Minimal jitter = good desktop.

Reducing jitter seems central for many things.
First of all keypresses seem faster. (less jitter = less latency).
Doom 3 and similar jittersensitive OpenGL applications run smoothly, and better than windows. Doom 3 was also my main app to get running well, and measuring jitter in the signalpath of OpenGL improved the overall computing experience.

Even youtube videos who are not synced to refresh, with a refresh of 60, and a videofps of 30, runs quite well. http://paradoxuncreated.com/Blog/wordpress/?p=3221

System is responsive, and I have only had one problem, and that is packing seems not to work at the moment.

For a fast test in Ubuntu, try http://paradoxuncreated.com/Blog/wordpress/?p=2268

Definately a recommended config on the desktop.

Peace Be With You.

less jitter != less latency

you could (in theory) eliminate jitter by delaying every keypress processed for exactly 1 second by having the code paths that process keypresses faster insert delays before implementing the results.

that would result in zero jitter, but horrific latency.

latency is how long it takes to do something, jitter is how much the latency varies.

normally, if you optimize for one you make the other worse.

If you optimize for latency, you try to finish everything as soon as you can. Since some things take longer than others, jitter increases.

David Lang

In the context of os-jitter, delay/latency is measured as jitter.

Peace Be With You.

Uwaysi Bin Kareem <uwaysi.bin.kareem <at> paradoxuncreated.com> writes:

[sorry for cutting out the context], but it's been topposted]

But the problem is, we cannot measure 'jitter' directly. There is no reliable benchmark that produces results adherent to what someones' definition of 'jitter' is.

At software level we only have a notion of latency, and that leads to jitter as david said, but as the kernel is not real-time, you cannot guarantee every opengl command/fb transfer will be finished in time for next frame to be drawn.

Maybe if someone could get the information of % finished frames (or % dropped frames) within one slice of userspace, that would be something to build on, but it's still a derivative and with unknown bias level.

Lukasz

This is really simple, and I don`t care about top posting, down posting, in the middle comments or whatever. Do whatehver you like, and have no other rule that what is in your soul. That is what drives ultimately society. Look up Aristotle natural-law. Which actually is based in divine nature.

Now jitter, is really easy. Jitter-sensitive OpenGL applications will show visible jitter. Doom 3 is extremely sensitive. I have tried to make it run well many times, but it wasn`t until I became aware of more unintuitive behaviour not according to theory with some settings, and I started trying reversing them. And then I found 90hz to be optimal, and giving a perfectly running doom 3. Someone actually suggested I try 10000hz BFS patch, because "it would reduce latency." Which I did. But then I also tried 20hz, and there was little difference on BFS. Ultimately I arrived at 90hz with CFS, and tweaking it`s granularity a bit, and it worked well. (Better than BFS). So in that case, JITTER is solved. Also a lot of low-jitter configs use low hz. So that seems to back it up. And everything on my computer seems to be running better. Smoother, more responsive. Even the ads in my browser ;(

I also appreciate those who can measure small jitter in the uS range, and mitigate it. But I would also like for those, to check if a simple hold-logic would be better. For the 10ms filter I mentioned. Say hold for 1ms at 0, and then to regular peak values. It seems that would be a better filter. This just me being a perfectionist ofcourse.

So yes, according to the general definition of "os-jitter" it seems highly reduced.

I don`t know at all why you are mentioning opengl calls. Obviously games, do run quite well. Atleast now. It is also going to be great to test new games coming, and keep iterating knowledge and tuning. Also ofcourse OpenGL is a great part of Wayland, and I hear more h/w is used there, and hopefully it doesn`t stop performance in games, so one can have an effectful desktop, without worrying about game-performance. Some of the GUI in doom3, running completely smooth, shows some great potential for GUI-ideas aswell :)

Peace Be With You.

The natural law on most mailing lists is : avoid top-posting and actively discourage it. That includes LKML.

Question isn't of how you 'see' or 'feel' it, question is: how do you give representative hard numbers of it.

Can you guarantee the same will apply in all hardware combinations? What if it doesn't work for e.g. someone with 4 core i5 and ATI gpu?

I know how filters work, thank you very much.
But with same outcome one could argue, that someone could put a regulator there instead, to auto-tune to a chosen benchmark; Maybe my personal favorite, GPC?
But since you cannot measure jitter, you can't minimize that automatically... or can you ?
Since we're in Control Theory, what we have accessible as 'measurements' is either a derivative of jitter or jitter derives from it, with bias unknown. IOW nice for looking at, but worthless.
So the goal, according to what I derive from looking at LKML, is not to minimize the latency, but rather to make it as precise as possible,

My thought experiment is as follows:
Imagine all the layers of the opengl aware application.
A thread or a few, to calculate the scene (spatial, physical, network threads). Also one to govern them all. This is scheduled in the application itself, kernel only sees one process.
The scene composition thread, produces new frames as fast as it can run (or if synchronized to vsync, produces just one and then waits).
An application competes for execution time with other applications. A thread (or a few) receiving data from app itself, recognizing what calls it should make, calling glibc/gl/kernel functions. This is still in userspace and competes with the application itself (say it's the X server)
A thread or a few, in kernel space, receiving calls from userspace, translating into calls into hardware. (kernel driver)
This competes for execution time with other kernel level threads, scheduler, other kernel services.
Now the pipeline in the GPU receives the data and manipulates
GPU memory; Say, we can generate 2 to 4 frames in between gpu's internal vsync (exxageration here) so the data in gpu memory changes 2 to 4 times before the video buffer swap occurs and the output(s) start pushing the buffer content into the monitor bitstream.
Say we have comfortably enough time to push 2 scene updates, but the third is dubious: not all calls to gpu arrive before vsync HAS TO happen. The pipeline will not be reset; and if in the mean time, os scheduler decides something else should be ran instead of the application or xserver, it will be alerted of this fact only when its next slice comes. But then, it will try to finish the remaining calls, which will only add to injury. Because it will push the remaining calls for the scene which is delayed because of the caller was preempted during vsync, instead of recalculating the whole scene with most recent timing.

If we could predict that all the required calls will NOT make it into GPU in time for 3rd frame update, we just wouldn't run it; but we can't predict that. This is what I would call 'graphic buffer underrun'.

Hope it's with you too.

Lukasz

I also compiled a 3.6.1 with a local shave (only components I want) + my low jitter config and tweaks. (most notably 90hz timer, which is where many go wrong.)
Quake 2, with software renderer, in wine, went from 15/30 fps, to 60fps with some jitter, with full distro low-jitter kernel, to perfect on the local shaved.

Wine seems indeed also to be extremely jitter-sensitive.

So low-jitter seems to be good, for many things. Also stopping ubuntu-daemons was neccesary.

Peace Be With You.