[Impeller] Document threading configuration with Vulkan. (#44874)
Moving bits and pieces of the presentation this morning into docs so we can keep them up to date.
diff --git a/impeller/docs/vulkan_threading.md b/impeller/docs/vulkan_threading.md
new file mode 100644
index 0000000..0537386
--- /dev/null
+++ b/impeller/docs/vulkan_threading.md
@@ -0,0 +1,61 @@
+# Threading in Vulkan
+
+The Vulkan backend uses a dedicated concurrent worker pool that is created along
+with the creation of the Vulkan context.
+
+Unlike other pools such as the IO worker pool, long running tasks may **not** be
+posted to this pool. This is because frame workloads can and often are
+distributed to workers in this pool. Having a potentially long running task
+(such as texture decompression) may potentially block frame critical tasks. The
+limitation of a separate pool for frame critical tasks is working around the
+separate limitation of not being able to specify a QoS to specific tasks and may
+be lifted in the future.
+
+There is also a separate component called the fence waiter which operates on its
+own thread. The purpose of the fence waiter is to ensure that the resource
+reference count lives at least as long as the GPU command buffer(s) that access
+this resource.
+
+Resource collection and pooling also happens on another thread called the
+resource manager. This is because touching the allocators is a potentially
+expensive operation and having the collection be done in a frame workload, or on
+the fence waiter thread may cause jank.
+
+With this overview, the total number of thread used by the Impeller Vulkan
+backend is the number of workers in the concurrent worker pool, and the two
+threads for the fence waiter and resource manager respectively.
+
+A summary of the interaction between the various threads is drawn below:
+
+
+```mermaid
+sequenceDiagram
+ participant rt as Render Thread
+ participant worker1 as Concurrent Worker 1
+ participant worker2 as Concurrent Worker 2
+ participant fence_waiter as Fence Waiter
+ participant resource_manager as Resource Manager
+ participant gpu as GPU
+ rt->>+worker1: Setup PSO 1
+ rt->>+worker2: Setup PSO n
+ worker1-->>-rt: Done
+ worker2-->>-rt: Done
+ Note over rt,resource_manager: Application launch
+ loop One Frame
+ activate rt
+ rt->>+worker2: Frame Workload
+ activate fence_waiter
+ rt->>fence_waiter: Resource 1 owned by GPU
+ worker2-->>-rt: Done
+ rt->>fence_waiter: Resource 2 owned by GPU
+ rt->>gpu: Submit GPU Commands
+ deactivate rt
+ end
+ activate gpu
+ gpu-->>fence_waiter: GPU Work Done
+ fence_waiter->>resource_manager: Collect/Pool Resources
+ deactivate fence_waiter
+ activate resource_manager
+ deactivate gpu
+ deactivate resource_manager
+```
