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2011-07-20 12:00 Age: 3 Years

Linux mainline real-time takes the next level

By: Carsten Emde

3.0.0-rt is out!

 Did you ever ask why the Linux kernel version number suddenly went to 3.0? Rumors were that Linus Torvalds wanted to avoid a version number that signifies the answer to life, universe and everything. Coincidentally, Thomas Gleixner finished a first version of the long awaited update of the PREEMPT_RT patch which is a major milestone of integrating it into mainline. For details please check out Thomas's LKML announcement. Don't you think that this is another good reason to increment the major release number of the Linux kernel?

Some interesting numbers from the release announcement

Size of the 3.0.0-rt patch:
  223 patches (size 680054)
  374 files changed, 8950 insertions(+), 2099 deletions(-)

Size of the patch for 2.6.33:
  462 patches (size 1396505)
  690 files changed, 15994 insertions(+), 5123 deletions(-)

This considerable reduction in size partly is due to new approaches, but a large part of it is the result of upstreaming infrastructure patches. The RT community gratefully appreciates all that work, and special thanks go to Peter Zijlstra for solving the preemptible mmu gather problem and lots of scheduler issues.

No, we still do not have a roadmap, but ...

Some of the patches already are or shortly will be on their way to mainline. These patches represent 40% of the current patch:

Upstream submitted: Already on LKML
  4 patches (size 9741)
  4 files changed, 81 insertions(+), 119 deletions(-)

Upstream fixes: Mainline bugs found and fixed when running the RT kernel
  8 patches (size 8892)
  7 files changed, 59 insertions(+), 51 deletions(-)

Upstream ready: Nearly ready for upstream submission, just needs some polishing
  79 patches (size 232566)
  192 files changed, 1204 insertions(+), 1097 deletions(-) 

Thus, there is hope that the patch will continue to shrink already in the next future.

How good is the code?

While testing Linux 3.0.0-rt, we were surprised to see the latency plot at the left. The plot was created from 8x100 million cyclictest cycles on a 4x2 way Intel i7-2600K CPU running @3400 MHz. This and some other systems of the OSADL QA farm were upgraded some days ago to Linux 3.0.0-rt and helped to prepare the release. BTW: Click on the "Kernel" header line in the overview about the various kernels of the OSADL QA farm systems to sort the list according to kernel version; the 3.0.0-rt systems will then be displayed at the bottom of the list.

So, enjoy Linux real-time at its best. Thanks Thomas for this invaluable and outstanding work!