[cairo] XShm

[Behdad Esfahbod]

On Mon, 2007-10-22 at 16:04 -0400, Chris Wilson wrote:
> Well, I've been playing some more with XShm code and implementing the
> suggestions that have been made - thanks!
> 
> What became most apparent when trying Vlad's
> cairo_surface_acquire_image() suggestion for real with swfdec was the
> lack of synchronisation with the XShmPutImage and especially whilst
> using the XShmPixmaps (this ugly fact was hidden earlier by the
> XSynchronize() within cairo-test). Adding the XSync() after the
> XShmPutImage dramatically affects cairo-perf results.
> 
> After spending a short time tuning, the results are still quite mixed:
> Speedups
> ========
>  xlib-rgb&  paint_similar_rgba_source-256    0.43 0.31% ->   0.19 6.62%:
> 2.54x speedup
> █▌
>  xlib-rgb&        text_image_rgb_over-64     1.32 0.52% ->   0.54 0.23%:
> 2.47x speedup
> █▌
>  xlib-rgb&     fill_image_rgba_source-128    1.37 1.02% ->   0.58 0.91%:
> 2.34x speedup
> █▍
>  xlib-rgba&   stroke_image_rgba_source-256    8.23 0.35% ->   3.52
> 0.89%:  2.34x speedup
> █▍
> ...
> Slowdowns
> =========
>  xlib-rgb&      fill_image_rgb_source-256    1.76 0.29% ->   4.09 0.30%:
> 2.32x slowdown
> █▍
>  xlib-rgba&       text_image_rgba_over-128    0.84 0.11% ->   1.96
> 1.42%:  2.32x slowdown
> █▍
>  xlib-rgba&     text_image_rgba_source-128    1.38 0.80% ->   3.21
> 1.55%:  2.32x slowdown
> █▍
>  xlib-rgba&     text_solid_rgba_source-128    1.30 1.78% ->   3.01
> 1.19%:  2.32x slowdown
> 
> But better than the initial 20x slowdown for text with the paranoid
> XSync'ing.

Good.  Any idea how to "fix" the slowdowns?  Maybe just bypassing Xshm
stuff for small images is all you need?

> The branch adds two new APIs:
> 
> 1) cairo_image_surface_create_similar() [what I previously called
> cairo_image_surface_create_for_target(), but Benjamin Otte suggested a
> better name] - allows the application to create an image surface that
> purports to be optimized for the backend. For example, it may be placed
> in a shared memory segment with the xlib backend (win32 support should
> be even easier...). This is used internally within Cairo when allocating
> image surfaces for gradient patterns.

Awesome.  My main and very serious problem with your first proposal was
the very hacky name cairo_image_surface_create_for_target() that seemed
to totally miss cairo's API feeling, but the new name goes very nicely.
The API is also useful when we add for example 565 image surfaces, etc.


> 2) cairo_surface_acquire_image()/cairo_surface_release_image() - Vlad's
> suggestion for a convenient API to directly manipulate the bits
> representing a raster surface (or failing that degrade appropriately to
> pulling and pushing the pixels). Having experimented in using this API
> with swfdec, it does provide an easier method for loading raw data into
> Pixmaps.

I'm still not convinced that we need to expose these, but I need to
first check the patch...  If we can get an API more like the
push/pop_group, that may help.

> For instance:
>     image = cairo_image_surface_create_similar (cairo_get_target (cr),
>                                                 ...);
>     load_pixel_data (cairo_image_surface_get_data (image), ...)
>     surface = cairo_surface_create_similar (cairo_get_target (cr), ...);
>     cr2 = cairo_create (surface);
>     cairo_set_source (cr2, image, 0, 0);
>     cairo_paint (cr2);
>     cairo_destroy (cr2);
>     cairo_surface_destroy (image);
> reduces to:
>     surface = cairo_surface_create_similar (cairo_get_target (cr), ...);
>     cairo_surface_acquire_image (surface);
>     load_pixel_data (cairo_image_surface_get_data (surface), ...)
>     cairo_surface_release_image (surface);
> 
> Whilst the cairo_surface_acquire_image() is in effect it redirects Cairo
> drawing operations on the surface to the backing image, so that the
> application is free to mix normal Cairo commands with direct pixel
> manipulation - at the expense of forcing the software implementation.

So, hopefully clipping helps allocating a smaller image, right?

> The patches have grown a little too large to be conveniently attached
> and reviewed inline, so I've pushed it to my git repo at
> git://annarchy.freedesktop.org/~ickle/cairo in the xshm branch. If you
> can spare a few moments to review the API additions or find better ways
> of synchronizing the images with the Xserver, please be my guest.
> --
> Chris Wilson
-- 
behdad
http://behdad.org/

"Those who would give up Essential Liberty to purchase a little
 Temporary Safety, deserve neither Liberty nor Safety."
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