20160721

Re Twitter: Thoughts on Vulkan Command Buffers

Because twitter is too short ...

API Review
In Vulkan the application is free to create multiple VkCommandPools each of which can be used to allocate multiple VkCommandBuffers. However effectively only one VkCommandBuffer per VkCommandPool can be actively recording commands at a given time. The intent of this design is to avoid having a mutex when command buffer recording needs to allocate new CPU|GPU memory.

Usage/Problem Case
The following hypothetical situation is my best understanding of the usage case as presented in fragmented max 140 character messages on twitter. Say one had a 16-core CPU, where each core did a variable amount of command buffer recording. The application will need at a minimum 16 VkCommandPools in order to have 16 instances of command buffer recording going in parallel (one per core). Say the application has a peak of 256 command buffers generated per frame, and cores pull a job to write a command buffer from some central queue. Now given CPU threading and preemption is effectively random, it is possible in the worst case that only one thread on the machine has to generate all 256 command buffers. In Vulkan there are two obvious methods one could attempt to manage this situation,

(1.) Could pre-allocate 256 VkCommandBuffers on the 16 VkCommandPools, resulting in needing 4096 VkCommandBuffer objects total. Unfortunately AMD's Vulkan driver currently has higher than desired minimum allocated memory for each VkCommandBuffer. On the plus side there is an active bug, number 98777 (if you want to reference this in an email to AMD), for resolving this issue.

(2.) Could alternatively allocate then free VkCommandBuffers at run-time each frame.

Once bug 98777 is resolved with a driver fix, option (1.) would be the preferred solution from the above two options.

Digging Deeper
Part of what concerns me personally about this usage case is that it implies building an engine where VkCommandPool is effectively pinned to a specific CPU thread, and then randomly asymmetrically loading each VkCommandPool! For example, say in typical case each CPU thread builds on average the same amount of command buffers in terms of CPU and GPU memory consumption. In this mostly symmetrical load pattern, the total memory utilization of each VkCommandPool will be relatively balanced. Now say at some frequency one of the threads chosen randomly, and it's associated VkCommandPool, is loaded with 50% of the frame's command buffers in terms of memory utilization. If VkCommandPools "pool" memory and keep it, then over time each VkCommandPool would end up "pooling" 50% of the memory required for all the frame's command buffers. Which in this case would be roughly 8 times what is required.

This problem isn't really Vulkan specific, it is a fundamental problem on anything which does deferred freeing of a resource. The amount of over-subscription in random asymmetrical load is a function of the delay before deferred free. Which ultimately becomes a balancing act between the overhead in run-time or synchronization cost for dynamic allocation, against the extra memory required.

Possible Better Solution?
Might be better to un-pin VkCommandPool from CPU thread. Then instead use a few more VkCommandPools than CPU threads, and have each CPU grab exclusive access to a random VkCommandPool at run-time to use to build command buffers for jobs until after a set timeout, at which point it releases a given VkCommandPool, and then chooses the next free one to start work again. Note there is no mutex in here for acquire/release pool, but rather a lock-free atomic access to a bit array in say a 64-bit word.

In this situation, assuming CPU/GPU memory overhead for a command buffer scales roughly with CPU load of filling said command buffer, regardless of how asymmetrical the mapping is of jobs to CPU threads, the VkCommandPools get loaded relatively symmetrically.

Another thing about CPU threading which is rather important IMO, is that the OS will preempt CPU threads randomly after they have taken a job, which can cause random pipeline bubbles. As long as this is a problem, it might be desirable to preempt the OS's preemption and instead manually yield execution to another CPU thread at a point which ensures no pipeline bubbles (ie after finishing a job and releasing a lock on a queue, etc). The idea being to transform the OS's perception of the thread from being "compute-bound" thread (something which always runs until preemption) to something which looks like an interactive "IO-bound" thread (something which ends in self blocking). Maybe it is possible to do this by having more worker threads than physical/virtual CPU threads, and waking another worker, then blocking until woken again. Something to think about...

Transferring Command Buffers Across Pools?
I'll admit here I've been so Vulkan focused that I'm current out of touch with how exactly DX12 works. Seems like the twitter claim is that the Vulkan design is fundamentally flawed because VkCommandBuffer is locked to a VkCommandPool at allocation-time, instead of being set at begin-recording-time like DX12. This sounds to me the same as (2.) at the top of this post, effectively making "Allocate" and "Free" very fast for command buffers in a given pool, just "Allocate" is now effectively "Begin Recording" in the DX12 model. Meaning just shuffling work around to different API entry points. Assigning the Pool at "Begin Recording" time does not do anything to solve the asymmetric Pool loading problem caused by the desire to have Pools pinned to CPU threads for this usage case.

Baking Command Buffers - And Replaying
As the number of command buffers increases, one is effectively factoring out the sorting/predication of commands which would otherwise be baked into one command buffer, and deferring that sorting/predication until batch command buffer submit time. As command buffer size gets smaller, it can cross the threshold where it becomes more expensive to generate the tiny command buffers, than to cache them and just place them into the batch submit. So if say one had roughly 256 command buffers in effectively everything outside of shadow generation and drawing, meaning everything from compute based lighting through post processing, it is likely better to just cache baked command buffers instead of always regenerating them.

My personal preference is effectively "compute-generated-graphics", rending with compute only, mixed with fully baked command buffer replay (no command buffer generation after init time), and indirect dispatch to manage adjusting amount of work to run per frame ...

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