[Nouveau] OpenGL context switching with Noveau

[Sampsa Riikonen]

Hi,

Thank you: this confirmed my suspicions about context switching.

A have a small follow up question..

I actually solved the problem for the moment, by dropping openGL 
completely.  My initial tests show that Xvideo handles video dumping ~ 
10 times more effectively than openGL and does not crash.

One can demonstrate this running ~ 10 vlc clients with 720p at 25fps and 
trying both Xvideo and OpenGL output (remember to put all fullscreen).

However, I am suspecting that Xvideo uses, under the hood, openGL, so 
let's see if I am getting this right ..

Both Xvideo and GLX are extension modules of X11, but:

(A) Dumping video through Xvideo:
   * XVideo considers the whole screen as a device port and manipulates 
it directly (it has these god-like powers, being an extension to X11)
   * .. so it only needs a *single* context from the graphics driver.  
Lets call it context 1.
   * Process 1 requests Xvideo services for a certain window .. Xvideo 
manages it into a certain portion of the screen, using context 1.
   * Process 2 requests Xvideo services for a certain window .. Xvideo 
manages it into a certain portion of the screen, using context 1.

(B) Dumping video "manually" through GLX:
   * Process 1 requests a context from glx, gets context 1 and starts 
dumping textures with it.
   * Process 2 requests a context from glx, gets context 2 and starts 
dumping textures with it.

Is there any way to achieve, using openGL directly, situation (A) ?  .. 
one might have to drop GLX completely, which starts to be a bit hard-core.

Sorry if this is OT for this list - I think I'm going to ask this 
somewhere else as well.  (from where?  suggestions welcome ..!)


Kind Regards,

Sampsa


On 08.02.2017 22:39, Roy Spliet wrote:
> For details see my presentation on day 3 of XDC2016. TL;DW: A context 
> switch on average takes ~25 microseconds, but depending on the display 
> resolution and the load on the card times up to 130 microseconds have 
> been observed. The average does not appear to differ much between 
> cards as the (growing) size of the context is in balance with the 
> increased DRAM bandwidth. Measured worst cases appear to get better 
> for higher end cards, but no hard real-time guarantees given.
>
> Roy
>
> Op 08-02-17 om 18:41 schreef 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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