Re: Performance drop on XDrawRectangles()

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jan 08, 2014 at 04:57:52PM +0100, Thomas Richter wrote:
> Hi folks,
> 
> during the changes from 3.12rc7 to 3.13rc4, the performance of
> XDrawRectangles() dropped considerably. Interestingly, it is not the
> raw rectangles drawing operations that are slow, but it seems that
> the "per-call" overhead has increased by one magnitude. In specific,
> if you use the unmodified "x11pref" program:
> 
> x11pref -rect10
> 
> no substiantial changes are visible. However, if the rectangles are
> drawn one by one by changing:
> 
> /* snip: old version, lines 86ff of do_rects.c of the x11perf program */
> 
> void
> DoRectangles(XParms xp, Parms p, int reps)
> {
>     int i;
> 
>     for (i = 0; i != reps; i++) {
>         XFillRectangles(xp->d, xp->w, pgc, rects, p->objects);
>         if (pgc == xp->bggc)
>             pgc = xp->fggc;
>         else
>             pgc = xp->bggc;
> 	CheckAbort ();
>     }
> }
> 
> /* to the following : */
> 
> void
> DoRectangles(XParms xp, Parms p, int reps)
> {
>     int i;
>     int j;
> 
>     for (i = 0; i != reps; i++) {
>         for(j = 0;j < p->objects;j++) {
>           XFillRectangles(xp->d, xp->w, pgc, rects+j, 1);
>         }
>         if (pgc == xp->bggc)
>             pgc = xp->fggc;
>         else
>             pgc = xp->bggc;
> 	CheckAbort ();
>     }
> }
> 
> by instead drawing the rectangles one by one, the performance is
> decreased to one eigths of the original performance:
> 
> 400000 trep @   0.0687 msec ( 14600.0/sec): 10x10 rectangle (new)
> 2500000 trep @   0.0107 msec ( 93900.0/sec): 10x10 rectangle (old)
> 
> Thus, apparently, not the actual hardware acceleration degraded, but
> there is something in the call path that slowed down the call
> considerably.
> 
> Any idea what changed?

If the ddx is indeed the same and Chris doesn't have any ideas I guess a
full bisect of the kernel should help a lot. Could very well be that some
seemingly unrelated change in e.g. the scheduler caused havoc ...
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch