Before a kernel is deemed stable enough for, say, server use, it goes through a long beta testing process. This gets an odd-numbered minor revision number - in this case 2.5. When it's stable enough (no obvious major bug or instabilities) it is feature frozen (after going through a 2.6-test phase, which is where we're at now) and becomes a "stable" kernel. Development does not stop here - there are still going to be bugs and the testers cant possibly test things on every platform and chipset. This requires a lot of work, and as soon as a kernel becomes "stable", the rejected or unstable features from the previous beta cycle are forked again into another tree, and the minor revision is incremented again. 

  

 Due to the heavy workload placed on a kernel maintainer ([LinusTorvalds] being the most memorable, but [AlanCox] is another), the maintainers handle one branch each - at the moment Alan is handling the 2.2 "stable" branch, Marcelo Tosatti is in charge of the 2.4 "stable" brach, and Andrew Morton has been handed the 2.6 branch so that Linus can work on 2.7 and beyond. At least for the time being, you can identify a beta kernel by right of it having an odd minor revision, and a stable kernel by an even minor revision. 

  

  

 The 2.6 tree adds two features that can lead to signifigant performance gains. The first one is reverse mapping of physical to virtual memory, which provides a better solution for paging as the kernel can now reuse pages allocated for a particular task rather than having to reallocate each time said task requires swap. The second one allows processes to be preempted anywhere in the kernel which reduces the average response time to 1ms, meaning the kernel spends less time idle under pressure, and so visible performance is, under certain circumstances, greatly improved. Other cool things added in the new kernel include the merging of [ALSA], which has finally superceded OSS/Free] as the preferred sound driver for Linux. For those of you unfamiliar with the Advanced Linux Sound Architecture, it beats down OSS in pretty much every respect, as well as adding support for multiple channel muxing on sound cards which support it, and supports a whole lot more chipsets then OSS could ever dream of doing. 

  

 !!Caveat Emptor 

  

 It took kernel 2.4 about 14 patches and three different VM systems before a "stable" stable version. 2.6 has gone through several months of test kernels, but the design methodology still says "release it to primetime and let the users do the testing" (otherwise, no-one would!) Running a beta kernel is not reccomended for any server or otherwise important box. If in doubt, wait six months. It should be fine to run on a workstation until then. There are some cases, however, when a [RAID] or [SCSI] controller isn't supported by the 2.4 series kernels, so if you want to use one of them in a server, your best bet is upgrading (carefully) to 2.6. 

  

  

 !!Getting and compiling the kernel 

  

 (Lazy RedHatLinux users can see KernelNotes for prepackaged RH kernels). 

  

 !!Nvidia notes 

  

  

 !! ModuleInitTools 

  

 Starting with 2.5.51, Linus merged a new way of handling modules. He's added versioning on every symbol inside a module, with the intent that the kernel can read the module and what hooks it uses, compare it to an internal database of whats critical and whats not, and can decide whether the module could be used even if it wasnt compiled specifically for the current running kernel. This adds a lot more flexibility for people forced to run binary-only proprietary modules to support their hardware. What this means for the user is a completely rewritten set of module tools (modprobe(8), insmod(8), lsmod(8)). On [Debian] you can get these with a quick 

  

  apt-get install module-init-tools 

  

 Module-init-tools is backwards compatible with 2.4.x kernels, so if you're dual booting between a stable and a beta kernel, you've got module functionality on both. (Read ModuleInitTools for an important caveat!) 

  

 __Important:__ module-init-tools no longer reads modules.conf and instead uses a new file called modprobe.conf(5) which has a different syntax. 'man 5 modprobe.conf' will be your friend here. It's possible to set modprobe.conf to handle modules for both 2.4.x and 2.5.x kernels, which is pretty cool if you're a dualbooter. 

 'I have tried (2.5).62 through .66. .62 appears to be the most stable with .66 second. I do run a lot of [GLX] apps (read: [Quake] III Arena) though, and it's more likely to be the Nvidia driver thats causing my system instability. As far as performance goes: 2.5.65 with the ingo-linus scheduler was by far the fastest in interactive applications. In general I get about a 30% increase in framerate (under glx) but of course this comes at the cost of stability. [WineX] also feels a lot faster (thought there is no framerate gain according to ingame displays - I've tried CS and Ghost Recon] - [TomHibbert]' 

  

 'Tried a generic 2.6.0 from kernel.org on Fedora Core release 1 (Yarrow) using SoftwareRaid. When running the make install it complains when creating the initrd image and the raid1 module not being present. The bzImage loads fine when testing it manually, and seems to run sofar. -- GerwinVanDeSteeg' 

  

  

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 [1] Originally unusual; now Gnome and Evolution (to name a few) use it.