# Pardon the interruption: Loop preemption in Go 1.14

迭代点:
- 1.5
    - concurrent garbage collector
- 1.6
    - concurrent finalizer scans
    - dirty stack tracking
    - global write barriers
- 1.7
    - concurrent stack shrinking
- 1.8
    - hybrid write barrier

抢占:
- 1.0 
    - voluntary preemption only
- 1.2
    - function call preemption
        - function entry -> grow stack/preempt -> Y -> call runtime
- 1.4
    - asynchronous preemption
        - +0.3% binary size
        - 0 performance overhead
        - ~25us typical preemption bound


一些技术尝试:
- preempt back-edges
    - adding instructions to loops isn't viable
        - 每次loop判断preempt
            - 8% slowdown
- "fowward simulation"
- use signals, like an OS kernal
    - zero overhead, but doesn't work with GC
        - 一次切换寄存器地址变了
- signals + gc stack maps at every instruction
    - +10% binary size
    - stack maps everywhere bloats binaries and doesn't "keep it simple"
- **signal-based preemption with conservative innermost frame scanning**
    - 1.14
    - 实现
        - send a signal to the goroutine
            - 23 SIGURG
        - examine where it stopped/conservative scanning/precisely scanning
            - `runtime.asyncPreempt`
            - requires `non-moving GC`
                - https://en.wikipedia.org/wiki/Tracing_garbage_collection#Moving_vs._non-moving