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

[Anthony Liguori <anthony@codemonkey.ws>]

Jamie Lokier wrote:
> 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.
>   

It's been supported in gtk-vnc for a while now.

> 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 key point being you can avoid a copy even when using a shared memory 
extension because you don't necessarily have to back the shared memory 
with an XShmImage.  Even if you have to copy from shared memory to a 
hardware framebuffer, you avoid copying the data to the kernel socket 
buffer.

But copying data isn't super expensive assuming you're hitting the cpu 
cache (which you probably are). So I don't know how much it saves in 
real life.

Regards,

Anthony Liguori