Re: Snappy GUI response

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On 3 Jul 2003, Owen Taylor wrote:

> On Thu, 2003-07-03 at 15:52, Mark Vojkovich wrote:
> >    This is because people writing X applications (and particularly 
> > X toolkits) have little understanding of graphics performance issues
> > in the X-Window system.  They say they do, but the results (slower
> > and slower toolkits every year) imply that they don't.  Most of
> > these problems lie in the continued failure of toolkits to reduce
> > the round trip communication between the client and the server.
> 
> * Without any firm definition of *what* we are talking about,
>   I think placing blame is a bit silly.

  He did leave it a little open ended, but you can guess what
he's refering to - interactivity, and I know what the problems are.
I've watched what these apps are doing and they're not getting
better.

  The biggest problems I see with X apps in general:

1) Failure to compress events.  Eg. handling of exposures, resizes,
   etc... without checking if there are other events of the same
   type in the queue.

2) Failure to make use of window backgrounds.  X will do alot of
   work automatically for you if you let it.  If the contents of a
   window don't need to change, set them as the window background.
   At least set it to your clear color/pixmap so that you don't have to
   waste bandwidth for that.

3) Poor performance analysis on the apps themselves.  I see a whole lot
   of overdraw within apps.  Not only are they doing more exposes than
   they really need, but these exposes are very heavy handed.  I
   see so many app's widgets that don't get drawn once during an
   exposure, but several times.  I'm not kidding.  I spend alot of
   time under a dubugger trying to repro direct rendering bugs in my 
   work.  Just unmap an occluding window and watch an app respond to the
   single expose rectangle.  I sit there watching it draw and clear
   and draw and clear some widgets many times in response to a single
   expose event.  This type of behavior is widespread and much worse
   than it was back when people were using Motif.

   With FVWM I can move windows around and I never see the server 
repainting anything.  It's all seemless.  With more modern window
managers I get to watch things get repainted.  This isn't because
X is slow.  I can verify that it's not falling back to software for
anything.  This is slow because window managers are getting expose 
events for everything and handling them badly.  Maybe better scheduling
at the kernel level will help a little, but that's not the root of
the problem.


				Mark.

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