non-sequentially. This would include devices such as disks (where
you can read any sector you want) but not serial ports (because you
can only read what is at the end of the wire.) Extensions to this
concept (such as ejecting a disk, etc.) are handled in Linux
through ioctls (I/O Controls). The concept of block devices hasn’t
changed in quite a while and support for things such as IDE and
SCSI disk drives has been present since the first revisions of the
Linux kernel.

In Linux 2.4, all the block device drivers have been rewritten
somewhat as the block device API has been changed to remove legacy
garbage from the interface and to completely separate the block API
from the file API at the kernel level. The changes required for
this API rewrite have not been major. However, module maintainers
who have modules outside the main tree may need to update their
code. (One should never assume full API compatibility for kernel
modules between major revisions.)

On the desktop at least, disk devices that use the IDE bus are
most prevalent. Linux has supported IDE since the earliest kernels
but Linux 2.4 has improved on previous versions of Linux’s support
for these devices in a number of ways. First off, high-end systems
that have multiple IDE controllers may benefit as the number of
supported IDE controllers has been increased to a maximum of 10.
(Previous versions of Linux allowed you to have 4.) As most
motherboards are shipped with a maximum of two, this is not likely
to impact many desktop users. Secondly, there have been changes in
the IDE driver which will improve Linux 2.4’s support for PCI and
PnP IDE controllers, IDE floppies and tapes, DVDs and CD-ROM
changers. And finally, Linux 2.4 includes driver updates which
should work around bugs present in some IDE chipsets and better
support the advanced features of others, such as ATA66.

While it would seem that the SCSI subsystem has not changed as
much as the IDE subsystem, the SCSI subsystem has been largely
rewritten. Additionally, a number of new SCSI controllers are
supported in this release. A further SCSI cleanup is expected
sometime during the 2.5 development cycle.

One completely new feature in the Linux 2.4 kernel is the
implementation of a “raw” I/O device. A raw device is one whose
accesses are not handled through the caching layer, instead going
right to the low-level device itself. A raw device could be used in
cases where a sophisticated application wants complete control over
how it does data caching and the expense of the usual cache is not
wanted. Alternatively, a raw device could be used in data critical
situations where we want to ensure that the data gets written to
the disk immediately so that, in the event of a system failure, no
data will be lost. Previous incarnations of this support were not
fit for inclusion as they required literally doubling the number of
device nodes so that every block device would also have a raw
device node. (This is the implementation that many commercial
UNIXes use.) The current implementation uses a pool of device nodes
which can be associated with any arbitrary block device.

One huge area of improvement for Linux 2.4 has been the
inclusion of the LVM (Logical Volume Manager) subsystem into the
mainstream kernel. This is a system, standard in Enterprise-class
UNIXes such as HP-UX and Tru64 UNIX (formerly Digital UNIX), that
completely rethinks the way filesystems and volumes are managed.
Without going into too many details, the LVM allows filesystems to
span disks, be resized, and managed in a more flexible way than can
be done using the current partition table schemes. Some of the
features of the LVM subsystem can be replicated with the md
(multiple device) driver or some userspace tools. However, the LVM
subsystem offers this support in a (defacto) standards-compliant
manner and in a way that will be at least somewhat familiar to
users of commercial UNIXes.

In addition to many of the other block device changes, Linux 2.4
also features updated loopback and ramdisk drivers which fix some
bugs in certain situations.

Filesystems and Partition Tables

Block devices can be used in a number of ways. The most common
way to use a block device is to mount a filesystem on it.
(Internally, the filesystem code is like an overlay on the block
device driver.) Other ways that a block device can be used include
partitioning (which is a lot like a filesystem, just handled in a
completely different way), and using it raw.

Linux 2.4 includes all of the filesystems present in Linux 2.2.
Those filesystems include FAT (for the assorted DOSes), NTFS (for
Windows NT), VFAT and FAT32 (for Windows 9x), HFS (for MacOS), HPFS
(for OS/2), and a variety of others. New filesystems have been
added, most notably the UDF filesystem used on DVD disks and the
XFS (aka EFS) filesystem used by IRIX. All filesystems have been
rewritten to some extent to support the new page caching system and
will be more efficient because of this change. The only exception
to this appears to be the NTFS filesystem which is currently
lacking a solid maintainer and is not currently stable coming into
the home stretch before Linux 2.4.

There are a number of improvements which will improve
compatibility with other systems. OS/2 users will finally be able
to write to their filesystems from within Linux. NT users do not
have this luxury yet as their driver is still in the experimental
stage. NextStep users will be able to mount their CD-ROMs under
Linux as Linux supports an extension to the UFS filesystem that
NextStep uses. It should be noted that HFS+, the new Macintosh
filesystem, is not yet supported by Linux.

Linux 2.4 does not yet include a journaling filesystem, although
several projects are close to providing this functionality in a
stable fashion. While it is almost certain at this point that a
journaling filesystem will not be supported for Linux 2.4, it is
expected that at least one journaling filesystem will be added
during the 2.4 cycle (before the developers break off again to
begin work which will eventually lead to Linux 2.6 or
whatever.)

Additionally, the partition table handling code has been
rewritten and now allows for a much larger selection of non-native
partition table types to be used. This would be useful if you have,
for example, an external SCSI drive from a Macintosh and you want
to use it on your Linux PC. A number of new partition table types
have been added, including the format for IRIX machines.

Not all filesystems are mounted over block devices. Some, like
the proc and devfs filesystems, are completely virtual. Others are
“mounted” over the network. There are a number of ways to
accomplish this end and many OSes provide their own methods of
doing this.

The Windows world uses the Server Message Block (SMB) protocol
for their network filesystems. The new Linux kernel removes the
compile-time workaround that required you to choose whether you
would be mounting drives from Windows 9x or NT. This updated Linux
kernel will be able to auto-detect the remote system type and
enable bug fixes on an as-needed basis. This will vastly improve
Linux’s ability to operate in networks with multiple versions of
Windows.

In the UNIX world, the Network Filesystem (NFS) protocol is the
method of choice for sharing files. Linux 2.4 includes for the
first the the ability to access filesystem shares over the most
recent version of the NFS protocol, NFSv3. NFSv3 includes many
advantages over previous versions and is one of Linux’s most
requested features. It should be noted however the kernel NFS
daemon will still only be exporting NFSv2 shares from Linux.
(Support for NFSv4 has been announced to be under development.)

Character Devices – Keyboards, Mice, Consoles, and Ports

The class of devices which can only be accessed sequentially is
the character device. These are devices, such as serial devices,
which allow you to read from a stream or push data onto it, but not
to “skip” ahead or behind. This includes serial and parallel ports,
keyboards, mice, and terminal devices. There have been several
major improvements in this area for the latest incarnation of the
Linux kernel.

One of the largest improvements in this area is in regards to
Linux 2.4’s support for keyboards and mice. Previous incarnations
of Linux included support for serial and PS/2 mice and keyboards
(and ADB, for instance, on the Macintosh.) Linux 2.4 also supports
using keyboards and mice attached to the USB ports. Additionally,
Linux 2.4 also supports keyboards on some systems where the
keyboard is not initialized by the BIOS and systems that have
trouble determining whether a keyboard is attached or not. And
finally, Linux 2.4 includes expanded support for digitizer pads and
features an emulation option to allow them to be used as normal
mice, even when this is not directly supported in hardware.

Linux’s support for serial ports has not changed much since the
days of Linux 2.2. Linux 2.4 (and some later versions of Linux 2.2)
supports sharing IRQs on PCI serial boards; previously, this
feature was limited to ISA and on-board serial ports. Additionally,
Linux 2.4 has added a number of new drivers for multi-port serial
cards. It is hoped that these changes and others will make using
your serial ports under Linux 2.4 easier than before.

In a separate department, there has been some work since 2.2 on
supporting so-called “WinModems” (or “soft modems”). These are
modems which exist largely in software and whose drivers are often
only provided by the manufacturer for Windows. (Often the DSP or
other parts of the hardware must be implemented in software rather
than on the board.) While no code has been submitted to Linus for
the support of these beasts, several independent driver projects
have been working to get some support for these beasts in and the
first fruits of these labors are becoming usable outside the main
tree. While it will be a long time before we see most of these
devices supported under Linux, for the first time it actually
appears that the Open Source snowball is beginning to roll in this
direction.

Linux 2.4 also includes a largely rewritten parallel port
subsystem. One of the major changes in this area is support for
so-called “generic” parallel devices. This functionality can be
used by programs which access the parallel ports in unusual ways
or, more likely, just want to probe the port for PnP information.
Additionally, this rewrite allows Linux 2.4 users to access all the
enhanced modes of their parallel ports, including using UDMA (for
faster I/O) if supported by the hardware. Under the new Linux
kernel, it is also possible to direct all console messages to a
parallel port device such as a printer. This allows Linux to match
the functionality of many commercial UNIXes by being able to put
kernel and debug messages on a line printer.

Infra-red support has progressed since Linux 2.2 and there have
been many changes in this area, including better network support. I
have to confess to not having a lot of experience in this area.

Multimedia: Sound, TV, Radio, etc.

On the complicated side of the character device list, we have
some of the less essential devices to be supported by Linux. Linux,
in its emerging role as a desktop platform, tries very hard to
support sound cards, TV and radio tuners, and other sound and video
output devices. To be honest, Linux 2.4 does not include as many
ground-breaking changes as Linux 2.2 did in this respect. Linux 2.4
does however include updates and new drivers for a variety of sound
and video cards, including full duplex support. Linux 2.4 and some
later versions of Linux 2.2 also include code which will allow some
sound devices to more easily allocate memory in required ranges;
this should make the configuration and use of some cards much
easier.

Work is in progress on a completely rewritten sound subsystem
which will support many of the more advanced features of today’s
sound cards. This support will not be present in Linux 2.4, but may
make it into the kernel for Linux 2.6.

Video Cards and Framebuffer Devices

Another, more complicated variety of device is the frame-buffer.
A frame-buffer is simply a section of memory that represents (or
is) video memory to such an extent that writing to this memory
affects the colors of the pixels on a screen. This is more
complicated than some other devices because it supports ioctls to
change the palette and other functions associated to video, a
complicated beast.

Linux 2.4 includes a number of new drivers and improvements to
old drivers. Especially important here is Linux’s support for many
more “standard” VGA cards and configurations, at least in some
modes. (Even if the mode is only 16 colors– at least it works.)
Please remember that this feature can be bypassed and (on i386) is
only necessary for people with certain systems which cannot be
supported in any other way. At this time, the XFree project
provides many more drivers to many more video cards than the kernel
can support so it is not necessary to use this feature to get
support for the X Window System. (SVGAlib and other libraries allow
you to do direct video manipulation on supported hardware, however
the use of these libraries must be done carefully as there are some
security concerns and race conditions.)

One of the biggest changes in this respect is the addition of
the Direct Rendering Manager to the Linux kernel. The DRM cleans up
access to the graphics hardware and eliminates many ways in which
multiple processes which write to your video cards at once could
cause a crash. This should improve stability in many situations.
The DRM also works as an entry point for DMA accesses for video
cards. In total, these changes will allow Linux 2.4 (in conjunction
with Xfree4.x and other compatible programs) to be more stable and
more secure when doing some types of graphics-intensive work. These
changes should also make some kinds of television tuner cards more
workable under Linux.

Accessibility

When thinking of Linux, the words “user friendly” do not
generally come immediately to mind. Therefore, one might be
surprised to learn that Linux 2.4 (and some later editions of the
Linux 2.2 kernel) supports speech synthesizer cards. This driver
and the appropriate hardware will allow vision-impaired Linux users
to hear all Linux output, including messages very early in the boot
process. Very few operating systems can boast such low level
support for these devices. (There will be other patches and
utilities that will be required for full use of these devices, this
kernel driver is only a component of the system.)

Networking and Protocols

Networking and network hardware is one of the major areas where
Linux has always excelled. These devices are neither “character”
nor “block” but inhabit a special space free of the need for device
nodes. Linux 2.4 will include many improvements to this layer
including new drivers, bug fixes, and new functionality added on to
existing drivers.

The Linux model of network sockets is one which is standard
across most UNIX variants. Unfortunately however, the standard does
have some correctable deficiencies. Under Linux 2.2 and previous
versions, if you have a number of processes all waiting on an event
from a network socket (a web server, for instance), they will all
be woken up when activity is detected. So, for every web page
request received, Linux would wake up a number of processes which
would each try and get at the request. As it does not make sense
for multiple processes to serve the same request, only one will get
to the data; the remainder will discover nothing to process and
fall back asleep. Linux is quite efficient at making this all
happen as quickly as possible, however it could still be made more
efficient by removing the redundant wakeups. Linux 2.4 includes
changes which implement “wake one” under Linux which will allow us
to completely remove the “stampede effect” of multiple processes.
In short, “wake one” does exactly as its name indicates: wakes up
only one process in the case of activity. This will allow
applications such as Apache to be even more efficient and make
Linux an even better choice as a web server.

Linux 2.4 also includes a completely rewritten networking layer.
In fact, it has been made as unserialized as possible so that it
will scale far better than any previous version of Linux.
Additionally, the entire subsystem has been redesigned to be as
stable as possible on multiprocessor systems and many races have
been eliminated. In addition, it contains many optimizations to
allow it to work with the particular quirks of the networking
stacks in use in many common operating systems, including Windows.
It should also be mentioned at this point that Linux is still the
only operating system completely compatible with the letter of the
IPv4 specification (Yes, IPv4; the one we’ve been using all this
time) and Linux 2.4 boasts an IPv4 implementation that is much more
scalable than its predecessor.

As part of this major rewrite, the firewall and IP masquerading
functionality of the kernel has been completely rewritten again.
(Older users may remember that these same components were largely
rewritten for Linux 2.2 also.) The new subsystem has been split
into two parts: a packet filtering layer and a network address
translation (NAT) layer. These new subsystems are considerably more
generic than their predecessors, and it is now possible to do most
types of sophisticated routing through any Linux box. Previously,
this kind of functionality was largely only available with
dedicated and proprietary routing hardware. Unfortunately, this
major rewrite also includes a new userspace tool to manage the
available functionality. For compatibility, modules exist which
will allow you to use either the Linux 2.0 (ipfwadm) or Linux 2.2
(ipchains) tools without a major loss of functionality. This will
make the upgrade from either of these kernel versions relatively
seamless.

For Enterprise-level users, there are a number of features that
will better enable Linux to integrate into older and newer
components of existing network infrastructures. One important
addition in this respect is Linux 2.4’s new support for the DECNet
and ARCNet protocols and hardware. This allows for better
interoperation with specialized systems, including older
Digital/Compaq ones. Also of special interest to this class of
users, Linux 2.4 will include support for ATM network adapters for
high-speed networking.

For the low-end desktop users, PPP is an important part of day
to day life. Linux 2.4 includes some major rewrites and
modularization of much of the code, including a long awaited
combination of the PPP layers from the ISDN layer and the serial
device PPP layer, such as for dial-up connections with modems. In
addition to the modularity, ISDN has been updated to support many
new cards. The PLIP (PPP over parallel ports) layer has also been
improved and uses the new parallel port abstraction layer. And
finally, PPP over Ethernet (PPPoE, used by some DSL providers)
support has been added to the kernel.

Although not present in Linux 2.4, there is work now on
supporting the NetBEUI protocol used by MS operating systems. While
Microsoft will be moving away from this protocol in its products
and towards TCP/IP, this protocol is still important for a number
of Windows-based network environments. (Previously, kernel
developers had commented that the protocol is too convoluted and
buggy to be supported in the kernel. Now that an implementation has
surfaced, it remains to be seen whether it will be stable enough to
ever be in an official kernel.)

Binary Types

One often overlooked portion of the Linux kernel is the program
loader; the bit that takes your programs, loads them properly, and
runs them. Many people are not aware however that Linux 2.2 added
support for a “miscellaneous” binary loader, a flexible module
designed to allow you to associate binary types (based on extension
or file header information) with “helper” applications in much the
same way as Windows or a comparable operating system would. This
would, for example, allow you to associate all Windows applications
on your machine with WINE (Windows Emulator) so that when you typed
“./notepad.exe” the right thing would happen. (However it is
generally not a good idea to take this concept to the extreme at
the kernel level and many of the “associations” provided by Windows
would be best left handled by your file manager or desktop
environment. It would be a bad idea, for example, to be able to run
“/etc/passwd” and have it come up in a text editor. My personal
recommendation is to use this functionality only when the file type
is best imagined as “executable.”) This was considered a big win by
many because it allowed many different groups, such as the WINE
(Windows non-Emulator) and Dosemu (DOS Emulator) groups, to publish
instructions for making their programs run “native” by the
kernel.

Linux 2.2 and Linux 2.0 included built-in support for starting a
Java interpreter (if present) whenever a Java application was
executed. (It was one of the first OSes to do this at the kernel
level.) Linux 2.4 still includes support for loading Java
interpreters as necessary, but the specific Java driver has been
removed and users will need to upgrade their configurations to use
the “Misc.” driver.

The Kernel Web Daemon

One of the most striking features in the Linux 2.4 kernel is the
kernel web daemon, or khttpd. It’s true, Linux actually supports a
kernel module which can process HTTP requests without having to
communicate with any user space servers (such as Apache.) This
feature is often misunderstood– it is not designed to replace
Apache or any other web server and it can be used only when serving
raw files (no CGI.) If it receives a request for something that it
cannot handle, it will “pass through” the request to user space
where a web server can snatch it up and process it without ever
being able to know the difference. This feature will make Linux an
even better choice for rapid-fire web serving of static content,
such as dedicated image servers.

Kernel Crypto

Although export laws in the US have been changed since Linux
2.2, Linux 2.4 is not likely to include support for cryptography in
the main distribution. Import and export regulations for
cryptography are different around the world and many Linux
developers are loth to make it harder for anyone in the world to
develop for the Linux kernel. Already however, patches are
available to add encryption to several applicable kernel
subsystems. It remains to be seen how official these patches will
be made in the future.

Linux 2.4 is almost ready and already has shown itself to be the
best Linux kernel yet in a number of respects. Although I do not
expect many new features before release, I will attempt to keep
this up to date. If you have a suggestion, please email me and I’ll
think about adding it. I tend to lose suggestions over time (I do
get paid for something else, and I don’t always have time to add
things to this document in a timely fashion) so if I don’t respond
or miss out on you in the next release, you should probably try
again. I’m not even going to tell you how much mail I get
everyday…

This is the probably not the final version, but it’s close. If
you haven’t commented on it by now and I missed something that you
feel is important, please email me at [email protected] or
[email protected] and
let me know.