The 2.6.25 kernel: Does CFS provide incr. responsivenes?

Nordog

I just slapped together a box for my collegue, composed of hardware slightly inferior to my own Gentoo machine, and just to test whether everything was working correctly I installed ubuntu (could have been anything, but a Ubuntu Hardy Heron beta CD was lying around the office). I felt that the new box with Ubuntu was significantly snappier than my own despite lagging behind in hardware, and I was wandering if it had anything to do with the fact that it has Linux 2.6.25 and thus the CFS?
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BradN · Posted: Mon Apr 21, 2008 7:34 pm Post subject:

It might help a little, or maybe even a lot under specific circumstances, the only way to be sure is to test it

Another thing to look at is what nice level the X server is run at - traditionally the X server was given a bit higher priority than normal apps, but I believe the recommendation now is to not do that.

GUI performance can also be impacted by the video card's acceleration support, but this is more of an issue on really old hardware.

There's really a ton of factors that can influence the actual (or percieved) performance. Another one might be the kernel's tick rate (or using the tickless kernel option). This may be a bigger deal for video playback since you want to make sure the video player gets scheduled at least once per frame, and plain 100hz with a few other apps using the CPU might not be enough to accomplish this.

Nordog · Posted: Tue Apr 22, 2008 12:05 am Post subject:

ah, so it's probably just more likely that the ubuntu guys have done some tweaking of their own (in the Xorg niceness and kernel ticks like you say).

I'm sorta the basic Gentoo user: I use some appropriate profile, add some use flags, compile my kernel in accordance with the hardware and that's it. I have never done any tweaking of my own, probably just because I wasn't aware of what was available to tweak

I just had a quick look but I couldn't see any recent Gentoo tweaking for the desktop. After seeing users of other distros getting all preachy about Gentoo not being faster than other distros, perhaps people have become sky and reluctant to write such a guide. Just a thought.
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Baronek · n00b Joined: 22 Apr 2008 Posts: 1

Akkara · Posted: Tue Apr 22, 2008 9:06 am Post subject:

aidanjt · Posted: Tue Apr 22, 2008 9:20 am Post subject:

doubt it's fs releated, more likely is the ubuntu kernel configuration/patchset is better geared towards desktops, since that's the primary target. And the last two kernel versions have made large strides in more dynamic scheduling advancement which is handy for desktops.
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Paapaa · l33t Joined: 14 Aug 2005 Posts: 955 Location: Finland

Nordog · Posted: Tue Apr 22, 2008 11:59 am Post subject:

Well, I would say that both boxes start up around as fast. Programs start up much faster on the Ubuntu box (a difference of 1-2 seconds in the case of Amarok) and the UI is snappier also, but all in all it's the fast loading of the programs that stands out. I should note that is a difference in filesystems: The gentoo box users reiserfs but the Ubuntu box uses the default one, Ext3 I think.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

One thing that has been going awry with recent kernels is CONFIG_FAIR_GROUP_SCHED. It sounds like you'd want to have it set, but experience seems to be showing that for a desktop/laptop (non-server) machine you clearly want it off.

I would also check the "swappiness" setting in /etc/sysctl.conf as a frequent "desktop tweak." The default, and good server value is 100, but many desktop/laptop tweaks set it to 10, or even 0. (I've got it at 25, myself, no logic, just conservatism and not wanting to deviate too far from default.)
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Paapaa · l33t Joined: 14 Aug 2005 Posts: 955 Location: Finland

BradN · Posted: Tue Apr 22, 2008 3:42 pm Post subject:

There's also prelinking which can speed up program startup, especially for things with tons of libraries like KDE apps, etc.

About the comment about a fresh install vs fragmented filesystem... I notice this on gentoo systems I install - after a fresh format and copy, things start a bit faster. It's not as bad as windows with regards to the eventual slow-down, but it is there.

Nordog · Posted: Tue Apr 22, 2008 4:21 pm Post subject:

depontius · Advocate Joined: 05 May 2004 Posts: 3509

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Evincar · Apprentice Joined: 13 Feb 2007 Posts: 217 Location: Madrid

widremann · Veteran Joined: 14 Mar 2005 Posts: 1314

Ubuntu? Tweaking to be faster? You must be kidding me!

I installed Hardy Heron this morning and in addition to having a serious lack of QA, it's also even slower than old Ubuntu. Maybe it's just my drivers, but I don't think they should get SLOWER with newer releases. My God, it's like using Windows 95.

Nordog · Posted: Fri Apr 25, 2008 7:47 pm Post subject:

gimpel · Posted: Sat Apr 26, 2008 7:17 am Post subject:

Ubuntu does other things beside kernel tweaking. Like linking things with -Wl,-O1 all over the place (--as-needed too, as well as DT_GNU_HASH, not sure)

I guess that has more influence on how fast apps load.
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energyman76b · Advocate Joined: 26 Mar 2003 Posts: 2048 Location: Germany

back to CFS.

When Ingo Molnar posted the first CFS patches I started to test them (and I mean test. With feedback and additional 'could you try this' patches).

And holy shit, CFs made the desktop a lot 'better'. Games - no real difference. But the desktop? That was a very pleasant improvement. Idle&under load.

But for ubuntu being faster:
I am sure they use prelink
Maybe they use preloading
They patch around a lot.

Prelink is great. Preloading - I don't know.

Hm, oh - and in 2d intel hardware (aka onboard graphics) is reported to be faster than nvidia hardware.
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depontius · Advocate Joined: 05 May 2004 Posts: 3509

energyman76b · Advocate Joined: 26 Mar 2003 Posts: 2048 Location: Germany

[ ] Group CPU scheduler

as you can see 'off'. Everything with 'group' is off

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depontius · Advocate Joined: 05 May 2004 Posts: 3509

For now that's what I'm doing.

Here's a better reference on the "scheduling daemon" thing: http://lwn.net/Articles/240474/

Next time I build a new kernel, I may try enabling all of the pieces, and see if it feels degraded. From what I've read, if you fully enable the pieces and then don't start a scheduling daemon, it will act just like all of the group scheduling stuff is turned off. That would be interesting to see, but it will be even more interesting to see when scheduling policy daemons start to show up.
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Evincar · Apprentice Joined: 13 Feb 2007 Posts: 217 Location: Madrid