Add closed-loop latency control for Vulkan vsync

Depends on VK_KHR_present_wait.

Source/Core/Core/CoreTiming.cpp

@@ -26,6 +26,9 @@
 #include "VideoCommon/VideoBackendBase.h"
 #include "VideoCommon/VideoConfig.h"
 
+// We should factor out video backpressure into a shared_ptr<PendingTimeOffset> later.
+#include "VideoCommon/PendingTimeOffset.h"
+
 namespace CoreTiming
 {
 // Sort by time, unless the times are the same, in which case sort by the order added to the queue
@@ -346,6 +349,8 @@ void CoreTimingManager::Advance()
 
 void CoreTimingManager::Throttle(const s64 target_cycle)
 {
+  using namespace std::chrono;
+
   // Based on number of cycles and emulation speed, increase the target deadline
   const s64 cycles = target_cycle - m_throttle_last_cycle;
 
@@ -358,9 +363,38 @@ void CoreTimingManager::Throttle(const s64 target_cycle)
   const double speed =
       Core::GetIsThrottlerTempDisabled() ? 0.0 : Config::Get(Config::MAIN_EMULATION_SPEED);
 
-  if (0.0 < speed)
-    m_throttle_deadline +=
-        std::chrono::duration_cast<DT>(DT_s(cycles) / (speed * m_throttle_clock_per_sec));
+  DT applied_offset = DT::zero();
+
+  if (0.0 < speed) {
+    ASSERT(cycles >= 0);
+
+    const DT sleep_dur = std::chrono::duration_cast<DT>(DT_s(cycles) / (speed * m_throttle_clock_per_sec));
+
+    DT pending_offset;
+    {
+      std::unique_lock lk{s_pending_time_offset.Lock};
+      pending_offset = s_pending_time_offset.Offset__;
+    }
+    if (pending_offset == DT::zero()) {
+      m_throttle_deadline += sleep_dur;
+    } else {
+      // The clocks used to measure vkWaitForPresentKHR() are imprecise and noisy.
+      // We want to exponentially approach vsync at a velocity proportional to
+      // remaining distance, with a time constant equal to 100ms.
+      double velocity = duration_cast<DT_us>(pending_offset) / DT_us(100'000);
+
+      // Limit velocity to at most 0.5ms per (frame≈16ms).
+      constexpr double MAX_VELOCITY = 1. / 32.;
+      velocity = std::clamp(velocity, -MAX_VELOCITY, MAX_VELOCITY);
+
+      constexpr DT MAX_FRAME_DUR = duration_cast<DT>(DT_ms(16));
+
+      // d=vt. Clamp t <= 1 frame to avoid unusual cases.
+      const DT_us distance = velocity * std::min(sleep_dur, MAX_FRAME_DUR);
+      applied_offset = duration_cast<DT>(distance);
+      m_throttle_deadline += sleep_dur + applied_offset;
+    }
+  }
 
   // A maximum fallback is used to prevent the system from sleeping for
   // too long or going full speed in an attempt to catch up to timings.
@@ -398,6 +432,11 @@ void CoreTimingManager::Throttle(const s64 target_cycle)
     const TimePoint time_after_sleep = Clock::now();
     g_perf_metrics.CountThrottleSleep(time_after_sleep - time);
   }
+
+  if (applied_offset != DT::zero()) {
+    std::unique_lock lk{s_pending_time_offset.Lock};
+    s_pending_time_offset.Offset__ -= applied_offset;
+  }
 }
 
 void CoreTimingManager::ResetThrottle(s64 cycle)

Source/Core/VideoBackends/Vulkan/CommandBufferManager.cpp

@@ -16,9 +16,13 @@
 
 namespace Vulkan
 {
-CommandBufferManager::CommandBufferManager(bool use_threaded_submission)
-    : m_use_threaded_submission(use_threaded_submission)
+
+CommandBufferManager::CommandBufferManager(bool use_threaded_submission, bool vsync)
+  : m_vsync(vsync)
+  , m_use_threaded_submission(use_threaded_submission)
 {
+  std::unique_lock lk{s_pending_time_offset.Lock};
+  s_pending_time_offset.Offset__ = DT::zero();
 }
 
 CommandBufferManager::~CommandBufferManager()
@@ -223,7 +227,7 @@ VkDescriptorSet CommandBufferManager::AllocateDescriptorSet(VkDescriptorSetLayou
 bool CommandBufferManager::CreateSubmitThread()
 {
   m_submit_thread.Reset("VK submission thread", [this](PendingCommandBufferSubmit submit) {
-    SubmitCommandBuffer(submit.command_buffer_index, submit.present_swap_chain,
+    SubmitCommandBuffer(submit.command_buffer_index, submit.present_swap_chain, submit.vsync,
                         submit.present_image_index, submit.frame_id);
     CmdBufferResources& resources = m_command_buffers[submit.command_buffer_index];
     resources.waiting_for_submit.store(false, std::memory_order_release);
@@ -329,14 +333,14 @@ void CommandBufferManager::SubmitCommandBuffer(bool submit_on_worker_thread,
   {
     resources.waiting_for_submit.store(true, std::memory_order_relaxed);
     // Push to the pending submit queue.
-    m_submit_thread.Push({present_swap_chain, present_image_index, m_current_cmd_buffer, frame_id});
+    m_submit_thread.Push({present_swap_chain, m_vsync, present_image_index, m_current_cmd_buffer, frame_id});
   }
   else
   {
     WaitForWorkerThreadIdle();
 
     // Pass through to normal submission path.
-    SubmitCommandBuffer(m_current_cmd_buffer, present_swap_chain, present_image_index, frame_id);
+    SubmitCommandBuffer(m_current_cmd_buffer, present_swap_chain, m_vsync, present_image_index, frame_id);
     if (wait_for_completion)
       WaitForCommandBufferCompletion(m_current_cmd_buffer);
   }
@@ -389,6 +393,7 @@ void CommandBufferManager::SubmitCommandBuffer(bool submit_on_worker_thread,
 
 void CommandBufferManager::SubmitCommandBuffer(u32 command_buffer_index,
                                                VkSwapchainKHR present_swap_chain,
+                                               bool vsync,
                                                u32 present_image_index, u64 frame_id)
 {
   CmdBufferResources& resources = m_command_buffers[command_buffer_index];
@@ -451,7 +456,7 @@ void CommandBufferManager::SubmitCommandBuffer(u32 command_buffer_index,
     if (g_vulkan_context->SupportsPresentWait())
     {
       present_info.pNext = &present_id;
-      PresentQueued(frame_id, present_swap_chain);
+      PresentQueued(frame_id, present_swap_chain, vsync);
     }
 
     m_last_present_result = vkQueuePresentKHR(g_vulkan_context->GetPresentQueue(), &present_info);

Source/Core/VideoBackends/Vulkan/CommandBufferManager.h

@@ -14,23 +14,28 @@
 #include <utility>
 #include <vector>
 
+#include <Common/CommonTypes.h>
 #include <Common/WorkQueueThread.h>
 #include "Common/BlockingLoop.h"
 #include "Common/Flag.h"
 #include "Common/Semaphore.h"
 
+#include "VideoCommon/PendingTimeOffset.h"
 #include "VideoBackends/Vulkan/Constants.h"
 
 namespace Vulkan
 {
+
 class CommandBufferManager
 {
 public:
-  explicit CommandBufferManager(bool use_threaded_submission);
+  explicit CommandBufferManager(bool use_threaded_submission, bool vsync);
   ~CommandBufferManager();
 
   bool Initialize();
 
+  void SetVSync(bool vsync) { m_vsync = vsync; }
+
   // These command buffers are allocated per-frame. They are valid until the command buffer
   // is submitted, after that you should call these functions again.
   VkCommandBuffer GetCurrentInitCommandBuffer()
@@ -102,6 +107,7 @@ private:
 
   void WaitForCommandBufferCompletion(u32 command_buffer_index);
   void SubmitCommandBuffer(u32 command_buffer_index, VkSwapchainKHR present_swap_chain,
+                           bool vsync,
                            u32 present_image_index, u64 frame_id);
   void BeginCommandBuffer();
 
@@ -138,6 +144,7 @@ private:
     return m_command_buffers[m_current_cmd_buffer];
   }
 
+  bool m_vsync;
   u64 m_next_fence_counter = 1;
   u64 m_completed_fence_counter = 0;
 
@@ -150,6 +157,7 @@ private:
   struct PendingCommandBufferSubmit
   {
     VkSwapchainKHR present_swap_chain;
+    bool vsync;
     u32 present_image_index;
     u32 command_buffer_index;
     u64 frame_id;

Source/Core/VideoBackends/Vulkan/PresentWait.cpp

@@ -8,6 +8,7 @@
 #include <tuple>
 
 #include "Common/WorkQueueThread.h"
+#include "VideoCommon/PendingTimeOffset.h"
 
 #include "VideoBackends/Vulkan/VulkanContext.h"
 #include "VideoBackends/Vulkan/VulkanLoader.h"
@@ -22,17 +23,25 @@ struct Wait
 {
   u64 present_id;
   VkSwapchainKHR swapchain;
+  bool vsync;
 };
 
 static Common::WorkQueueThread<Wait> s_present_wait_thread;
 
-void WaitFunction(Wait wait)
+static void WaitFunction(Wait wait)
 {
+  using namespace std::chrono;
+
   do
   {
+    const TimePoint begin = Clock::now();
+
     // We choose a timeout of 20ms so can poll for IsFlushing
     VkResult res = vkWaitForPresentKHR(s_device, wait.swapchain, wait.present_id, 20'000'000);
 
+    const TimePoint end = Clock::now();
+    DT block_dur = end - begin;
+
     if (res == VK_TIMEOUT)
     {
       WARN_LOG_FMT(VIDEO, "vkWaitForPresentKHR timed out, retrying {}", wait.present_id);
@@ -47,6 +56,20 @@ void WaitFunction(Wait wait)
     if (res == VK_SUCCESS)
       g_perf_metrics.CountPresent();
 
+    if (wait.vsync)
+    {
+      block_dur = std::min(block_dur, duration_cast<DT>(DT_us(2 * TARGET_VSYNC_BLOCK_US)));
+
+      std::unique_lock lk{s_pending_time_offset.Lock};
+
+      // pray DT is signed.
+      s_pending_time_offset.Offset__ =
+        block_dur - duration_cast<DT>(DT_us(TARGET_VSYNC_BLOCK_US));
+    } else {
+      std::unique_lock lk{s_pending_time_offset.Lock};
+      s_pending_time_offset.Offset__ = DT::zero();
+    }
+
     return;
   } while (!s_present_wait_thread.IsCancelling());
 }
@@ -62,9 +85,9 @@ void StopPresentWaitThread()
   s_present_wait_thread.Shutdown();
 }
 
-void PresentQueued(u64 present_id, VkSwapchainKHR swapchain)
+void PresentQueued(u64 present_id, VkSwapchainKHR swapchain, bool vsync)
 {
-  s_present_wait_thread.EmplaceItem(Wait{present_id, swapchain});
+  s_present_wait_thread.EmplaceItem(Wait{present_id, swapchain, vsync});
 }
 
 void FlushPresentWaitQueue()

Source/Core/VideoBackends/Vulkan/PresentWait.h

@@ -12,7 +12,7 @@ namespace Vulkan
 void StartPresentWaitThread();
 void StopPresentWaitThread();
 
-void PresentQueued(u64 present_id, VkSwapchainKHR swapchain);
+void PresentQueued(u64 present_id, VkSwapchainKHR swapchain, bool vsync);
 void FlushPresentWaitQueue();
 
 }  // namespace Vulkan

Source/Core/VideoBackends/Vulkan/VKGfx.cpp

@@ -395,6 +395,7 @@ void VKGfx::OnConfigChanged(u32 bits)
   if (m_swap_chain && (bits & CONFIG_CHANGE_BIT_VSYNC))
   {
     ExecuteCommandBuffer(false, true);
+    g_command_buffer_mgr->SetVSync(g_ActiveConfig.bVSyncActive);
     m_swap_chain->SetVSync(g_ActiveConfig.bVSyncActive);
   }
 

Source/Core/VideoBackends/Vulkan/VKMain.cpp

@@ -198,7 +198,7 @@ bool VideoBackend::Initialize(const WindowSystemInfo& wsi)
   UpdateActiveConfig();
 
   // Create command buffers. We do this separately because the other classes depend on it.
-  g_command_buffer_mgr = std::make_unique<CommandBufferManager>(g_Config.bBackendMultithreading);
+  g_command_buffer_mgr = std::make_unique<CommandBufferManager>(g_Config.bBackendMultithreading, g_ActiveConfig.bVSyncActive);
   if (!g_command_buffer_mgr->Initialize())
   {
     PanicAlertFmt("Failed to create Vulkan command buffers");

Source/Core/VideoCommon/CMakeLists.txt

@@ -106,6 +106,8 @@ add_library(videocommon
   OnScreenUIKeyMap.h
   OpcodeDecoding.cpp
   OpcodeDecoding.h
+  PendingTimeOffset.cpp
+  PendingTimeOffset.h
   PerfQueryBase.cpp
   PerfQueryBase.h
   PerformanceMetrics.cpp

Source/Core/VideoCommon/PendingTimeOffset.cpp

@@ -0,0 +1,4 @@
+#include "PendingTimeOffset.h"
+
+// export
+PendingTimeOffset s_pending_time_offset;

Source/Core/VideoCommon/PendingTimeOffset.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <mutex>
+
+#include "Common/CommonTypes.h"
+
+constexpr u64 TARGET_VSYNC_BLOCK_US = 2'000;
+
+struct PendingTimeOffset
+{
+  std::mutex Lock;
+  DT Offset__;
+};
+
+extern PendingTimeOffset s_pending_time_offset;