Synchronous (Zero-Copy) Outlet Mode

[cboulay]

This PR adds a new transp_sync_blocking transport flag that enables synchronous, zero-copy data transfer for stream outlets. Instead of copying sample data into an internal buffer for async delivery, sync mode writes directly from the user's buffer to connected sockets, eliminating memory allocation and copy overhead.

It is intended to be a replacement for #170 , which has not been updated in some time.

Motivation

For high-channel-count, high-sample-rate applications (e.g., 1000+ channels at 30kHz), the async outlet's per-sample memory allocation and copying becomes a significant CPU bottleneck. Sync mode addresses this by:

• Zero-copy transfer: Sample data is written directly from the user's buffer
• No per-sample allocation: Timestamps are stored in a small metadata buffer, but sample data isn't copied
• Blocking semantics: push_sample/push_chunk blocks until data is sent to all consumers

This makes all the difference for me on a lower power embedded system

Usage

lsl::stream_info info("MyStream", "EEG", 1000, 30000, lsl::cf_float32);
lsl::stream_outlet outlet(info, 0, 360, transp_sync_blocking);

std::vector<float> sample(1000);
outlet.push_sample(sample);  // Blocks until sent to all consumers

Limitations

• String format not supported: Sync mode requires fixed-size samples
• Blocking: Push operations block until all connected consumers receive data
• Latency scales with consumers: More consumers = longer push latency

Benchmark Results

Test configuration: 1000 channels, 30kHz sample rate, macOS (Apple Silicon)

CPU Usage by Chunk Size (1 consumer)

Chunk Size	Async (µs/sample)	Sync (µs/sample)	CPU Savings
1	31.18	15.65	50%
4	9.05	5.06	44%
8	7.76	6.37	18%
16	9.76	6.88	30%
30	9.82	8.34	15%

Scaling with Multiple Consumers (chunk=4)

Consumers	Async (µs/sample)	Sync (µs/sample)	CPU Savings
1	10.15	4.95	51%
2	21.95	9.84	55%
3	33.82	15.91	53%

CPU savings remain significant (~50%) across consumer counts. However, push latency increases linearly with consumers in sync mode (async latency stays constant).

Implementation Details

• New sync_write_handler class manages connected sockets grouped by endianness
• tcp_server hands off sockets to sync handler after protocol negotiation
• Optimized enqueue_chunk_sync() batches timestamp+data buffers for efficient gather-write
• sync_timestamps_ uses std::deque to ensure pointer stability during buffer accumulation