hiyoyo Posted on May 31 Why MTP Batch Transfers Slow Down Between Files # android # mtp # tauri # rust All tests run on an 8-year-old MacBook Air. You're transferring a batch of large files over MTP. The first one flies at 45 MB/s. Then the second file starts — and you're at 30 MB/s. The third is slower still. Nothing changed. Same cable, same device, same app. So what's happening? The Cause Is in the Protocol Itself Between every file, MTP requires a full negotiation cycle — SendObjectInfo followed by SendObject . This isn't an implementation detail you can optimize away. It's how MTP works. During that gap, a few things happen in sequence: The Android device's flash controller is still committing the previous file to storage The USB pipe is flushed and re-established for the next object The device's MTP stack is processing metadata before it's ready to receive data again The result is a speed dip at every file boundary. The longer the previous file, the longer the device needs to catch up. What I Tried Building HiyokoMTP, I went through the obvious candidates: Tokio thread pool exhaustion — sync Read/Write calls blocking async threads were a real issue. Fixing it improved overall stability, but didn't eliminate the inter-file dip. Chunk size tuning — adjusting the USB bulk transfer buffer (up to 4 MB per chunk) helped peak throughput, but not the boundary behavior. Intentional cooldown between files — adding a short pause actually helped in some cases, giving the device's flash controller time to breathe before the next transfer starts. Why It Can't Be Fully Fixed The inter-file overhead is structural. MTP was designed as a stateful, command-response protocol — not a streaming pipeline. Every file is a discrete transaction with its own negotiation. There's no mechanism to pre-stage the next file while the current one is still writing. Non-async bulk transfer pipelining (similar to io_uring or Zero Copy USB) could theoretically reduce this, but it would require dee
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