Vulkan and DX12 have very similar API's, but the way they handle their synchronization primitives seem to differ in the fundamental design.
Now, both Vulkan and DirectX 12 have resource barriers, so I'm going to ignore those.
DirectX 12 uses fences with explicit values that are expected to monotonically increase. In the simplest case, you have the swap chain present barrier. I can see two ways to implement fencing in this case:
1) You create N fences. At the end of frame N you signal fence N and then wait on fence (N + 1) % SwapBufferCount.
2) You create 1 fence. At the end of each frame you increment the fence and save off the value. You then wait for the fence to reach the value for frame (N + 1) % SwapBufferCount.
In general, it seems like the "timestamp" approach to fencing is powerful. For instance, I can have a page allocator that retires pages with a fence value and then wait for the fence to reach that point before recycling the page. It seems like creating one fence per command list submission would be expensive (maybe not? how lightweight are fences?).
Now compare this with Vulkan.
Vulkan has the notion of fences, semaphores, and events. They are explained in detail here. All these primitives are binary, it is signaled once and stay signaled until you reset it. I'm less familiar with how to use these kinds of primitives, because you can't do the timestamp approach like you can with DX12 fences.
For instance, to do the page allocator in Vulkan, the fence is the correct primitive to use because it involves synchronizing the state of the hardware queue with the host (i.e. to know when a retired page can be recycled).
In order to do this, I now have to create 1 fence for each vkSubmit call, and the page allocator receives a fence handle instead of a timestamp.
It seems to me like the DirectX-style fence is more flexible, as I would imagine that internally the Vulkan fence is using the same underlying primitive as the DirectX fence to track signaling. In short, it seems like the DirectX timestamp-based fencing allows you to use less fence objects overall.
My primary concern is thinking about a common backend between Vulkan and DX12. It seems like the wiser course of action is to support the Vulkan style binary fences because they can be implemented with DX12 fences. My concern is whether I will lose performance due to creating 1 fence per ExecuteCommandLists call vs 1 overall in DirectX.
For those who understand the underlying hardware and API's deeper than me, I would appreciate some insight into these design decisions.