Playing a torrent while it downloads: how I got start time from 41 s to 8 s

Description and use

Using anacrolix/torrent’s responsive reader and a three‑minute download window reduced median start time from 41 s to 8.3 s and seek time from 43 s to 3.8 s, meeting the 15‑second start‑time goal.

Problem

Streaming video from a torrent that is still downloading often stalls because the player requests bytes that are not yet available, especially the start and end of the file.

Outcome

Using anacrolix/torrent’s responsive reader and a three‑minute download window reduced median start time from 41 s to 8.3 s and seek time from 43 s to 3.8 s, meeting the 15‑second start‑time goal.

Workflow

• 1. Measure start and seek times by downloading first 2 MB and last 1 MB, then middle 2 MB and 85% 2 MB.
• 2. Test with Rust library – record median times.
• 3. Replace with Go service using anacrolix/torrent and enable responsive reader with readahead 16 MiB.
• 4. Serve HTTP range requests via http.ServeContent, setting Content‑Type manually.
• 5. Implement a three‑minute download window ahead of playback, downloading in 64 MiB batches.
• 6. Remove prewarm requests that read start and end immediately after torrent addition.
• 7. Measure and compare results.
• 8. Handle library callbacks and panics to avoid deadlocks and lock leaks.

Evidence

Median start: 41 seconds. Worse, it was random. The same file started in 2 seconds on one run and 57 on the next.

Source

I swapped it for a small Go service on top of anacrolix/torrent. The library handles the hard stuff (peers, DHT, picking pieces), and my code is just the HTTP API, the stream, file selection and storage.

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For Range requests I let http.ServeContent do the work: 206, 416, Content-Range, Content-Length, all handled.

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So while you watch, only a window ahead of the position gets downloaded: roughly three minutes of video, worked out from the file's average bitrate and kept between 64 MB and 1 GiB.

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The prewarm requests were fighting the player's own requests for the same peers. I took it out, and it stays out until a benchmark says otherwise.

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Library callbacks run under its lock. anacrolix calls some callbacks while it holds the client lock. Touch the client from inside one, and the engine deadlocks.

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