9c995fc4dd
The torrent reader used a static 5 MiB readahead — about 1.9s of a 20 Mbps 4K stream — so streaming a torrent while it downloaded outran the download and stalled. anacrolix's reader already prioritises the pieces in the readahead window ahead of the playhead (and re-prioritises on seek); the window was just too small. dynamicReadahead sizes it to ~30s of video (clamped 8-96 MiB, 24 MiB default when bitrate is unknown). The torrent provider probes the bitrate asynchronously so stream start never blocks on ffprobe; readers created after the probe resolves pick up the accurate size. Real 4K (20.7 Mbps) -> 73 MiB.
32 lines
1.1 KiB
Go
32 lines
1.1 KiB
Go
package engine
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// Torrent stream readahead sizing.
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//
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// anacrolix's Reader (SetResponsive + SetReadahead) already prioritises the
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// pieces in a window ahead of the read position and re-prioritises on Seek —
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// so the playhead→piece-priority feedback is built in. The problem was the
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// window: a static 5 MiB is only ~1.6s of a 25 Mbps 4K stream, so playback
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// outran the download and stalled. Sizing the window by bitrate (~30s of video)
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// keeps a real buffer ahead of the playhead.
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const (
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readaheadSeconds = 30
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minReadahead = 8 << 20 // 8 MiB
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maxReadahead = 96 << 20 // 96 MiB — cap so a seek doesn't waste a huge fetch
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defaultReadahead = 24 << 20 // 24 MiB — when bitrate is unknown (still ~5x the old 5 MiB)
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)
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// dynamicReadahead returns the bytes-ahead window for a torrent reader given the
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// stream's bitrate (bits/sec). Unknown/zero bitrate → a generous default.
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func dynamicReadahead(bitrateBps int64) int64 {
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if bitrateBps <= 0 {
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return defaultReadahead
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}
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ra := bitrateBps / 8 * readaheadSeconds
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if ra < minReadahead {
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return minReadahead
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}
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if ra > maxReadahead {
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return maxReadahead
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}
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return ra
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}
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