[Nouveau] OpenGL context switching with Noveau

[Ilia Mirkin]

Context switching = very slow on NVIDIA. Don't do it if you can avoid
it. Each context is like a megabyte of data, if not more. Each time it
has to get saved off and restored.

On Wed, Feb 8, 2017 at 1:38 PM, Sampsa Riikonen <sampsa.riikonen at iki.fi> wrote:
> Dear Devs,
>
> (I hope this question is not that much OT for this list..)
>
> My question is about fast OpenGL context switching, i.e. when there are
> several processes using the same nvidia card, each one with their own OpenGL
> context.  In my specific case, I am trying to dump 720p video simultaneously
> to multiple windows using OpenGL textures.
>
> So, to begin with, I have a process that spans child processes (*not*
> threads, i.e. using the fork system call)
>
> Each one of these (multi)processes has a video decoder that produces bitmaps
> at 25 fps rate.  I am dumping the bitmaps to openGL using pixel buffer
> objects (PBOs) and textures.
>
> Each time when uploading a PBO and later on, when showing the texture, each
> (multi)process needs to change to its own context by performing the GLX call
>
> glXMakeCurrent(display_id, window_id, context_id)
>
> So there is quite a bit of context switching going on..!  If there are 30
> windows, that's > 750 context switches per second.  I am able to pull this
> off with intel and ati graphics cards, but not with any nvidia card (tried a
> few of them), neither with the proprietary driver nor with noveau.
>
> Are there any specific tricks to achieve this?  Or should I just use one
> context from one process, i.e. use multithreading instead of multiprocessing
> and avoid context switching alltogether?  (is there something fundamentally
> wrong in my approach of multiprocesses and context switching?)
>
> Interestingly, when using N>2 multiprocesses (spanned with fork), only one
> of the textures is updating - the others freeze.  However, when using the
> same code as independently launched programs, I can achieve N=10+.  However,
> this seems not to be always stable (whole X server freezes sometimes).
>
> On the other hand, I can also use, say, several vlc clients to create a same
> kind of situation and it seems to work.
>
> Ay insight highly appreciated..
>
> Regards,
>
> Sampsa Riikonen
>
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