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New Kernel 2.4 Development Branch (-mjc)
Ivo writes: "kerneltrap is reporting: Michael Cohen announced to the lkml his intention to begin a new 2.4 development tree. The first release of his -mjc branch includes a number of performance enhancing patches, including Robert Love's preemptible kernel patch, Rick van Riel's reverse mapping patch and George Anzinger's real time scheduler patch. Michael says of this patch, "I feel that there's need for a rapidly developing '-ac [like]' tree, and so, here we go. Feel free to test it""
Most of the performace patches (pre-emptible, etc) are just patches to the current kernels that the stable kernel's maintainer hasn't accepted, for one reason or another. Chances are, they will go straight into 2.5 so that they can be tested and improved upon.
The reason for -mjc is to allow people to use the performance patches without having to worry about conflicts between the patches, applying them, etc - it looks just like a normal kernel package, except with -mjc at the end.
The -ac series follows the same guidelines - it tends to have slightly fancier features, newer drivers, etc. Every once in a while, when the stable kernel's maintainer decides that the -ac series is stable enough, a lot of the patches that went into -ac are put into the stable series, to get all the new drivers in there.
I doubt that will happen the same way for -mjc, since the patches are more along the lines of getting every little bit of performance possible, instead of having a wide testing ground for new drivers, vm's, etc.
So how much gain in performance (or apparent performance) should one expect after applying this combined patch? Are the performance gains only applicable under special circumstances? Are they focused more on desktop apps than server?
I doubt you will see ANY performance enhancements with this patch - in fact, under most circumstances, performance will be worse.
The patch MOSTLY addresses a need to have shorter latency responses under linux. So the real benefit will be seen if you, say, run xmms, browse the web on a java intensive site, and do a make -j10 bzImage at the same time. On most machines this will cause xmms to stutter a little - either an audio skip or the text in the scrolling windows will stop and start. With the patch you can expect perfect xmms performance under broader circumstances.
This has the most significant implications for audio and video under linux - things that require short latencies to perform properly. This is questionably the most needed area of improvement for the linux kernel for desktop use.
However, if you time kernel compiles or run lmbench, you'll probably see slightly (but not hugely) worse results. You can expect that changes to address these issues will be incorporated in mainline kernels eventually, although not necessarily in the form that these patches take. Maybe - it will be interesting to see it sort out.