Rendering performace vs. line-spacing

[Herman , Géza]

Hi all,

I noticed that emacs's performance can depend on the font. For example, 
Consolas has a much worse performance (scrolling is sluggish) than 
BitStream Mono. I profiled emacs, and the main difference is 
"draw_glyphs". When emacs is slow (using consolas), this function takes 
50-60% of CPU time (measured by "perf record -g"). When emacs is fast 
(using BitStream), this function takes only ~2-3%.

I played with my font's ascent and descent settings to have more line on 
the screen (as emacs doesn't support negative line-spacing). Originally, 
"draw_glyphs" takes 2-3% with my font. But if I decrease the height of 
the font by modifying ascent/descent, then the same thing happens: 
draw_glyphs takes 50-60% CPU time. If I set line-spacing to 2, 
draw_glyphs become normal, 2-3% CPU time.

(I'm using a master build from yesterday)

Any ideas why this happens? Maybe this happens because lines may overlap 
and some caching mechanism gets disabled?

Thanks,
Geza