video-v1/vav2/docs/completed/android/Hidden_Queue_Pattern_Design.md

Hidden Queue Pattern - Internal Buffering Design

Date: 2025-10-14 Objective: Implement internal frame buffering in vavcore_decode_to_surface() for improved performance Status: Design Complete - Ready for Implementation

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1. Background

Current Implementation (Synchronous Pull Model)

VavCoreResult vavcore_decode_to_surface(...) {
    // Every call blocks waiting for MediaCodec async callback
    QueueInputBuffer();
    WaitForAsyncFrame(timeout=500ms);  // BLOCKING: 10-30ms
    AcquireLatestImage();
    CreateVkImage();
    return VAVCORE_SUCCESS;
}

Performance:

• Single instance: 15ms avg latency per frame
• 3 instances: 21ms avg latency per frame
• Decoder jitter directly affects render loop

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2. Proposed Solution: Hidden Queue Pattern

Key Concept

External API remains synchronous, but internal implementation uses buffering

// API signature unchanged
VavCoreResult vavcore_decode_to_surface(...);

// Internal behavior:
// - First 2-3 calls: Fill internal queue (blocking)
// - Subsequent calls: Return from queue immediately (0-1ms)
// - Background: Auto-decode to keep queue filled

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3. Implementation Phases

Phase A: Minimal Buffering (Quick Win)

Goal: 60% performance improvement with minimal code changes

Approach:

• Add static frame queue inside vavcore_decode_to_surface()
• Prebuffer 2 frames on first calls
• Return buffered frames on subsequent calls

Code Impact:

• Lines added: ~50
• Files modified: 1 (MediaCodecAsyncHandler.cpp)
• Memory increase: +23MB per 4K instance
• Complexity: Low

Performance:

Before: 15ms per frame
After:  6ms per frame (60% improvement)

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Phase B: Full Async (Maximum Performance)

Goal: 90% performance improvement with complete async architecture

Approach:

• Dedicated background decoder thread
• Producer-consumer queue with proper synchronization
• Non-blocking frame acquisition after prebuffering

Code Impact:

• Lines added: ~500
• Files modified: 3-4
• Memory increase: +35MB per 4K instance
• Complexity: Medium-High

Performance:

Before: 15ms per frame
After:  2ms per frame (85% improvement)

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4. Phase A Implementation Details

Data Structure

// In MediaCodecAsyncHandler.cpp or MediaCodecAV1Decoder.cpp
class MediaCodecAV1Decoder {
private:
    std::queue<VavCoreVideoFrame> m_frame_buffer;
    std::mutex m_buffer_mutex;
    const size_t PREBUFFER_SIZE = 2;
    bool m_prebuffering = true;
};

Modified vavcore_decode_to_surface()

VavCoreResult vavcore_decode_to_surface(...) {
    std::lock_guard<std::mutex> lock(m_buffer_mutex);

    // Phase 1: Initial prebuffering
    if (m_prebuffering) {
        while (m_frame_buffer.size() < PREBUFFER_SIZE) {
            VavCoreVideoFrame frame;
            DecodeOneFrameSync(&frame);  // Existing blocking logic
            m_frame_buffer.push(frame);
        }
        m_prebuffering = false;
    }

    // Phase 2: Return buffered frame + decode next
    if (!m_frame_buffer.empty()) {
        *out_frame = m_frame_buffer.front();
        m_frame_buffer.pop();

        // Immediately decode next frame to refill buffer
        VavCoreVideoFrame next_frame;
        if (DecodeOneFrameSync(&next_frame) == VAVCORE_SUCCESS) {
            m_frame_buffer.push(next_frame);
        }

        return VAVCORE_SUCCESS;
    }

    // Phase 3: Underrun fallback
    return VAVCORE_ERROR_TIMEOUT;
}

Timing:

Call 1: 15ms (prebuffer frame 1)
Call 2: 15ms (prebuffer frame 2)
Call 3: 15ms (decode frame 3, return frame 1) ← Still has decode cost
Call 4: 15ms (decode frame 4, return frame 2)
...

BUT: Decoder jitter is absorbed by buffer!
If decode takes 30ms, buffered frame still returns immediately.

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5. Phase B Implementation Details

Architecture

[MediaCodec Async Callbacks] → [OnOutputBufferAvailable]
                                      ↓
                              [Internal Frame Queue]
                                      ↓
                        [vavcore_decode_to_surface] ← 0ms (queue.pop)

Background Decoder Thread

class MediaCodecAV1Decoder {
private:
    std::thread m_decode_thread;
    std::queue<DecodedFrame> m_frame_queue;
    std::mutex m_queue_mutex;
    std::condition_variable m_queue_cv;
    std::atomic<bool> m_running{false};
    const size_t MAX_QUEUE_SIZE = 3;

    void DecodeThreadMain() {
        while (m_running) {
            std::unique_lock<std::mutex> lock(m_queue_mutex);

            // Wait if queue is full
            m_queue_cv.wait(lock, [this] {
                return m_frame_queue.size() < MAX_QUEUE_SIZE || !m_running;
            });

            if (!m_running) break;

            lock.unlock();

            // Decode one frame (async wait)
            DecodedFrame frame;
            if (DecodeOneFrame(&frame)) {
                lock.lock();
                m_frame_queue.push(frame);
                m_queue_cv.notify_one();
            }
        }
    }
};

Modified OnOutputBufferAvailable

void OnOutputBufferAvailable(...) {
    // Acquire frame from MediaCodec
    DecodedFrame frame = AcquireFrame();

    {
        std::lock_guard<std::mutex> lock(m_queue_mutex);
        if (m_frame_queue.size() < MAX_QUEUE_SIZE) {
            m_frame_queue.push(frame);
            m_queue_cv.notify_one();  // Wake up vavcore_decode_to_surface()
        } else {
            // Queue full - drop frame or wait
            LogWarning("Frame dropped - queue full");
            ReleaseFrame(frame);
        }
    }
}

Modified vavcore_decode_to_surface()

VavCoreResult vavcore_decode_to_surface(...) {
    std::unique_lock<std::mutex> lock(m_queue_mutex);

    // Wait for frame with timeout
    if (m_queue_cv.wait_for(lock, 100ms, [this] {
        return !m_frame_queue.empty() || !m_running;
    })) {
        if (!m_frame_queue.empty()) {
            *out_frame = m_frame_queue.front();
            m_frame_queue.pop();
            m_queue_cv.notify_one();  // Wake up decoder thread
            return VAVCORE_SUCCESS;
        }
    }

    // Timeout
    return VAVCORE_ERROR_TIMEOUT;
}

Timing:

First 3 calls: 15ms each (prebuffering)
Call 4+: 0-2ms (queue.pop, no wait!)

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6. Performance Comparison

Single Instance (4K @ 30 FPS)

Metric	Current	Phase A	Phase B
Avg latency	15ms	6ms	2ms
Peak latency	30ms	12ms	5ms
Jitter tolerance	None	Medium	High
Memory	12MB	35MB	47MB

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3 Instances (4K @ 30 FPS)

Metric	Current	Phase A	Phase B
Avg latency	21ms	8ms	2ms
Achieved FPS	39	52	60
Frame drops	36%	15%	0%
Memory	36MB	105MB	141MB

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7. Implementation Plan

Step 1: Phase A (Minimal Buffering)

Timeline: 4-6 hours

Tasks:

1. Add frame buffer queue to MediaCodecAV1Decoder
2. Modify DecodeFrameAsync() to implement buffering logic
3. Test with single instance
4. Test with 3 instances
5. Measure performance improvement

Files to modify:

• MediaCodecAV1Decoder.h - Add buffer members
• MediaCodecAsyncHandler.cpp - Add buffering logic

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Step 2: Phase B (Full Async)

Timeline: 1-2 days

Tasks:

1. Create background decoder thread
2. Refactor OnOutputBufferAvailable to push to queue
3. Modify vavcore_decode_to_surface to non-blocking queue access
4. Add proper lifecycle management (start/stop thread)
5. Test with single and multiple instances
6. Stress test with seeking, pause/resume

Files to modify:

• MediaCodecAV1Decoder.h - Add thread, queue, CV
• MediaCodecAV1Decoder.cpp - Thread implementation
• MediaCodecAsyncHandler.cpp - Queue-based decode
• MediaCodecSurfaceManager.cpp - Queue integration

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8. Risk Assessment

Phase A Risks

Risk	Likelihood	Impact	Mitigation
Increased memory usage	High	Low	Acceptable for 4K playback
Seek latency increase	Medium	Low	Clear buffer on seek
Queue overflow	Low	Medium	Limit queue size to 2

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Phase B Risks

Risk	Likelihood	Impact	Mitigation
Thread synchronization bugs	Medium	High	Extensive testing, use proven patterns
Deadlock on cleanup	Medium	High	Proper thread shutdown protocol
Memory leak	Low	High	RAII, smart pointers
Race conditions	Medium	High	Mutex protection, atomic operations

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9. Testing Strategy

Phase A Tests

1. Single video playback - Verify smooth 30 FPS
2. 3 concurrent videos - Measure FPS improvement
3. Seek operations - Verify buffer is cleared
4. Pause/Resume - Verify no buffer corruption
5. End of stream - Verify graceful handling

Phase B Tests

1. All Phase A tests
2. Thread lifecycle - Start/stop 100 times, check for leaks
3. Queue overflow - Send frames faster than consumption
4. Queue underrun - Slow decoder, verify fallback
5. Concurrent access - Multiple threads calling decode_to_surface
6. Memory profiling - Run for 1 hour, check for leaks

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10. Metrics

Success Criteria

Phase A:

• ✅ Latency reduced by 50%+
• ✅ 3-instance FPS improved to 50+ FPS
• ✅ No memory leaks
• ✅ API compatibility maintained

Phase B:

• ✅ Latency reduced by 80%+
• ✅ 3-instance FPS sustained at 60 FPS
• ✅ No deadlocks or race conditions
• ✅ Memory usage within 150MB for 3 instances

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11. Rollout Plan

Week 1: Phase A Implementation

• Day 1-2: Implementation
• Day 3: Testing
• Day 4: Code review and merge

Week 2: Phase B Implementation

• Day 1-3: Implementation
• Day 4-5: Testing and debugging

Week 3: Validation

• Full regression testing
• Performance benchmarking
• Production deployment

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12. Future Enhancements

Priority 1: Adaptive Buffer Size

• Dynamically adjust buffer size based on decoder performance
• Small buffer (2 frames) for fast decoders
• Large buffer (4 frames) for slow/jittery decoders

Priority 2: GPU Fence Integration

• Pass VkFence through queue
• Enable proper GPU synchronization with buffered frames

Priority 3: Frame Dropping Strategy

• Smart frame dropping on buffer overflow
• Prioritize I-frames over P-frames

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13. References

• Current implementation: MediaCodecAsyncHandler.cpp:DecodeFrameAsync()
• Tutorial pattern: Vulkan+Image+Tutorial.md
• GPU synchronization: Phase 1-3 implementation (completed 2025-10-14)

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Document Status: ✅ Ready for Implementation Reviewed By: Architecture Team Approved Date: 2025-10-14 Implementation Start: Immediate