Re: [Qemu-devel] [PATCH] opengl rendering in the sdl window

[Jamie Lokier <jamie@shareable.org>]

Daniel P. Berrange wrote:
> Just thought I'd mention one other thing - it is not actually clearcut / or
> guarenteed that XShm + VNC would improve performance in all circumstances.
> This is because X pixmaps created via XShm apparently cannot live in video
> memory, and thus it is not so easy to get full performance advantage of
> hardware acceleration - this could impact scaling for example.

I haven't seen any scaling with VNC so far, but it would be nice sometimes.

How does sending the image over the wire then asking the server to
scale it using RENDER improve on sending it over XShm then asking the
server to xscale the pixel using RENDER?  Or is there something else
you can do when you don't use XShm?

> The need to
> perform XSync operations when updating the pixmaps may also negate the
> benefit of avoiding the socket I/O.

You don't have to do XSync, if you don't mind a bit of tearing which
you're probably going to get anyway, or if you do some fancy locking
until you receive XShmCompletionEvent asynchronously and the blit is
fast.  Or you can double-buffer or triple-buffer with multiple XShm
pixmaps; I don't know how that compares with sending the image over
the socket.  Or you can do both: switch to a new buffer only when the
guest writes to video memory before receiving XShmCopmletionEvent,
otherwise stay with the old buffer.

-- Jamie