EXA and migration [was: shrink xrender featureset]

Michel Dänzer michel at tungstengraphics.com
Wed Nov 26 01:31:17 PST 2008

On Tue, 2008-11-25 at 20:38 +0100, Clemens Eisserer wrote: 
> > That's not 'strong vocabulary' but simply baseless flamebait.
> Would it make sence to implement some fallback-optimizations like:
> - Copy pictures without drawables (gradients) to a temporary surface,
> if the driver supports composition?

This should be straightforward to do with solid pictures for someone
interested, but I'm not sure about gradients; wouldn't they possibly
need to be re-pre-generated depending on the destination picture? Then
again, the worst case of that might not be worse than what we have
currently... However, that would still be just damage control compared
to accelerating the gradients.

> - Support solid write-only operations (X11 core drawing) for which EXA
> does not provide hooks (e.g. diagonal lines) through a temporal mask,
> if the driver supports composition?

Sure, things like that could be done by interested people. It's just a
tradeoff between effort/interest/... and gain. If someone's interested
in tackling things like these though, I think there'd be some lower
hanging fruit in using the driver Composite hooks with temporary
pictures for GC state like tiled or stippled fills.

OTOH, if we start going down that road, I can't help wondering if
something like GLucose wouldn't be a better long term solution, possibly
targeting Gallium directly.

> For both cases I saw ping-pong migration killing performance, and I
> had to implement work-arrounds in my application myself which are
> built on assumptions about the accaleration architecture's behaviour
> and sometimes cause degraded performance.
> By the way thanks a lot for the EXA improvements in 1.5/1.6 :)

I can definitely see how diagonal lines would cause migration ping-pong,
but for gradients without blending, as of xserver 1.6 they should just
get rendered by the CPU to the system memory copy without any previous
migration necessary. If you're not seeing that, please provide more

Earthling Michel Dänzer           |          http://tungstengraphics.com
Libre software enthusiast         |          Debian, X and DRI developer

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