Real time kernel - useful for a desktop user?

bitpicker

I have just read that as of 2.6.18 the kernel will include real time support by default. I'd like to know whether this is something a normal desktop user should be looking forward to; does it have any impact on performance other than when using, for instance, MIDI software such as Rosegarden?

Robin

yabbadabbadont · Posted: Mon Oct 16, 2006 8:01 am Post subject:

gentoo-sources already has patches for this. (I think that is what the PREEMPT stuff is for) It makes the desktop nice and snappy even when the system is under load (glibc, gcc, xorg upgrades come to mind

)
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bitpicker · Posted: Mon Oct 16, 2006 8:18 am Post subject:

I'm not sure that's the same thing; there must be a reason why for instance the pro-audio overlay offers a specific real-time kernel. I have googled a bit on preemption and it doesn't quite seem to be the same thing. My kernel is set to 'preemptible kernel' for a low-latency desktop already, but when I start Rosegarden I get the following warning:

yabbadabbadont · Posted: Mon Oct 16, 2006 9:01 am Post subject:

make menuconfig
"Processor type and features"->"Timer frequency"->1000HZ

Also, make sure that "dev.rtc.max-user-freq = 1024" is added to /etc/sysctl.conf
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fangorn · Veteran Joined: 31 Jul 2004 Posts: 1886

realtime means you get a GUARANTEED reaction of the system within a given time frame. Preemption AFAIK is not quite such a strong promise.

The Downside is, that the task switching eats much of the cpu power in extreme situations.

For desktops this is not a preferred behaviour, as the user does not recognize, if the system reacts within .5 seconds or in less than .01 seconds. An audio application in recording mode or live filtering mode will surely recognize the difference.
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bitpicker · Posted: Mon Oct 16, 2006 4:14 pm Post subject:

I am just in the process of compiling the real-time kernel from the pro-audio overlay to see whether there is a noticeable difference. It has an additional setting for real-time in the PREEMPT section, so it does go further than what is included in the standard Gentoo sources. I also found the timer entry, which was, in fact, disabled.

Robin

chrbecke · Posted: Mon Oct 16, 2006 4:50 pm Post subject:

If you don't want to use an overlay-kernel, you might want to look at sys-apps/realtime-lsm. It's a kernel module that provides realtime capabilities e.g. to a specific group - I use it with sys-kernel/gentoo-sources to run jack in realtime mode.

PaulSorensen

bitpicker · Posted: Mon Oct 16, 2006 7:49 pm Post subject:

bitpicker · Posted: Tue Oct 17, 2006 10:00 pm Post subject:

There has been an interesting and annoying side effect to using the real time kernel, or at least that's the only reason I can trace it to: the computer ran all night using the real time kernel, downloading some torrents, and this morning I shut it down when I left the house. When I started it again this afternoon (using the normal kernel) I found that my system time, which is regulated by chrony, was off by almost one and a half hours; BIOS time was UTC, the time zone was correct, but somehow the drift values had been warped completely. I had to delete them and rebooted, and within a couple of hours the time was back on track.

Robin

Bobnoxous · Apprentice Joined: 03 May 2005 Posts: 240