Add per-Task cpu time metric (#56320) #194
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Close JuliaLang#47351 (builds on top of JuliaLang#48416) Adds two per-task metrics: - running time = amount of time the task was actually running (according to our scheduler). Note: currently inclusive of GC time, but would be good to be able to separate that out (in a future PR) - wall time = amount of time between the scheduler becoming aware of this task and the task entering a terminal state (i.e. done or failed). We record running time in `wait()`, where the scheduler stops running the task as well as in `yield(t)`, `yieldto(t)` and `throwto(t)`, which bypass the scheduler. Other places where a task stops running (for `Channel`, `ReentrantLock`, `Event`, `Timer` and `Semaphore` are all implemented in terms of `wait(Condition)`, which in turn calls `wait()`. `LibuvStream` similarly calls `wait()`. This should capture everything (albeit, slightly over-counting task CPU time by including any enqueuing work done before we hit `wait()`). The various metrics counters could be a separate inlined struct if we think that's a useful abstraction, but for now i've just put them directly in `jl_task_t`. They are all atomic, except the `metrics_enabled` flag itself (which we now have to check on task start/switch/done even if metrics are not enabled) which is set on task construction and marked `const` on the julia side. In future PRs we could add more per-task metrics, e.g. compilation time, GC time, allocations, potentially a wait-time breakdown (time waiting on locks, channels, in the scheduler run queue, etc.), potentially the number of yields. Perhaps in future there could be ways to enable this on a per-thread and per-task basis. And potentially in future these same timings could be used by `@time` (e.g. writing this same timing data to a ScopedValue like in JuliaLang#55103 but only for tasks lexically scoped to inside the `@time` block). Timings are off by default but can be turned on globally via starting Julia with `--task-metrics=yes` or calling `Base.Experimental.task_metrics(true)`. Metrics are collected for all tasks created when metrics are enabled. In other words, enabling/disabling timings via `Base.Experimental.task_metrics` does not affect existing `Task`s, only new `Task`s. The other new APIs are `Base.Experimental.task_running_time_ns(::Task)` and `Base.Experimental.task_wall_time_ns(::Task)` for retrieving the new metrics. These are safe to call on any task (including the current task, or a task running on another thread). All these are in `Base.Experimental` to give us room to change up the APIs as we add more metrics in future PRs (without worrying about release timelines). cc @NHDaly @kpamnany @d-netto --------- Co-authored-by: Pete Vilter <[email protected]> Co-authored-by: K Pamnany <[email protected]> Co-authored-by: Nathan Daly <[email protected]> Co-authored-by: Valentin Churavy <[email protected]>
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NHDaly
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PR Description
Backports JuliaLang#56320.
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