Xorg input thread (2)

Tiago Vignatti vignatti at c3sl.ufpr.br
Thu Jul 5 20:05:32 PDT 2007

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"

2) 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 

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.


[1] http://lists.freedesktop.org/archives/xorg/2007-June/025610.html

[2] http://lists.freedesktop.org/archives/xorg/2007-June/025612.html

[3] http://lists.freedesktop.org/archives/xorg/2005-August/009626.html

Tiago Vignatti
C3SL - Centro de Computação Científica e Software Livre

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