• Peter Goodspeed-Niklaus's avatar
    Add metrics timing message passing from OverseerSubsystemContext to Overseer::route_message (#2201) · 004dc50d
    Peter Goodspeed-Niklaus authored
    * add timing setup to OverseerSubsystemContext
    * figure out how to initialize the rng
    * attach a timer to a portion of the messages traveling to the Overseer
    This timer only exists / logs a fraction of the time (configurable
    by `MESSAGE_TIMER_METRIC_CAPTURE_RATE`). When it exists, it tracks
    the span between the `OverSubsystemContext` receiving the message
    and its receipt in `Overseer::run`.
    * propagate message timing to the start of route_message
    This should be more accurate; it ensures that the timer runs
    at least as long as that function. As `route_message` is async,
    it may not actually run for some time after it is called (or ever).
    * fix failing test
    * rand_chacha apparently implicitly has getrandom feature
    * change rng initialization
    The previous impl using `from_entropy` depends on the `getrandom`
    crate, which uses the system entropy source, and which does not
    work on `wasm32-unknown-unknown` because it wants to fall back to
    a JS implementation which we can't assume exists.
    This impl depends only on `rand::thread_rng`, which has no documentation
    stating that it's similarly limited.
    * remove randomness in favor of a simpler 1 of N procedure
    This deserves a bit of explanation, as the motivating issue explicitly
    requested randomness. In short, it's hard to get randomness to compile
    for `wasm32-unknown-unknown` because that is explicitly intended to be
    as deterministic as practical. Additionally, even though it would never
    be used for consensus purposes, it still felt offputting to intentionally
    introduce randomness into a node's operations. Except, it wasn't really
    random, either: it was a deterministic PRNG varying only in its state,
    and getting the state to work right for that target would have required
    initializing from a constant.
    Given that it was a deterministic sequence anyway, it seemed much simpler
    and more explicit to simply select one of each N messages instead of
    attempting any kind of realistic randomness.
    * reinstate randomness for better statistical properties
    This partially reverts commit 0ab8594c.
    `oorandom` is much lighter than the previous `rand`-based implementation,
    which makes this easier to work with.
    This implementation gives each subsystem and each child RNG a distinct
    increment, which should ensure they produce distinct streams of values.