UPDATED: Wonderful World of Linux 2.4 - Final Candidate #3 - 4/10/00 (aka the "Sleeping in the Flowers" Update)Apr 11, 2000, 16:36 (18 Talkback[s])
(Other stories 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.
< This work has absolutely nothing to do with Lycos, my employer. The views here are all mine and this article does not constitute an endorsement from Lycos or anything of the sort. I do enjoy working for them however. Reproduction or translation of this article is fine, with permission. Email me. >
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 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.
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.)
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.
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@example.com or firstname.lastname@example.org and let me know.