Bug 73531 - Extreme slowness drawing dashed lines
Summary: Extreme slowness drawing dashed lines
Alias: None
Product: cairo
Classification: Unclassified
Component: general (show other bugs)
Version: unspecified
Hardware: Other All
: medium normal
Assignee: Chris Wilson
QA Contact: cairo-bugs mailing list
Depends on:
Reported: 2014-01-13 03:14 UTC by M Welinder
Modified: 2014-01-20 03:16 UTC (History)
0 users

See Also:
i915 platform:
i915 features:


Description M Welinder 2014-01-13 03:14:16 UTC
First off, apologies for a bug report with precious few details.  I am
hoping this rings a bell for some cairo developer.

The Gnumeric team has received a few reports of severe cpu usage when we
do our "walking ants" animation that marks a copied selection.  I do not
see this on my Linux machine; I might see it with our win32 binary under
wine, but that's a whole lot of extra variables.

To see the walking ants animation in action, start Gnumeric, select a
large area, and press ctrl-c.  Alternatively, watch

Here's what the animation code does:

1. We set a two-item dashed line style using cairo_set_dash.
2. We set a line width using cairo_set_line_width
3. We set one of the two ant colours.
4. We set a rectangle path using cairo_rectangle
5. We draw a line along that path using cairo_stroke_preserve.
   [That draws half the ant pattern, say the black bits.]
6. We set the other ant colour.
7. We set the stipple with an offset of one "ant" length
8. We draw a line along that path using cairo_stroke.
   [That draws half the ant pattern, say the wite bits.]

Repeat that every 150ms with the two colours swapped.

The actual code is at
near line 450.

Is there any reason why this should bring a modern machine to its knees?

Further random info:

Both my Linux and win32 cairo are 1.12.16.

One original bug report for this: https://bugzilla.gnome.org/show_bug.cgi?id=381011 starting at comment 12.
Comment 1 Uli Schlachter 2014-01-18 18:26:38 UTC
I don't really understand the code, but here is what I think I found.

The 150ms timeout is implemented in item_cursor_realize() in item-cursor.c. For items with ->style == GNM_ITEM_CURSOR_ANTED it does this:

  ic->animation_timer = g_timeout_add (150, (GSourceFunc) cb_item_cursor_animation, ic);

Looking at the callback function, we see that it just toggles ic->state and calls goc_item_invalidate() on the item. I haven't actually found that function's implementation, but debian code search gave me the following (goc_item_invalidate(item) just calls goc_item_maybe_invalidate(item, FALSE);):


This calculates the bounds of the items and invalidates that rectangle on the canvas.

TL;DR: gnumeric redraws "basically all of its content" about 7 times per second and then wonders that this is slow. (Or did I miss something?)

Re "this used to work fine for over a decade": The implementation a decade ago was more intelligent (I guess instead of redrawing things, it just drew the ant line ones and then used an XOR operator to "toggle the dashes/ants").
Comment 2 Uli Schlachter 2014-01-18 18:57:30 UTC
And valgrind --tool=callgrind says that 65% of CPU time is spent inside gnm_style_borders_row_draw() (unrelated code?) which itself spends most of its cpu time in cairo_stroke(). So the high cpu usage seems to come from some unrelated code which just happens to be executed more often due to the "ant lines".

I don't see why this would be a cairo problem. Could you provide more information, perhaps some self-contained C code that shows cairo being slow?
Comment 3 M Welinder 2014-01-20 03:16:54 UTC
Thanks for the analysis and the effort.  I am now able to reproduce the
problem myself and work on it.

Apologies for having suspected cairo: this was hitting a random set of machines
suggesting something below gtk+.  In fact, I now believe this is something as
simple as different screen resolution.  More pixels, more work.  And pixels
go up with resolution squared.

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