[Impeller] Add keep alive for 4 frames in render target cache. (flutter/engine#57020)

Improve cache usage by keeping textures alive for 4 frames after the last usage. This improves cache usage in scenarios such as repeatidly dragging the android overscroll functionality.

THis isn't expected to have a negative impact on memory, because a texture cannot be _immediately_ deleted anyway.

Author: Jonah Williams

engine/src/flutter/impeller/entity/render_target_cache.cc

@@ -8,8 +8,10 @@
 
 namespace impeller {
 
-RenderTargetCache::RenderTargetCache(std::shared_ptr<Allocator> allocator)
-    : RenderTargetAllocator(std::move(allocator)) {}
+RenderTargetCache::RenderTargetCache(std::shared_ptr<Allocator> allocator,
+                                     uint32_t keep_alive_frame_count)
+    : RenderTargetAllocator(std::move(allocator)),
+      keep_alive_frame_count_(keep_alive_frame_count) {}
 
 void RenderTargetCache::Start() {
   for (auto& td : render_target_data_) {
@@ -20,9 +22,12 @@ void RenderTargetCache::Start() {
 void RenderTargetCache::End() {
   std::vector<RenderTargetData> retain;
 
-  for (const auto& td : render_target_data_) {
+  for (RenderTargetData& td : render_target_data_) {
     if (td.used_this_frame) {
       retain.push_back(td);
+    } else if (td.keep_alive_frame_count > 0) {
+      td.keep_alive_frame_count--;
+      retain.push_back(td);
     }
   }
   render_target_data_.swap(retain);
@@ -49,10 +54,11 @@ RenderTarget RenderTargetCache::CreateOffscreen(
       .has_msaa = false,
       .has_depth_stencil = stencil_attachment_config.has_value(),
   };
-  for (auto& render_target_data : render_target_data_) {
-    const auto other_config = render_target_data.config;
+  for (RenderTargetData& render_target_data : render_target_data_) {
+    const RenderTargetConfig other_config = render_target_data.config;
     if (!render_target_data.used_this_frame && other_config == config) {
       render_target_data.used_this_frame = true;
+      render_target_data.keep_alive_frame_count = keep_alive_frame_count_;
       ColorAttachment color0 =
           render_target_data.render_target.GetColorAttachment(0);
       std::optional<DepthAttachment> depth =
@@ -69,10 +75,12 @@ RenderTarget RenderTargetCache::CreateOffscreen(
   if (!created_target.IsValid()) {
     return created_target;
   }
-  render_target_data_.push_back(
-      RenderTargetData{.used_this_frame = true,
-                       .config = config,
-                       .render_target = created_target});
+  render_target_data_.push_back(RenderTargetData{
+      .used_this_frame = true,                            //
+      .keep_alive_frame_count = keep_alive_frame_count_,  //
+      .config = config,                                   //
+      .render_target = created_target                     //
+  });
   return created_target;
 }
 
@@ -99,10 +107,11 @@ RenderTarget RenderTargetCache::CreateOffscreenMSAA(
       .has_msaa = true,
       .has_depth_stencil = stencil_attachment_config.has_value(),
   };
-  for (auto& render_target_data : render_target_data_) {
-    const auto other_config = render_target_data.config;
+  for (RenderTargetData& render_target_data : render_target_data_) {
+    const RenderTargetConfig other_config = render_target_data.config;
     if (!render_target_data.used_this_frame && other_config == config) {
       render_target_data.used_this_frame = true;
+      render_target_data.keep_alive_frame_count = keep_alive_frame_count_;
       ColorAttachment color0 =
           render_target_data.render_target.GetColorAttachment(0);
       std::optional<DepthAttachment> depth =
@@ -120,10 +129,12 @@ RenderTarget RenderTargetCache::CreateOffscreenMSAA(
   if (!created_target.IsValid()) {
     return created_target;
   }
-  render_target_data_.push_back(
-      RenderTargetData{.used_this_frame = true,
-                       .config = config,
-                       .render_target = created_target});
+  render_target_data_.push_back(RenderTargetData{
+      .used_this_frame = true,                            //
+      .keep_alive_frame_count = keep_alive_frame_count_,  //
+      .config = config,                                   //
+      .render_target = created_target                     //
+  });
   return created_target;
 }
 

engine/src/flutter/impeller/entity/render_target_cache.h

@@ -16,7 +16,8 @@ namespace impeller {
 ///        Any textures unused after a frame are immediately discarded.
 class RenderTargetCache : public RenderTargetAllocator {
  public:
-  explicit RenderTargetCache(std::shared_ptr<Allocator> allocator);
+  explicit RenderTargetCache(std::shared_ptr<Allocator> allocator,
+                             uint32_t keep_alive_frame_count = 4);
 
   ~RenderTargetCache() = default;
 
@@ -59,11 +60,13 @@ class RenderTargetCache : public RenderTargetAllocator {
  private:
   struct RenderTargetData {
     bool used_this_frame;
+    uint32_t keep_alive_frame_count;
     RenderTargetConfig config;
     RenderTarget render_target;
   };
 
   std::vector<RenderTargetData> render_target_data_;
+  uint32_t keep_alive_frame_count_;
 
   RenderTargetCache(const RenderTargetCache&) = delete;
 

engine/src/flutter/impeller/entity/render_target_cache_unittests.cc

@@ -50,8 +50,8 @@ class TestAllocator : public Allocator {
 };
 
 TEST_P(RenderTargetCacheTest, CachesUsedTexturesAcrossFrames) {
-  auto render_target_cache =
-      RenderTargetCache(GetContext()->GetResourceAllocator());
+  auto render_target_cache = RenderTargetCache(
+      GetContext()->GetResourceAllocator(), /*keep_alive_frame_count=*/0);
 
   render_target_cache.Start();
   // Create two render targets of the same exact size/shape. Both should be
@@ -73,10 +73,41 @@ TEST_P(RenderTargetCacheTest, CachesUsedTexturesAcrossFrames) {
   EXPECT_EQ(render_target_cache.CachedTextureCount(), 1u);
 }
 
+TEST_P(RenderTargetCacheTest, CachesUsedTexturesAcrossFramesWithKeepAlive) {
+  auto render_target_cache = RenderTargetCache(
+      GetContext()->GetResourceAllocator(), /*keep_alive_frame_count=*/3);
+
+  render_target_cache.Start();
+  // Create two render targets of the same exact size/shape. Both should be
+  // marked as used this frame, so the cached data set will contain two.
+  render_target_cache.CreateOffscreen(*GetContext(), {100, 100}, 1);
+  render_target_cache.CreateOffscreen(*GetContext(), {100, 100}, 1);
+
+  EXPECT_EQ(render_target_cache.CachedTextureCount(), 2u);
+
+  render_target_cache.End();
+  render_target_cache.Start();
+
+  // The unused texture is kept alive until the keep alive countdown
+  // reaches 0.
+  EXPECT_EQ(render_target_cache.CachedTextureCount(), 2u);
+
+  for (auto i = 0; i < 3; i++) {
+    render_target_cache.Start();
+    render_target_cache.End();
+    EXPECT_EQ(render_target_cache.CachedTextureCount(), 2u);
+  }
+  // After the countdown has elapsed the texture is removed.
+  render_target_cache.Start();
+  render_target_cache.End();
+  EXPECT_EQ(render_target_cache.CachedTextureCount(), 0u);
+}
+
 TEST_P(RenderTargetCacheTest, DoesNotPersistFailedAllocations) {
   ScopedValidationDisable disable;
   auto allocator = std::make_shared<TestAllocator>();
-  auto render_target_cache = RenderTargetCache(allocator);
+  auto render_target_cache =
+      RenderTargetCache(allocator, /*keep_alive_frame_count=*/0);
 
   render_target_cache.Start();
   allocator->should_fail = true;
@@ -89,8 +120,8 @@ TEST_P(RenderTargetCacheTest, DoesNotPersistFailedAllocations) {
 }
 
 TEST_P(RenderTargetCacheTest, CachedTextureGetsNewAttachmentConfig) {
-  auto render_target_cache =
-      RenderTargetCache(GetContext()->GetResourceAllocator());
+  auto render_target_cache = RenderTargetCache(
+      GetContext()->GetResourceAllocator(), /*keep_alive_frame_count=*/0);
 
   render_target_cache.Start();
   RenderTarget::AttachmentConfig color_attachment_config =
@@ -114,8 +145,8 @@ TEST_P(RenderTargetCacheTest, CachedTextureGetsNewAttachmentConfig) {
 }
 
 TEST_P(RenderTargetCacheTest, CreateWithEmptySize) {
-  auto render_target_cache =
-      RenderTargetCache(GetContext()->GetResourceAllocator());
+  auto render_target_cache = RenderTargetCache(
+      GetContext()->GetResourceAllocator(), /*keep_alive_frame_count=*/0);
 
   render_target_cache.Start();
   RenderTarget empty_target =