Nvidia talk Vulkan in a developer blog post, they say Vulkan supplements OpenGL

If they need something simpler for most developers, I don't see the advantage of using OpenGL instead of a Vulkan-based rendering library.

Vulkan is appealing because it has a good chance of solving the current driver situation. I guarantee you there will be plenty of libraries popping up in no time and even if there wasn't, they could easily develop an OpenGL-styled library of their own to bridge the gap and have it work far more consistently across different drivers.

OpenGL has a fair amount of hurdles that make it fundamentally slow. The two major ones from my experience is the state baggage and it realtime processing of commands. When an OpenGL command is made the CPU instantly goes off and starts communicating with the GPU. It's like a postman being given a letter and going to the house straight away to deliver it.

Vulkan on the other hand batches, you issue and command and it waits, it waits till the buffer is full and only starts processing them when you've told it to. This is like giving the postman a large box of letters and telling them to go deliver them all.

The two processing styles have advantages and disadvantages. But for games that issue a lot of commands the round-trip approach of OpenGL starts to add up in relation to time. To make things worse OpenGL retains a lot of state, uploading new geometry will deny other commands from being processed until the upload is complete, this happens as it's likely that the next command will be impacted by the new geometry.

Vulkan gets round this by using Command Buffers. You can upload geometry in one buffer while issuing commands in another buffer. If a command is dependent on the geometry add it to the Command Buffer that is uploading the geometry. This approach allows the developer to inform the GPU what is dependent on what. Side Note: This part is usually when a driver vendor would step in and start taking shortcuts.

I look forward to hearing more. :)

So basically, we will end up with dev's coding to suite NV hardware again ?

Wasn't Kronos making sure that all the tools are available out the door on release of the final spec?

I think you missed the point of these extensions. There are solely meant for making the progression from OpenGL to Vulkan, for example when porting an existing engine, easier. They are not meant to be used in finished programs, it really wouldn't make much sense to have a clean Vulkan rendering path for AMD and Intel (and consoles, and mobile devices, and whichever device also supports Vulkan) and a separate rendering path for Nvidia when there is no obvious benefit in performance or features and the Nvidia driver can use the clean Vulkan rendering path also. No sane developer would increase their workload without such benefits, and when the major engines have a clean Vulkan path there is absolutely no need anymore for these extensions anyways, since their purpose, to ease up porting, is fulfilled.


Last edited by TobiSGD on 15 January 2016 at 6:41 pm UTC

There may be multiple queues with today's hardware (one for graphics, one or two for compute, one for DMA transfers), but it's important to remember that command buffer creation is not command buffer submission.

And Vulkan can address using multiple queues in parallel as well, while OpenGL can't if I understood correctly. Can't there be more than one queue for graphics? Even if single GPU wouldn't have it, multi-GPU scenario for sure would. And Vulkan address it as well.


Last edited by Shmerl on 15 January 2016 at 6:55 pm UTC

The main benefit with Vulkan should come from the developer being in control of synchronization. This means a thread does not need to stall due to a queue/buffer being locked and can continue work on other things. Later on it can come back and check the lock again. This is a much more efficient approach than OpenGL where most commands will completely stall the thread while claiming the synchronization lock which often leads to a context switch between threads, which is crazy expensive!

Looking forward to spending a few weekends playing around with Vulkan :)


Last edited by Donkey on 15 January 2016 at 8:50 pm UTC

nVidia is only saying that to assure current users of OpenGL that they won't be abandoned. But down the road there won't be a need for OpenGL once there is stable libraries/engines built on top of Vulcan to provide simplicity.

OpenGL will still provide a lot of applications with 3d acceleration where it's not that important, as they perfectly well state in example in Window Managers - that's a direct hint. The benefit of Vulkan backends is questionable to the amount of work required for Window Managers. Even just having one OpenGL context, it's pretty much enough for a lot of use cases (going to two threads in the backend anyway, but ok, they left that fact out, but it's just one rendering thread, that's true).

So OpenGL is here to stay. Vulkan has just a different audience, and NVidia is here to say that OpenGL will live on.

What's really cool is this one:

Furthermore we leverage our industry leading OpenGL driver and allow you to run Vulkan inside an OpenGL context and presenting Vulkan Images within it. This allows you to use your favorite windowing and user-interface libraries and some of our samples will make use of it to compare OpenGL and Vulkan seamlessly.

Will be very interesting how this will be implemented, since it hints to making the window managers believe they have a gl context there, properly updating it while it's actually vulkan doing it. No idea how they want to achieve that, but it sounds interesting.

OpenGL has a fair amount of hurdles that make it fundamentally slow. The two major ones from my experience is the state baggage and it realtime processing of commands. When an OpenGL command is made the CPU instantly goes off and starts communicating with the GPU. It's like a postman being given a letter and going to the house straight away to deliver it.

Vulkan on the other hand batches, you issue and command and it waits, it waits till the buffer is full and only starts processing them when you've told it to. This is like giving the postman a large box of letters and telling them to go deliver them all.

The two processing styles have advantages and disadvantages. But for games that issue a lot of commands the round-trip approach of OpenGL starts to add up in relation to time. To make things worse OpenGL retains a lot of state, uploading new geometry will deny other commands from being processed until the upload is complete, this happens as it's likely that the next command will be impacted by the new geometry.

Vulkan gets round this by using Command Buffers. You can upload geometry in one buffer while issuing commands in another buffer. If a command is dependent on the geometry add it to the Command Buffer that is uploading the geometry. This approach allows the developer to inform the GPU what is dependent on what. Side Note: This part is usually when a driver vendor would step in and start taking shortcuts.

I look forward to hearing more. :)

Good explanation thanks.

I suppose for games with lots of individual items like RTS games doing commands in batch rather than a singular stream could speed up swarm games or games with individual point lights really well.

If the game is rather demanding in the first place, then you will definitely notice a difference (if it's a DirectX game). Rewriting an engine, and optimizing the code takes time, and time is money. I guess that they (Feral, Aspyr, etc.) can not afford to spend that much time optimizing the code.

Unless you have optimized code, you can not compare DirectX to OpenGL. I don't believe that the games we're getting now are 100% optimized, not even close.

In a purely theoretical world (read perfect) OpenGL and DirectX might perform the same but, as we have seen, that is not typically the case in our practical world. Code is never 100% optimized. Currently ports seem to choke especially when it comes to multithreading due to technical differences between OpenGL and DirectX.

Simply put, you cannot write DirectX in OpenGL and expect it to perform well

I suppose one issue is that we never see it working the other way. Lots of things written in DirectX get half-assed ports to OpenGL. But if something was written in OpenGL in the first place there is no point porting it to DirectX because OpenGL is already cross-platform. So we don't see what the performance of half-assed ports from OpenGL to DirectX would look like, performance-wise.

I think NVidia are following the same logic that Microsoft applied with D3D12. Microsoft allow you to write a hybrid D3D11/D3D12 application, where you take advantage of D3D12 API to construct commands on multiple threads/cores of the CPU to get over the draw-call limit that both D3D and OpenGL had been suffering from. The rest of the application continues to be D3D11.

For a game whose performance suffers from draw-call limits, this is a lot less risk and a lot less work than redesigning your renderer, and possibly other parts of your engine to conform to a new development model.

But single-threaded draw-call preparation is far from the only reason why Linux games run slowly. The biggest problem is that a capable PCIe bus GPU is going to be stalled by almost ANY interaction with the CPU because the bus and CPU will both introduce latency. Submitting a draw call therefore has a cost, changing state or state blocks has a cost, and having to send or receive data to synchronise CPU and GPU memory has a much greater cost. These are costs that you can't really avoid if your engine splits processing between CPU and GPU as has normally been the case in the past.

The AZDO API calls added to OpenGL 4.x ( particularly multi-draw indirect and bindless textures/buffers ) along with compute shaders and OpenCL kernels, would in theory allow you to rewrite an engine such that after initial setup, almost all of the work is internal to the GPU. Trouble is, to make that change is a lot more work than you might think. There is only a small body of reference papers describing recent experiments in avoiding CPU interaction, so such a conversion is risky and slow to implement, which is why we have not really seen much benefit from AZDO yet.

I'm guessing that we will have the same issue with Vulkan. It will improve command submission and allow almost complete control over the GPU, but at an even greater development risk and cost than AZDO. I have to say that, apart from increasing draw-call submission, I think it will also be quite a while before game developers really work out the best way to use the benefits of Vulkan.