Wonderful World of Linux 2.4 - "Back on the Table Edition" - 2/1/00

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 several thousand 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 almost completed. Submitted for your approval, this document describes some of the new features in Linux 2.4. (This document is based on Linux 2.3.40, the most recent developers kernel to date. When the development series reaches maturity, it will become Linux 2.4.0.)

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. 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. (It is probably on the basis of quality that I personally have never gotten any sizable patches into the kernel myself.)

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 light code freeze but several new large features and API changes have been added relatively recently. My personal feeling is that we will soon enter a "real" code freeze and we may be able to expect a Linux 2.4 within a few months. (Keep in mind that it may be several months longer before distributions pick up the new kernel for wide-spread release as they have to do their own rounds of testing.)

A disclaimer: not all of the features in this document will be production quality when 2.4 is released. In the past, some drivers and other features have been flagged as experimental in the kernel configuration system. These drivers and features are available for anyone to use but are often not included in the pre-packaged distributions. These drivers and features also may not be up to the same high-quality as the rest of the Linux kernel however your feedback and bug reports can help them to mature.

This draft of the WWOL2.4 is subtitled the "Back on the Table" edition and will most likely be followed by several later drafts which will probably also be somewhat final. If I missed a change that you feel is important, please let me know and I'll think really hard about adding it.

Joe - jpranevich@linuxtoday.com   (Home) 
      jpranevich@lycos.com        (Work)

< Many thanks to linuxtoday for providing me with the spiffy email addy. Other than the email, I'm in no way associated with nor speak for Linux Today. This work also 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 any other such thing. They are however a pretty damn fine company to work for. >

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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. 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's ability to handle processes and users have also been vastly expanded in the 2.4. Older versions of Linux included relatively low limits on the number of simultaneously executing processes and number of unique users on a system-- two conditions that were definitely hurting Linux's entrance into Enterprise environments. Linux 2.4 fixes these problems by providing a scalable and configurable process limit and increasing the maximum number of users to about 4.2 billion (2^32.)

Linux 2.4 can also handle much larger "enterprise-class" hardware than could previous Linux kernel revisions. For example, Linux 2.4 now includes support for more than 4 gigabytes of RAM on any hardware that can support it, (including Intel hardware,) more than 16 ethernet cards, more than 10 IDE controllers, multiple IO-APICs, and other seemingly pointless abuses of good hardware. 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.)

Linux has become more ready for the Enterprise in more ways than in sheer numbers, however. Many components of Linux 2.4 including the all-important filesystem and network layers have been completely rewritten for scalability. Linux 2.4 should scale much better with many subsystems being optimized for 8 processors or more. Additionally, the work which began in Linux 2.2's removal of the global spin lock has continued and many kernel locks have become much more precise enabling even more scalability in multi-processor environments. Linux 2.4 will be more standards compliant and includes POSIX clocks support, the latest revision of the NFS filesystem (NFSv3), and other changes that will allow it to be more useful in enterprise environments. Linux 2.4 could be called the "Enterprise Linux" although an argument could be put forward for it being "the" Linux in several other categories, as well.

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 many vary. (Remember that most kernel components can be disabled at compile-time, unlike many other Operating Systems.)

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 very dependent on architecture, especially on hardware that is not applicable on more than one platform. 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 for it 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, Linux 2.4 adds support for two new processor families. I have no experience with these platforms so am unsure as to their level of hardware support, etc. (Or even what class 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 Merced (ia64)

While not yet delivered to the starving masses, Intel's 64-bit replacement to the x86 line is coming down the pipeline. While Linux 2.4 does not include any direct support for the chip (since it is not out yet, that would be difficult), there are multiple groups working to make sure that there will be Linux on Merced immediately after its release. This porting process is no doubt simplified by Linux's existing support for 64-bit processors (including Compaq's Alpha chips and the Sparc64) which are already merged into the main Linux tree. This means that much of the grunt work in making 64-bit Linux on Merced a reality (making sure that the kernel and all its drivers do not believe that they "know" what the size of datatypes are and how numbers are stored internally, etc.) is already completed.

I do not want to imply that there are no remaining problems that keep Linux from being "perfect" on 64-bit systems; that is not the case. However a vast majority of the difficult and subtle parts are completed and all that remains are problems derived from a legacy world.

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 similar (older) processors, including processors without MMUs. It has not yet been integrated with the mainstream Linux kernel but may eventually provide some "real Linux" functionality on older hardware.

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's 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 now 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, 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.

Much 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.

Block Devices - Disk Drives, RAID Controllers, etc.

In its simplest form, block devices are devices 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.

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 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.

The SCSI subsystem has not changed as much as the IDE subsystem at this point however a number of new SCSI controllers are supported. It is generally hoped that the SCSI subsystem will be cleaned up sometime in 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.

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

Filesystems and Partition Tables

Block filesystems can be used in many ways. The most common way to use a block devices 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 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.

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. (This change is a long time in coming and I hope that there are still OS/2 users out there to enjoy it.) 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.

Linux 2.4 does not yet include a journaling filesystem, although several projects are close to providing this functionality in a stable fashion. Of the two main ones, ReiserFS is the closest and may actually make it into 2.4. ext3, the long awaited next generation of the ext2 filesystem used standard in most Linux distributions, is not yet close enough to be feature complete. Once a journaling filesystem is in the kernel, Linux users will be able to have shorter fsck times and overall safer data.

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 are 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 however. Some, like the proc filesystem, 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 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 the bug fix 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.

Video Cards - Framebuffer Devices

Another, more complicated variety of block 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 block devices because it supports ioctls to change the palette and other functions associated to video. (Which it might be possible to "format" this device and mount a filesystem from it, I wouldn't recommend you try.)

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 X Windows support. (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.)

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.

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.

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" or "Linmodems"). 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 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. While it will be a long time before we see most of these devices supported under Linux, for the first time it actual appears that the Open Source snowball is beginning to roll in this direction.

In contrast to to the serial driver, Linux 2.4 includes a largely rewritten parallel port subsystem. One of the major changes in this area is support for so-called "generic" parallel devices. This could 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.

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.)

Multimedia: Sound, TV, Radio, etc.

On the complicated side of the character device list, we have some of the "fluffier" 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.

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 if 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 notice that it doesn't have anything 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 an unserialized as possible so that it will scale far better than any previous version of Linux. 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 Linux 2.2 became completely compatible with the specification, the use of "colon mode" for aliasing was deprecated. This functionality was completely removed in Linux 2.4 and may require some advanced users or distributions to partially rewrite scripts.

Next to the new network layer, the next most important improvement in the Linux 2.4 network layer is the addition of code to handle the DECNet and ARCNet protocols and hardware. This allows for better interoperation with specialized systems, including older Digital/Compaq ones.

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.

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 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 window or file manager. 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 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 suppports a kernel module which can process HTTP requests without having 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, 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.

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Linux 2.4 is continuing to shape up to be the best Linux kernel yet. As new features are added into the kernel, I will try and keep this document updated. If you have a suggestion, please do not hesitate to write me. Over the next several weeks, it is likely that this document will be completely rewritten as it's beginning to not flow as well as I'd like; the constant addition of new features piecemeal will do that to you, I guess.

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 jpranevich@linuxtoday.com or jpranevich@lycos.com and maybe I'll add in your suggestion.