Performance with System.Numerics and Multithreading about dotrecast

[ikpil]

I've added a new branch with a version that uses SIMD. While it needs testing on various architectures, it has already increased performance by over 50% on my current laptop. I'll continue with more R&D, and once I'm confident it's safe, I'll plan to merge it into the main branch.

@GabrielMotaAlexandre

[galvesribeiro]

ConcurrentQueue does a hard lock internally. So the Enqueue() call is essentially blocking the thread.

We use DotRecast in a server which is based on Microsoft Orleans. Locks in that context are extremely harmful. So I guess in our case the SemaphoreSlim would be better.

[ikpil]

Compatibility Issues

System.Numerics.Vector3 leverages SIMD (Single Instruction, Multiple Data) instructions for optimized vector operations, which can be very fast for certain operations. However, SIMD operations can significantly vary in performance depending on the data structures and types of operations involved.

For example, using Vector3 may actually be slower for performing operations on small-sized vectors or simple scalar operations. Furthermore, SIMD extensions may not be supported on all hardware, making it unavailable in certain environments.

[GabrielMotaAlexandre]

I couldn't find topics related to downsides, I could imagine very few case Vector3 would be slower but overall I thought it would be worth it.

[ikpil]

Could you please provide your environment?
Could you provide me with the DLL that you've built?
Would you like to try running the build artifacts on a different CPU environment?

[GabrielMotaAlexandre]

I edited my comment in case it was misinterpreted.

[ikpil]

See info

• dotnet/runtime#63354

[GabrielMotaAlexandre]

Thanks, grat news.

[galvesribeiro]

Hello folks!

Any updates on the SIMD support?

We are using it on the server side for an unannounced MMO and results are good with this port. However, we would like to indeed leverage SIMD on this.

On that subject - are DtCrowdAgent.RequestMoveTarget()/.AddAgent()/.RemoveAgent() thread-safe?

Thanks!

[ikpil]

1. SIMD will be supported. However, we are currently working on fixing the SOH issue first.

2. By default, RecastNavigation is not thread-safe. The ported DotRecast is also not thread-safe. Therefore, when using it, use isolation or a Query pool to use multiple instances.

@galvesribeiro

[galvesribeiro]

Thanks for the reply @ikpil

We only use DtCrowd and NavQuery/NavMesh. In that case, I guess we should have a SemaphoreSlim(1,1) being used whenever we need to call Add/Remove agent and Update.

Is that enough to protect the Write operations but Read from any thread without "lock"?

Again, thanks for the great work on this port!

[ikpil]

I haven't tested it, but just take a look at the feeling.
@galvesribeiro

        public class DtCrowdManager
        {
            private DtCrowd _crowd;
            private ConcurrentQueue<Action> _requests;

            public DtCrowdManager(DtCrowd crowd)
            {
                _crowd = crowd;
                _requests = new ConcurrentQueue<Action>();
            }

            // DtCrowdAgent -  should only read.
            // AddAsync - thread-safe
            public Task<DtCrowdAgent> AddAsync(RcVec3f pos, DtCrowdAgentParams option)
            {
                var tcs = new TaskCompletionSource<DtCrowdAgent>(TaskCreationOptions.RunContinuationsAsynchronously);
                _requests.Enqueue(() =>
                {
                    var ag = _crowd.AddAgent(RcVec3f.Zero, null); // ..
                    tcs.SetResult(ag);
                });

                return tcs.Task;
            }

            // RemoveAsync - thread-safe
            public Task<bool> RemoveAsync(DtCrowdAgent ag)
            {
                var tcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
                _requests.Enqueue(() =>
                {
                    _crowd.RemoveAgent(ag);
                    tcs.SetResult(true); // ...
                });

                return tcs.Task;

            }

            // It should be called only from one thread.
            public void Update(float dt)
            {
                while (_requests.TryDequeue(out var action))
                {
                    action.Invoke();
                }
                
                _crowd.Update(dt, null);
            }
        }