From 57a75d77c0aa8fa3a32a65362bc50fa88e78daf6 Mon Sep 17 00:00:00 2001
From: daemyung jang <dm86.jang@gmail.com>
Date: Sat, 16 Apr 2022 00:01:05 +0900
Subject: [PATCH] Summerize Chapter 24: Performance Optimization

---
 .../README.md                                 | 59 +++++++++++++++++++
 README.md                                     |  1 +
 2 files changed, 60 insertions(+)
 create mode 100644 24-synchronization-and-multithreading/README.md

diff --git a/24-synchronization-and-multithreading/README.md b/24-synchronization-and-multithreading/README.md
new file mode 100644
index 0000000..c64e104
--- /dev/null
+++ b/24-synchronization-and-multithreading/README.md
@@ -0,0 +1,59 @@
+# Chapter 24: Performance Optimization
+
+As I read this book, I summarize what I think is wrong. If you think my comments are wrong then please let me know. We can dicuss more and update your opinion.
+
+## CPU-GPU synchronization
+
+For the triple buffering, currentUniformIndex is used to point a current uniform data. However this logic is totally wrong because uniform data which is sent to GPU is invalid. Let's print uniform data and see what's wrong.
+
+In Scene.swift, in update(deltaTime:), add this code before currentUniformIndex is updated:
+
+```
+print("Update: \(uniforms[currentUniformIndex].viewMatrix)")
+```
+
+In Renderer.swift, in draw(in view:), add this line after declaring uniforms:
+
+```
+print("Draw: \(uniforms.viewMatrix)")
+```
+
+Build and run. You will see logs like below:
+
+```
+Update: simd_float4x4([[0.99999994, 0.0, 0.0, 0.0], [0.0, 0.99999994, 0.0, 0.0], [0.0, 0.0, 0.99999994, 0.0], [0.0, -0.99999994, 0.99999994, 0.99999994]])
+Draw: simd_float4x4([[0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0], [0.0, 0.0, 0.0, 0.0]])
+```
+
+We've updated uniform data in update(deltaTime:) but sent uniform data to GPU is the garbage. To fix this, we need to keep the uniform index for rendering.
+
+In Scene.swift, add this property:
+
+```
+var renderUniformIndex = 0
+```
+
+In Scene.swift, in update(deltaTime:), add this code before currentUniformIndex is updated:
+
+```
+renderUniformIndex = currentUniformIndex
+```
+
+In Renderer.swift, in draw(in view:), replace this code:
+
+```
+let uniforms = scene.uniforms[scene.currentUniformIndex]
+```
+
+With:
+
+```
+let uniforms = scene.uniforms[scene.renderUniformIndex]
+```
+
+Build and run. Now we send valid uniform data to GPU:
+
+```
+Update: simd_float4x4([[0.99999994, 0.0, 0.0, 0.0], [0.0, 0.99999994, 0.0, 0.0], [0.0, 0.0, 0.99999994, 0.0], [0.0, -0.99999994, 0.99999994, 0.99999994]])
+Draw: simd_float4x4([[0.99999994, 0.0, 0.0, 0.0], [0.0, 0.99999994, 0.0, 0.0], [0.0, 0.0, 0.99999994, 0.0], [0.0, -0.99999994, 0.99999994, 0.99999994]])
+```
diff --git a/README.md b/README.md
index 5899d10..1837515 100644
--- a/README.md
+++ b/README.md
@@ -37,6 +37,7 @@ You can see color images in this book at [here](https://www.raywenderlich.com/bo
  * [Chapter 21: Metal Performance Shaders](https://github.com/daemyung/metal-by-tutorials/tree/main/21-metal-performance-shaders)
  * [Chapter 22: Integrating SpirteKit & SceneKit](https://github.com/daemyung/metal-by-tutorials/tree/main/22-integrating-spritekit-scenekit)
  * [Chapter 23: Debugging & Profiling](https://github.com/daemyung/metal-by-tutorials/tree/main/23-debugging-and-profiling)
+ * [Chapter 24: Performance Optimization](https://github.com/daemyung/metal-by-tutorials/tree/main/24-synchronization-and-multithreading)
 
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