CompositorBase: Reimplement frame events with a ring buffer.

Restricts the maximum number of pipelined frames, but it gets rid of the locks and simplifies the code greatly.

Author: CrossVR

Revive/CompositorBase.cpp

@@ -61,16 +61,21 @@ CompositorBase::CompositorBase()
 , m_MirrorTexture(nullptr)
 , m_OverlayCount(0)
 , m_ActiveOverlays()
-, m_FrameMutex()
-, m_FrameEvent()
+, m_FrameEvents()
 {
 // We want to handle all graphics tasks explicitly instead of implicitly letting WaitGetPoses execute them
 vr::VRCompositor()->SetExplicitTimingMode(vr::VRCompositorTimingMode_Explicit_ApplicationPerformsPostPresentHandoff);
-m_FrameEvent[0] = CreateEvent(nullptr, true, true, nullptr);
+
+// Create a ring buffer of events
+for (int i = 0; i < MAX_QUEUE_AHEAD; i++)
+m_FrameEvents[i] = CreateEvent(nullptr, true, i == 0, nullptr);
 }
 
 CompositorBase::~CompositorBase()
 {
+for (int i = 0; i < MAX_QUEUE_AHEAD; i++)
+CloseHandle(m_FrameEvents[i]);
+
 if (m_MirrorTexture)
 delete m_MirrorTexture;
 }
@@ -121,22 +126,9 @@ ovrResult CompositorBase::WaitToBeginFrame(ovrSession session, long long frameIn
 {
 MICROPROFILE_SCOPE(WaitToBeginFrame);
 
-// Find the event for the last frame
-void* handle = nullptr;
-{
-std::unique_lock<std::mutex> lk(m_FrameMutex);
-handle = m_FrameEvent[frameIndex - 1];
-
-// If the last frame hasn't started yet, create a new event object for it
-if (!handle)
-{
-handle = CreateEvent(nullptr, true, false, nullptr);
-m_FrameEvent[frameIndex - 1] = handle;
-}
-}
-
 // Wait on the last frame completion with a 500ms timeout
-bool timeout = WaitForSingleObject(handle, 500) != WAIT_OBJECT_0;
+assert(frameIndex - session->FrameIndex < MAX_QUEUE_AHEAD);
+bool timeout = WaitForSingleObject(m_FrameEvents[(frameIndex - 1) % MAX_QUEUE_AHEAD], 500) != WAIT_OBJECT_0;
 
 // Wait for the actual frame start
 if (!session->Details->UseHack(SessionDetails::HACK_WAIT_ON_SUBMIT))
@@ -151,34 +143,8 @@ ovrResult CompositorBase::BeginFrame(ovrSession session, long long frameIndex)
 {
 MICROPROFILE_SCOPE(BeginFrame);
 
-{
-std::unique_lock<std::mutex> lk(m_FrameMutex);
-
-// Find the event from the last frame
-auto it = m_FrameEvent.find(frameIndex - 1);
-
-if (m_FrameEvent[frameIndex])
-{
-// If this frame already has an event, close the last one if it exists
-if (it != m_FrameEvent.end())
-{
-CloseHandle(it->second);
-m_FrameEvent.erase(it);
-}
-}
-else if (it != m_FrameEvent.end())
-{
-// Reset the event from the last frame and reuse it for the next frame
-ResetEvent(it->second);
-m_FrameEvent[frameIndex] = it->second;
-m_FrameEvent.erase(it);
-}
-else
-{
-// Create a new event for this frame
-m_FrameEvent[frameIndex] = CreateEvent(nullptr, true, false, nullptr);
-}
-}
+// Reset the event in the frame ring buffer
+ResetEvent(m_FrameEvents[frameIndex % MAX_QUEUE_AHEAD]);
 
 session->FrameIndex = frameIndex;
 vr::VRCompositor()->SubmitExplicitTimingData();
@@ -289,14 +255,7 @@ ovrResult CompositorBase::EndFrame(ovrSession session, long long frameIndex, ovr
 }
 
 // Frame now completed so we can let anyone waiting on the next frame call WaitGetPoses
-{
-std::unique_lock<std::mutex> lk(m_FrameMutex);
-void* handle = m_FrameEvent[frameIndex];
-if (handle)
-SetEvent(handle);
-else
-m_FrameEvent[frameIndex] = CreateEvent(nullptr, true, true, nullptr);
-}
+SetEvent(m_FrameEvents[frameIndex % MAX_QUEUE_AHEAD]);
 
 if (m_MirrorTexture && error == vr::VRCompositorError_None)
 RenderMirrorTexture(m_MirrorTexture);

Revive/CompositorBase.h

@@ -5,8 +5,8 @@
 
 #include <openvr.h>
 #include <vector>
-#include <mutex>
-#include <map>
+
+#define MAX_QUEUE_AHEAD 5
 
 class CompositorBase
 {
@@ -51,6 +51,5 @@ class CompositorBase
 std::vector<vr::VROverlayHandle_t> m_ActiveOverlays;
 
 // Call order enforcement
-std::mutex m_FrameMutex;
-std::map<long long, void*> m_FrameEvent;
+void* m_FrameEvents[MAX_QUEUE_AHEAD];
 };