I switched to Vulkan from OpenGL to use multi-threading improvements.
In OpenGL, I was able to load dynamically object to the scene (buffer, textures, etc) while rendering by using a waiting system. I was loading all app-side stuffs in a thread, then when it was ready, just before a frame render in the main thread, I was sending everything into the video memory. That was fine.
With Vulkan, I know I can call some functions between threads without provoking the well known segfault from OpenGL. But, this doesn't works with vkQueueSubmit(). I already know, I tried the naive way. To me, it seems logical you can't bother a queue from multiple threads.
I came with some ideas, but I don't know which one is good or bad.
First, I would go the OpenGL way, I will prepare everything I can from the CPU/App side, then just before render a frame, I will submit buffers (with transfer queue) to the video memory. But I feel there is no a real improvement from OpenGL way...
Second, I will try to use the synchronization mechanism to be able to send buffers in a thread and render from an other. But I keep reading there is a lot of way to slow down everything by causing irrelevant locks or by using incorrectly semaphores and fences.
So my question, is basically what path to pick to solve this problem ? How can I load a buffer dynamically from an other thread while the main thread is rendering without making too much pain to performances ? How Vulkan can help ?
CodePudding user response：
If you want to stream resources for immediate use (i.e. the main render cannot proceed without them), then you're pretty much going to either block the main thread waiting, or have it spin doing something visually interesting (e.g. an animated loading screen) waiting for the resources to load.
If you want to stream resources while the app is doing real rendering then the main trick here is to load resources asynchronously in the background and only switch to using those resources in the main thread once they are already loaded. If the main thread ever ends up actually blocked on a semaphore then you've probably already started dropping frames, so your "engine" design needs to ensure that never happens. A lot of game use simple low-detail proxy objects as stand-in versions while the high-detail version is loading in the background.
None of this is particularly related to the graphics API - both GL and Vulkan need the same macro-scale behavior. Vulkan API features don't particularly help because the major bottlenecks which cause problems here are storage/network/CPU which have nothing to do with the graphics part of the problem.
CodePudding user response：
I decided to trust threads !
In the first place it seems to work, I get a lot of :
[MESSAGE:Validation Error: [ UNASSIGNED-Threading-MultipleThreads ] Object 0: handle = 0x56414228bad8, type = VK_OBJECT_TYPE_QUEUE; | MessageID = 0x141cb623 | THREADING ERROR : vkQueueSubmit(): object of type VkQueue is simultaneously used in thread 0x7f6b977fe640 and thread 0x7f6bc2bcb740]
But it works !
So, the basic idea is to have a thread for loading objects while the engine is drawing. This thread takes care of creating the UBO for the location of the object, then when the geometry is loaded from RAM, it creates the VBO and IBO (I left material with image/UBO on hold for now), then creates the graphics pipeline (with layout, descriptor layout, shaders compiled with GLSLang on the fly) (The next idea is to reuse pipeline for similar needs) and finallly sets a flag to say the object is ready to use. In the other hand, I have my main thread rendering and takes new objects when they shows up ready.
I think it works because I have a gentle video card (GTX 1070) with multiple queues setup, I had one for graphics and an other one for transfer setup.
I'm pretty sure, this will crash or goes poorly with a GPU with a single queue, and this should be why the validation layers tolds me these messages.