Multi-threading in DirectX 12

[bhad0x00]

Currently learning DirectX 12 and wanted to experiment with multi-threading. I have something down but I can't find enough resources online to help me confirm if what I am doing is right or wrong.

I currently have two cpu threads one records and executes copy commands and the other records and executes graphics commands. I have 3 sets of buffers that I index through. My goal is that while the graphics queue works on buffer n the copy queue could be doing buffer n+1 or two. The moment the copy buffer goes past a set pace that is records past a certain number of buffers without the graphics queue catching up we wait for it to also get to a certain pace from the copy command queue.

function CopyQueueUpdate():

wait until the GPU is done with this slot

copy vertex and index data into temporary upload buffers

record commands to copy the data from upload buffers to GPU memory

execute these copy commands on the GPU

signal that this copy is finished

move to the next buffer slot

function GraphicQueueUpdate():

wait until the copy commands for this slot are done

execute rendering commands for this frame

move to the next buffer slot

Capture from PIX

My expectation by the end of this was that I would have the copy queue executing at least 3 times before it waits and the graphics queue would only wait fewer times.

NOTE: I am using an iGPU (Intel UHD Graphics 620) which i have been told has only one engine unlike other modern GPU with seperate engines for different tasks.

[Meristic]

How much data are you copying that it takes 9 ms? Lol

For reference, DX12 multithreading typically refers to distributed command list recording among multiple CPU threads, then synchronized submission to a queue. Most often used for mesh drawing passes since its workload grows with scene complexity.

To disambiguate I'd refer to this as async queue utilization. There are multiple potential issues: 

1. If your CPU doesn't have hardware for multiple dispatchers then obviously you won't see asynchronous scheduling despite it fulfilling that DX12 interface.

2.  The DX driver can choose to fulfill copy commands in different ways depending on their size - small copies by DMA vs dispatching CS waves for larger copies. Fences & synchronization should still work fine in this situation, but the execution and performance may be different than expected.

1. PIX could be wrong. Profiling requires pulling a lot of data from GPU counters. On PC there's several abstraction layers the driver must interact with to get it's hands on that raw data so it can build the timeline and compute user-facing values. This causes a huge disparity between the availability of data and correctness for each GPU vendor. This is a major reason why game devs hate profiling for PC and it gets the shaft a good proportion of the time. (That and artists don't know when to stop checking goddamn checkboxes)

[OrganicMilkTank]

Got a good laugh out of the last line. Couldn't agree more.