Re: [Caml-list] OpenGL and LWT

["Török Edwin" <edwin+ml-ocaml@etorok.net>]

On 03/17/2014 12:09 PM, Jeremie Dimino wrote:
> Hi,
> 
> On Sat, Mar 15, 2014 at 5:37 PM, Richard Neswold <rich.neswold@gmail.com <mailto:rich.neswold@gmail.com>> wrote:
> 
>     Hello,
> 
>     What would be the appropriate way to use LWT and LabGL together?
> 
>     My gut feeling would be to have the core model and logic to be LWT with a 'detached' thread executing the OpenGL main loop. The thread would get events through the reactive module and would update the display accordingly.
> 
>     The problem is I'm still trying to wrap my head around LWT and I'm not sure a detached thread can participate with the LWT environment it was spawned from. It appears the detached thread is supposed to do something and eventually return a value.
> 
> 
> Yes, the purpose of Lwt_preemptive.detach is to execute a blocking action in a separate system thread in order not to block all lwt threads. And from another system thread you cannot run lwt code directly, you have to use Lwt_preemptive.run_in_main.

Be careful to have an OpenGL context set as current on the thread(s) from which you make GL calls. See:
https://www.opengl.org/wiki/OpenGL_and_multithreading
http://www.equalizergraphics.com/documentation/parallelOpenGLFAQ.html

I don't think you'll get a performance improvement from launching multiple GL calls in different (Lwt) threads.

>  
> 
>     Is the correct approach to use a detached thread to make the individual OpenGL calls?
> 
> 
> I think that would be very slow, as in involves system thread synchronization. 
>  
> 
>     Any recommendations or resources on the subject would be appreciated.
> 
> 
> You could try to assume  that OpenGL calls are fast and perform them in the same thread as lwt.

Well there are a few that are slow, like swapping buffers which can wait for vsync.

I think its best to make GL calls from just *one* thread, either your application's main thread, or a dedicated rendering thread.
You can probably build a Lwt.t that waits for the rendering thread to finish one frame using
Lwt_unix.make_notification/send_notification for thread-safe Lwt notification, coupled with Lwt.wait/Lwt.wakeup_result.

Best regards,
--Edwin