Freeing buffers on Metal can be slower than expected

[AkiSakurai]

This issue is not observed when using the OpenGL renderer. I have not tested it on other operating systems.

2023-07-03 13:44:04 [verbose]: Duration:133ms Freed Vertex Buffers: 4496
2023-07-03 13:44:09 [verbose]: Duration:30ms Freed Vertex Buffers: 1116

The versions of osu! and osu!framework used were from the GitHub repositories located at https://github.com/ppy/osu/tree/1f3d833b058e90dbfdb901f61149b1fc0135542f and 19cbb8f, respectively.

These modifications were used to generate the log.

diff --git a/osu.Framework/Graphics/Rendering/Renderer.cs b/osu.Framework/Graphics/Rendering/Renderer.cs
index 360bf9d3a..08bba7468 100644
--- a/osu.Framework/Graphics/Rendering/Renderer.cs
+++ b/osu.Framework/Graphics/Rendering/Renderer.cs
@@ -18,6 +18,7 @@
 using osu.Framework.Graphics.Textures;
 using osu.Framework.IO.Stores;
 using osu.Framework.Lists;
+using osu.Framework.Logging;
 using osu.Framework.Platform;
 using osu.Framework.Statistics;
 using osu.Framework.Threading;
@@ -820,11 +821,17 @@ private void freeUnusedVertexBuffers()
             if (ResetId % vbo_free_check_interval != 0)
                 return;
 
+            var start = System.Diagnostics.Stopwatch.GetTimestamp();
+            var freedVertexBuffers = 0;
             foreach (var buf in vertexBuffersInUse)
             {
-                if (buf.InUse && ResetId - buf.LastUseResetId > vbo_free_check_interval)
+                if (buf.InUse && ResetId - buf.LastUseResetId > vbo_free_check_interval){
                     buf.Free();
+                    freedVertexBuffers ++;
+                }
             }
+            var duration = System.Diagnostics.Stopwatch.GetTimestamp() - start;
+            Logger.Log($"Duration:{duration/1000/1000}ms Freed Vertex Buffers: {freedVertexBuffers}");
 
             vertexBuffersInUse.RemoveAll(b => !b.InUse);
         }

[peppy]

We are very aware of metal performance issues across the board and are actively working on improving the situation. Freeing buffers is just one of many pain points.

The issue is going to lie within veldrid, FWIW.