Q: 2.8 from buildbot don't recognize PREEMPT_RT kernel

You should take Todd's advice and download that image, boot it from a USB and check the latency without installing it, that way you eliminate the HDD issues if there are any.

HDD is WD SSD, and it is fine.

At first, I supposed it was problem with nVidia GT630 and nouveau driver.
I replaced GT630 with Radeon HD6450, as suggested here (post #418):
www.cnc-club.ru/forum/viewtopic.php?f=15&t=1078&start=400

Latency is still horrible.
I disabled onboard audio, firewire, tweaked grub kernel params - all same,

Finally, I found this thread, which describe latency spikes on Gigabyte P35-DS3 and DS4.
forums.tomshardware.com/threads/audio-la...ith-p35-ds3r.411939/

Its a buggy m/b BIOS after all.

I'll simply trash this m/b, its very old anyway, and buy something else.

Hope this info will be useful for someone.

Had the same processor on an Asus P5B Deluxe, it is still working perfectly well overclocked at 3.2 i think, latency was never over 40000.
I remember i did change the BIOS as it would not work with that processor.
Did you try flashing the BIOS ?

Yes, I re-flashed latest available F14, no changes.

Here is another thread with similar latency issues, looks like the whole P35 family of Gigabyte m/b have been problematic.

forums.anandtech.com/threads/gigabyte-bo...-dpc-latency.171880/

Replacement m/b cost close to nothing, the most cumbersome part is a re-assembly.

Oh well, been there done that........a lot of times....like a lot !
Glad you got to the bottom of it, so now it is only up from here.

Connected my SSD with same OS install to another m/b - everything fine with latency. So yes, culprit is a specific family of m/b.