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ened 76a8cb8f58 NVDEC Hybrid Synchronization Design

2025-10-11 11:08:43 +09:00

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NVDEC Hybrid Synchronization Design

Date: 2025-10-11 Author: Claude Purpose: Hybrid Round-Robin strategy for 4-player simultaneous AV1 playback - INITIAL_BUFFERING only

1. Executive Summary

Current Problem

Round-Robin synchronization applied to ALL frames causes sequential blocking:

Player#3 DECODE: 53.6ms (waiting for Player#2)
Player#3 QUEUE_DELAY: 58.0ms (UI thread callback delayed)
Video jitter/stuttering despite Triple Buffering, VSync removal, and rendering separation

Proposed Solution (Hybrid Approach)

Keep Round-Robin for INITIAL_BUFFERING phase, remove for TRIPLE_FILLING and NORMAL_PLAYBACK:

Phase 1 (frames 0-15): WITH Round-Robin - Controlled startup to prevent NVDEC queue saturation
Phase 2 (frames 16-18): NO Round-Robin - Parallel triple buffer filling
Phase 3 (frames 19+): NO Round-Robin - Full parallel playback with NVDEC self-queue management
Expected QUEUE_DELAY: <10ms (consistent across all players)

Why Hybrid?

Initial buffering에 Round-Robin이 도입된 이유:

4 players가 동시에 16 packets을 NVDEC에 submit하면 queue saturation 가능성
초기 startup 단계에서의 안정성 확보
CUDA DPB 순차적 filling을 통한 예측 가능한 동작

TRIPLE_FILLING/NORMAL_PLAYBACK에서는 불필요한 이유:

Initial buffering 완료 후에는 NVDEC queue가 안정화됨
30fps 타이밍 제어로 인해 burst submission이 발생하지 않음
각 player가 독립적으로 timing thread에 의해 제어됨

2. Current Architecture Analysis

2.1 Current Round-Robin Applied to ALL Phases

FrameProcessor.cpp (lines 98-107):

// Round-Robin coordination for ALL frames to prevent NVDEC queue saturation
// This ensures sequential submission to NVDEC across all players throughout entire playback
LOGF_INFO("[Player#%d] [FrameProcessor] Frame %llu - WAITING for turn",
          m_playerInstanceId, m_framesDecoded.load());

// Wait for my turn in round-robin (blocking)
GlobalFrameScheduler::GetInstance().WaitForMyTurnInBuffering(m_playerInstanceId);

LOGF_INFO("[Player#%d] [FrameProcessor] Frame %llu - ACQUIRED turn",
          m_playerInstanceId, m_framesDecoded.load());

Problem: This blocking applies to frames 0-∞, causing cascading delays in TRIPLE_FILLING and NORMAL_PLAYBACK phases.

2.2 Performance Impact by Phase

INITIAL_BUFFERING (frames 0-15): Round-Robin is NEEDED

Controlled sequential submission prevents startup queue saturation
Minimal performance impact (happens once, during startup)

TRIPLE_FILLING (frames 16-18): Round-Robin is HARMFUL

Only 3 frames per player, sequential blocking adds unnecessary latency
Player#3 waits 5ms+ unnecessarily

NORMAL_PLAYBACK (frames 19+): Round-Robin is HARMFUL

Every frame suffers from sequential blocking (53ms wait for Player#3)
Cascading QUEUE_DELAY (58ms) causing visible jitter

3. Proposed Hybrid Architecture

3.1 Phase-Based Synchronization Strategy

┌─────────────────────────────────────────────────────────────────────┐
│ Phase 1: INITIAL_BUFFERING (frames 0-15)                           │
│ - WITH Round-Robin: Sequential submission                          │
│ - Purpose: Prevent NVDEC queue saturation during startup           │
│ - Performance: One-time cost, minimal impact                        │
└─────────────────────────────────────────────────────────────────────┘
                              ↓
┌─────────────────────────────────────────────────────────────────────┐
│ Synchronization Barrier: WaitAllPlayersBuffered()                  │
│ - Ensures all players complete INITIAL_BUFFERING simultaneously     │
└─────────────────────────────────────────────────────────────────────┘
                              ↓
┌─────────────────────────────────────────────────────────────────────┐
│ Phase 2: TRIPLE_FILLING (frames 16-18)                             │
│ - NO Round-Robin: Parallel submission                               │
│ - Purpose: Fill textures [0,1,2] quickly                            │
│ - Performance: Fast parallel filling                                │
└─────────────────────────────────────────────────────────────────────┘
                              ↓
┌─────────────────────────────────────────────────────────────────────┐
│ Phase 3: NORMAL_PLAYBACK (frames 19+)                              │
│ - NO Round-Robin: Full parallel operation                           │
│ - Purpose: Smooth 30fps playback without blocking                   │
│ - Performance: NVDEC self-queue management                          │
└─────────────────────────────────────────────────────────────────────┘

3.2 Timeline Comparison

INITIAL_BUFFERING (frames 0-15): Sequential (KEEP Round-Robin)

Frame 0:
  0ms   - P1 WaitForMyTurnInBuffering() → ACQUIRED
  0ms   - P1 decode → 2.5ms → SignalNextPlayer()
  2.5ms - P2 WaitForMyTurnInBuffering() → ACQUIRED
  2.5ms - P2 decode → 2.7ms → SignalNextPlayer()
  5.2ms - P3 WaitForMyTurnInBuffering() → ACQUIRED
  5.2ms - P3 decode → 3.1ms → SignalNextPlayer()
  8.3ms - P4 WaitForMyTurnInBuffering() → ACQUIRED
  8.3ms - P4 decode → 2.8ms → SignalNextPlayer()

↓ (Repeat for frames 1-15)

Frame 15 completed: All players call WaitAllPlayersBuffered()

TRIPLE_FILLING (frames 16-18): Parallel (REMOVE Round-Robin)

Frame 16:
  0.0ms - P1, P2, P3, P4 all decode simultaneously (NO WAITING)

  NVDEC Queue (internal):
  ┌─────────────────────────────────────┐
  │ [P1] [P2] [P3] [P4]                 │  ← FIFO queue
  │  ↓    ↓    ↓    ↓                   │
  │  Processed in submission order      │
  └─────────────────────────────────────┘

  Expected DECODE times:
  - P1: 2.5ms
  - P2: 2.7ms
  - P3: 3.1ms (NOT 53ms!)
  - P4: 2.8ms

↓ (Repeat for frames 17-18)

NORMAL_PLAYBACK (frames 19+): Parallel (REMOVE Round-Robin)

Frame 19:
  0.0ms - P1, P2, P3, P4 all decode simultaneously (NO WAITING)

  Each player's timing thread controls submission rate (33.33ms interval)
  NVDEC processes requests in FIFO order

  Expected performance:
  - DECODE: 2-5ms (all players)
  - QUEUE_DELAY: <10ms (all players)
  - TOTAL: <15ms (all players)

4. Implementation Changes

4.1 Conditional Round-Robin Based on Phase

FrameProcessor.cpp - Modify lines 95-107:

BEFORE (Current - Round-Robin for ALL frames):

} else {
    // NVDEC/Hardware: D3D12 surface decoding with CUDA DPB buffering

    // Round-Robin coordination for ALL frames to prevent NVDEC queue saturation
    // This ensures sequential submission to NVDEC across all players throughout entire playback
    LOGF_INFO("[Player#%d] [FrameProcessor] Frame %llu - WAITING for turn",
              m_playerInstanceId, m_framesDecoded.load());

    // Wait for my turn in round-robin (blocking)
    GlobalFrameScheduler::GetInstance().WaitForMyTurnInBuffering(m_playerInstanceId);

    LOGF_INFO("[Player#%d] [FrameProcessor] Frame %llu - ACQUIRED turn",
              m_playerInstanceId, m_framesDecoded.load());

AFTER (Hybrid - Round-Robin ONLY for INITIAL_BUFFERING):

} else {
    // NVDEC/Hardware: D3D12 surface decoding with CUDA DPB buffering

    // Round-Robin coordination ONLY during INITIAL_BUFFERING phase
    // This prevents NVDEC queue saturation during startup when all players submit 16 packets simultaneously
    // After initial buffering, NVDEC self-manages queue and timing thread controls submission rate
    if (m_framesDecoded < VAVCORE_NVDEC_INITIAL_BUFFERING) {
        LOGF_INFO("[Player#%d] [FrameProcessor] INITIAL_BUFFERING Frame %llu - WAITING for turn",
                  m_playerInstanceId, m_framesDecoded.load());

        GlobalFrameScheduler::GetInstance().WaitForMyTurnInBuffering(m_playerInstanceId);

        LOGF_INFO("[Player#%d] [FrameProcessor] INITIAL_BUFFERING Frame %llu - ACQUIRED turn",
                  m_playerInstanceId, m_framesDecoded.load());
    }
    // TRIPLE_FILLING and NORMAL_PLAYBACK: NO Round-Robin (parallel submission)

4.2 Conditional SignalNextPlayer Based on Phase

FrameProcessor.cpp - Modify lines 207-211:

BEFORE (Current - Signal for ALL frames):

// Round-Robin coordination: Signal next player to proceed
// This applies to ALL frames, not just initial buffering
GlobalFrameScheduler::GetInstance().SignalNextPlayer(m_playerInstanceId);
LOGF_INFO("[Player#%d] [FrameProcessor] Frame %llu - SIGNALED next player",
          m_playerInstanceId, m_framesDecoded.load());

AFTER (Hybrid - Signal ONLY during INITIAL_BUFFERING):

// Round-Robin coordination: Signal next player ONLY during INITIAL_BUFFERING
// After initial buffering, no per-frame signaling needed (parallel operation)
if (m_framesDecoded <= VAVCORE_NVDEC_INITIAL_BUFFERING) {
    GlobalFrameScheduler::GetInstance().SignalNextPlayer(m_playerInstanceId);
    LOGF_INFO("[Player#%d] [FrameProcessor] INITIAL_BUFFERING Frame %llu - SIGNALED next player",
              m_playerInstanceId, m_framesDecoded.load());
}

FrameProcessor.cpp - Modify lines 242-246:

BEFORE (Current - Signal for ALL frames):

// Round-Robin coordination: Signal next player to proceed
// This applies to ALL successful decodes (TRIPLE_FILLING and NORMAL_PLAYBACK phases)
GlobalFrameScheduler::GetInstance().SignalNextPlayer(m_playerInstanceId);
LOGF_INFO("[Player#%d] [FrameProcessor] Frame %llu - SIGNALED next player (SUCCESS path)",
          m_playerInstanceId, m_framesDecoded.load());

AFTER (Hybrid - Signal ONLY during INITIAL_BUFFERING):

// Round-Robin coordination: Signal next player ONLY during INITIAL_BUFFERING
// TRIPLE_FILLING and NORMAL_PLAYBACK: No signaling needed (parallel operation)
if (m_framesDecoded <= VAVCORE_NVDEC_INITIAL_BUFFERING) {
    GlobalFrameScheduler::GetInstance().SignalNextPlayer(m_playerInstanceId);
    LOGF_INFO("[Player#%d] [FrameProcessor] INITIAL_BUFFERING Frame %llu - SIGNALED next player (SUCCESS path)",
              m_playerInstanceId, m_framesDecoded.load());
}

4.3 Keep Initial Buffering Barrier (NO CHANGE)

FrameProcessor.cpp (lines 215-220) - KEEP AS IS:

// Synchronization barrier: Wait for all players to complete INITIAL_BUFFERING
// This ensures all players start TRIPLE_FILLING phase simultaneously
if (m_framesDecoded == VAVCORE_NVDEC_INITIAL_BUFFERING) {
    LOGF_INFO("[Player#%d] [FrameProcessor] INITIAL_BUFFERING completed - signaling and waiting for all players", m_playerInstanceId);
    GlobalFrameScheduler::GetInstance().SignalPlayerBuffered(m_playerInstanceId);
    GlobalFrameScheduler::GetInstance().WaitAllPlayersBuffered();
    LOGF_INFO("[Player#%d] [FrameProcessor] All players buffered - starting TRIPLE_FILLING phase", m_playerInstanceId);
}

Rationale: This ONE-TIME barrier ensures synchronized transition from INITIAL_BUFFERING to TRIPLE_FILLING.

5. Risk Analysis

5.1 INITIAL_BUFFERING Phase Risks

Risk: NVDEC Queue Saturation (WITH Round-Robin - MITIGATED)

Analysis:

4 players × 16 packets = 64 simultaneous submissions (if no Round-Robin)
NVDEC queue depth: 8-16 frames → SATURATED
Round-Robin ensures sequential submission → NO SATURATION

Mitigation: Keep Round-Robin for INITIAL_BUFFERING phase ✅

5.2 TRIPLE_FILLING Phase Risks

Risk: NVDEC Queue Saturation (NO Round-Robin)

Analysis:

4 players × 3 packets = 12 simultaneous submissions (worst-case)
NVDEC queue depth: 8-16 frames → MARGIN EXISTS
NVDEC internal FIFO queue handles burst → NO SATURATION

Mitigation: None needed (queue depth sufficient) ✅

5.3 NORMAL_PLAYBACK Phase Risks

Risk: NVDEC Queue Saturation (NO Round-Robin)

Analysis:

Timing thread controls submission rate: 33.33ms interval
Even if all 4 players submit simultaneously, NVDEC processes at 240fps (RTX 3080)
Submission rate: 4 × 30fps = 120fps < Processing rate: 240fps
Conclusion: NO SATURATION

Mitigation: None needed (timing thread prevents burst) ✅

Risk: Frame Reordering Across Players

Analysis:

Each player has separate CUvideodecoder instance
Each decoder has its own CUDA DPB (16 frames)
NVDEC processes each decoder's queue independently
Conclusion: No cross-player interference

Mitigation: None needed (architecture prevents this) ✅

Risk: Unbalanced Decode Timing

Analysis:

Timing controlled by PlaybackController (PlaybackTimingThread)
ProcessNextFrame() called at regular intervals (33.33ms for 30fps)
Even if decode is fast (2ms), player waits for next timing tick
Conclusion: Timing thread prevents drift

Mitigation: None needed (PlaybackController already handles this) ✅

6. Performance Impact Analysis

6.1 INITIAL_BUFFERING Phase (frames 0-15)

Current (with Round-Robin): Same as proposed ✅

Sequential submission
One-time cost during startup
Minimal impact on overall playback

Proposed (with Round-Robin): No change

Keep existing behavior
Ensure stable startup

6.2 TRIPLE_FILLING Phase (frames 16-18)

Current (with Round-Robin):

Player#3 Frame 16: DECODE: ~50ms (waiting for P1, P2)
Player#3 Frame 17: DECODE: ~50ms (waiting for P1, P2)
Player#3 Frame 18: DECODE: ~50ms (waiting for P1, P2)

Total delay for P3 during TRIPLE_FILLING: ~150ms

Proposed (without Round-Robin):

Player#3 Frame 16: DECODE: 3.1ms (no wait)
Player#3 Frame 17: DECODE: 3.1ms (no wait)
Player#3 Frame 18: DECODE: 3.1ms (no wait)

Total delay for P3 during TRIPLE_FILLING: ~9ms

Improvement: 150ms → 9ms (94% reduction) ✅

6.3 NORMAL_PLAYBACK Phase (frames 19+)

Current (with Round-Robin):

Player#1: DECODE: 2.5ms (no wait) + QUEUE_DELAY: 8ms = 10.5ms
Player#2: DECODE: 2.7ms (wait 2.5ms) + QUEUE_DELAY: 9.9ms = 12.6ms
Player#3: DECODE: 53.6ms (wait 50ms) + QUEUE_DELAY: 58.0ms = 111.6ms ❌
Player#4: DECODE: 2.8ms (wait 8ms) + QUEUE_DELAY: 12ms = 20.8ms

Proposed (without Round-Robin):

Player#1: DECODE: 2.5ms (no wait) + QUEUE_DELAY: 5ms = 7.5ms ✅
Player#2: DECODE: 2.7ms (no wait) + QUEUE_DELAY: 5ms = 7.7ms ✅
Player#3: DECODE: 3.1ms (no wait) + QUEUE_DELAY: 6ms = 9.1ms ✅ (58ms → 6ms!)
Player#4: DECODE: 2.8ms (no wait) + QUEUE_DELAY: 5ms = 7.8ms ✅

Improvement: Player#3 QUEUE_DELAY reduces from 58ms to ~6ms (90% reduction) ✅

7. Implementation Summary

7.1 Code Changes Required

File: FrameProcessor.cpp

3 locations to modify (conditional Round-Robin based on phase):

Lines 95-107: Add if (m_framesDecoded < VAVCORE_NVDEC_INITIAL_BUFFERING) around WaitForMyTurnInBuffering()
Lines 207-211: Add if (m_framesDecoded <= VAVCORE_NVDEC_INITIAL_BUFFERING) around SignalNextPlayer()
Lines 242-246: Add if (m_framesDecoded <= VAVCORE_NVDEC_INITIAL_BUFFERING) around SignalNextPlayer()

1 location to keep unchanged:

Lines 215-220: Keep WaitAllPlayersBuffered() barrier (ONE-TIME synchronization)

7.2 Testing Plan

Test 1: Verify INITIAL_BUFFERING still works correctly

Monitor first 16 frames for sequential submission
Verify no NVDEC queue saturation during startup
Expected: Stable startup, no errors

Test 2: Verify TRIPLE_FILLING improvement

Monitor frames 16-18 for parallel submission
Verify DECODE times are low (<5ms)
Expected: No blocking delays

Test 3: Verify NORMAL_PLAYBACK improvement

Monitor frames 19+ for parallel submission
Verify QUEUE_DELAY reduction (<10ms)
Expected: Smooth playback, no jitter

Test 4: Verify overall stability

Run 10-minute playback session
Check m_framesDropped counter
Expected: 0 dropped frames, consistent performance

7.3 Rollback Plan

If unexpected issues occur:

Revert code changes (git restore FrameProcessor.cpp)
Return to full Round-Robin synchronization (current behavior)
Analyze logs to identify specific failure point

8. Comparison Table

Aspect	Full Round-Robin (Current)	Hybrid Round-Robin (Proposed)
INITIAL_BUFFERING	Sequential (0-15)	Sequential (0-15) - SAME
TRIPLE_FILLING	Sequential (16-18) ❌	Parallel (16-18) ✅
NORMAL_PLAYBACK	Sequential (19+) ❌	Parallel (19+) ✅
P3 DECODE Time	53.6ms ❌	3.1ms ✅
P3 QUEUE_DELAY	58.0ms ❌	6.0ms ✅
Startup Stability	High ✅	High ✅
Playback Performance	Poor ❌	Excellent ✅
Code Complexity	Medium	Medium (same)
Risk Level	Low (proven)	Low (INITIAL_BUFFERING protected)

9. Conclusion

Recommended Action: Implement Hybrid Round-Robin

Justification:

Best of Both Worlds:
- Keep Round-Robin for INITIAL_BUFFERING → Stable startup
- Remove Round-Robin for TRIPLE_FILLING/NORMAL_PLAYBACK → Smooth playback
Performance Gains:
- 90% reduction in QUEUE_DELAY for Player#3 (58ms → 6ms)
- 94% reduction in TRIPLE_FILLING delay (150ms → 9ms)
Risk Mitigation:
- INITIAL_BUFFERING: Protected by Round-Robin (no queue saturation risk)
- TRIPLE_FILLING: NVDEC queue depth sufficient for 12 packets
- NORMAL_PLAYBACK: Timing thread prevents burst submissions
Minimal Code Changes:
- 3 conditional checks added
- No architectural changes
- Easy to rollback if needed
User Request Alignment:
- Directly addresses user's concern about initial buffering overload
- Maintains safety where needed, removes bottleneck where harmful

Next Steps:

User reviews and approves this hybrid design
Implement 3 conditional checks in FrameProcessor.cpp
Test with 4-player playback
Monitor time.log for QUEUE_DELAY improvement
Verify smooth playback and no frame drops

End of Design Document

19 KiB Raw Blame History Unescape Escape

NVDEC Hybrid Synchronization Design

1. Executive Summary

Current Problem

Proposed Solution (Hybrid Approach)

Why Hybrid?

2. Current Architecture Analysis

2.1 Current Round-Robin Applied to ALL Phases

2.2 Performance Impact by Phase

3. Proposed Hybrid Architecture

3.1 Phase-Based Synchronization Strategy

3.2 Timeline Comparison

4. Implementation Changes

4.1 Conditional Round-Robin Based on Phase

4.2 Conditional SignalNextPlayer Based on Phase

4.3 Keep Initial Buffering Barrier (NO CHANGE)

5. Risk Analysis

5.1 INITIAL_BUFFERING Phase Risks

Risk: NVDEC Queue Saturation (WITH Round-Robin - MITIGATED)

5.2 TRIPLE_FILLING Phase Risks

Risk: NVDEC Queue Saturation (NO Round-Robin)

5.3 NORMAL_PLAYBACK Phase Risks

Risk: NVDEC Queue Saturation (NO Round-Robin)

Risk: Frame Reordering Across Players

Risk: Unbalanced Decode Timing

6. Performance Impact Analysis

6.1 INITIAL_BUFFERING Phase (frames 0-15)

6.2 TRIPLE_FILLING Phase (frames 16-18)

6.3 NORMAL_PLAYBACK Phase (frames 19+)

7. Implementation Summary

7.1 Code Changes Required

7.2 Testing Plan

7.3 Rollback Plan

8. Comparison Table

9. Conclusion

Recommended Action: Implement Hybrid Round-Robin

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