Re: Analysis of redisplay performance on Windows

[Chong Yidong <cyd@stupidchicken.com>]

Jason Rumney <jasonr@gnu.org> writes:

> Chong Yidong wrote:
>
>> Could you verify that setting the overlaps argument unconditionally to 1
>> in draw_glyphs indeed removes the performance problem?  If so, we can
>> work around this.
>
> Doing this results in a blank frame.  #if 0 around the block that calls
> left_overwritten and right_overwritten does not solve the performance
> problems completely (redisplay can still not keep up with scrolling,
> whereas Emacs 22 can on the same PC), and without that code, the display
> becomes garbaged, so I don't think it is worth pursuing this workaround.

I wasn't suggesting this as a final solution, just as a starting point
to look for the solution.

What you wrote seems to imply that left_overwritten and
right_overwritten aren't really the problem; they have a performance
impact simply because they cause more glyphs to be drawn (i.e., for
redrawing overwritten glyphs).  But the ultimate problem is that drawing
glyphs is a much slower operation, compared to GNU/Linux.  The question
is, what's the reason for this slowness, and can we fix it?