Convert as much at once as the Mac can stand, and no more

[thousandflowers]

One number decided how much ran at once, and it was wrong in both directions: two videos at a time is one too many, because the media engine is a single piece of hardware and asking it for two only makes heat, while two images at a time is far too few on a machine with ten cores.

The limit is now per kind of work, computed from the cores, the memory and whether this is Apple silicon — images bounded by memory, since a decoded photograph is a quarter of a gigabyte and eight at once is how a batch takes a Mac down. Nothing here decides which chip does the work: Core Image goes to the GPU, Vision to the neural engine, VideoToolbox to the media engine, and Apple routes that without being asked.

It backs off mid-batch: a warm Mac, Low Power Mode or memory pressure halves the limits or drops them to one, and they come back when the pressure does not. The screen says so rather than just getting slower.

Pause stops new files starting and lets the ones in flight finish — a native encode cannot be frozen halfway and a button claiming otherwise would be lying. Cancel now reaches one file as well as all of them, and reaches a subprocess: a cancelled conversion used to drop the task and leave ffmpeg encoding a film nobody was waiting for.

Two bugs the tests caught on the way: two waiters could be let in for one slot, because a slot was counted when the waiter woke up rather than when it was given; and stopping a row that had not started yet marked it cancelled on screen and converted it anyway.

Not built, deliberately: a software/high-quality encode toggle. AVFoundation exposes no software-encoder switch — VideoToolbox is already the default path and the quality choice is the export preset. A toggle that commutes nothing would be worse than none.

Test plan

• swift test — green
• 13 tests: limits per workload, scaling with mocked cores/memory, the limit held under 10 racing tasks, one kind not blocking another, thermal and memory pressure shrinking and recovering, pause holding new starts, cancel leaving the original intact with no scratch behind, cancel-per-file leaving siblings alone
• Real 12-file batch through the CLI

🤖 Generated with Claude Code

https://claude.ai/code/session_018H9CZxWumc3ryumgEHd578