question about bad cairo performance on the olpc xo

[Sascha Silbe]

Excerpts from Erik Blankinship's message of Fri Sep 03 07:49:27 +0200 2010:

> I need help understanding why I am getting very poor frame rates when I use
> pycairo to move a large image around the screen on an olpc XO.  When I use
> pycairo to draw a rectangle of the same size on an XO, I get great
> performance!  This is confusing to me because in my experience on other
> platforms, redrawing vector graphics is much, much slower than blitzing
> bitmaps to the screen.

Summary: Graphics acceleration is probably disabled on the OLPC build
you are likely to be using, causing vector graphics to be slow.

Long story:

The relative performance of vector vs. bitmap graphics depends on quite
a few factors:
- bandwidth between main memory and video memory
- who renders the vector graphics: CPU ("unaccelerated") vs. graphics
  chip ("accelerated")
- CPU performance vs. graphics chip performance
- complexity of the vector graphics operation

"Regular" PCs nowadays often use some portion of the main memory as
video memory ("shared memory") with a high-bandwidth link between CPU
and video memory. It isn't surprising if using pre-rendered bitmaps with
this kind of setup is faster than using vector operations.

I haven't checked the bandwidths on XO-1 (which is probably the one
you're talking about; results will differ on XO-1.5), but I do know
that until very recently the geode graphics driver had several bugs
if acceleration was enabled, so it's rather likely to be disabled in
the OLPC builds (though I don't know if it worked in older builds).
This means that the CPU has to do the work of executing your vector
operations *and* transfer the result (a bitmap image) to video memory.
This is obviously slower than skipping the vector operations step and
transferring a bitmap image right away.

Sascha

-- 
http://sascha.silbe.org/
http://www.infra-silbe.de/
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