Xorg input thread - summary or something


This mail that I’ve sent to xorg mailing list tells the current state of my project.

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Hi guys.

As you might noted here [1], my GSoC’s project is to do a separate mouse thread for the X server. Now, I’m really stucked with it and I need some good ideas from you before go to the next steps.

Today the cursor lags in two situations on Xorg:

  1. lot of rendering on the server (CPU usage)

This lags the cursor only if the rendering is done by sw. So, if we’re worried only with hw cursor then CPU is definitely not our problem. Should we take care with the sw cursor for now? And the MPX case which only do sw cursor?

Q: How to reproduce 1)? A: “x11perf -putimagexy500”

  1. heavy memory loads

Under heavy memory usage we’ve got two problems: the X server process in the uninterruptible sleep (‘D’ state) and some parts of the server getting paged to the disk (which leads to the first). These two problems happens when all the physical memory has ended up.

The good news: since my approach of implementation is not using signals (SIGIO) in the input thread, the D state problem is the first which is over. The bad news here is that I didn’t note any performance difference on the cursor movement with heavy memory loads :(

Also, different from what was expected, the input thread is paging to disk. I tried the Jesse Barnes suggestion [2] to mlock the thread with no real success (with or without the input thread when I mlock some mice functions I obtained an unbelievable smooth movement. But I know that this isn’t an elegant solution).

Q: How to reproduce 2)? A: a malloc hog.

The small conclusion of 2): if the real focus of the input thread is to stop with the cursor’s lag then we must provide other ways to keep the cursor’s footprint in the physical memory. (Should I consider the Jesse’s suggestion to put this all inside DRM? I really don’t know how difficult this can be. Jesse, please?)

Also, if we’re running to achieve the 2) solution, the real interest will be systems with few memory (embedded and so on)? On this mobile systems people active the swap all the time (the OLPC’s laptop not, right?)? This leads to other question: would really advantageous to do the input thread only having in mind tiny systems?

So far, we’re not requiring any thread lock mechanism. (Yes, I already tested it on a SMP machine)

To end with a pessimist quote from Jim Gettys [3]: “And I don’t want all input events routed through a secondary input process, as that has bad effects on latency (we can’t guarantee that such a helper process gets scheduled at the right moment, and latency variability drives people nuts in interactive situations). So through such a module, the X server would call all the way down to the input device or socket (depending on input type), and not be subject to such variability.”

Well, the last patch you can see here (it’s tiny! Go ahead and tell me something about it!):

I’ll be really appreciating any comments on this mail, please.