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use std::time::{Duration, Instant};
#[cfg(windows)]
use winapi::um::timeapi;
use rand_distr::{Distribution, Exp};
/// Timer is an iterator that sleeps an appropriate amount of time between iterations
/// so that we can sample the process a certain number of times a second.
/// We're using an irregular sampling strategy to avoid aliasing effects that can happen
/// if the target process runs code at a similar schedule as the profiler:
/// https://github.com/benfred/py-spy/issues/94
pub struct Timer {
start: Instant,
desired: Duration,
exp: Exp<f64>,
}
impl Timer {
pub fn new(rate: f64) -> Timer {
// This changes a system-wide setting on Windows so that the OS wakes up every 1ms
// instead of the default 15.6ms. This is required to have a sleep call
// take less than 15ms, which we need since we usually profile at more than 64hz.
// The downside is that this will increase power usage: good discussions are:
// https://randomascii.wordpress.com/2013/07/08/windows-timer-resolution-megawatts-wasted/
// and http://www.belshe.com/2010/06/04/chrome-cranking-up-the-clock/
#[cfg(windows)]
unsafe {
timeapi::timeBeginPeriod(1);
}
let start = Instant::now();
Timer {
start,
desired: Duration::from_secs(0),
exp: Exp::new(rate).unwrap(),
}
}
}
impl Iterator for Timer {
type Item = Result<Duration, Duration>;
fn next(&mut self) -> Option<Self::Item> {
let elapsed = self.start.elapsed();
// figure out how many nanoseconds should come between the previous and
// the next sample using an exponential distribution to avoid aliasing
let nanos = 1_000_000_000.0 * self.exp.sample(&mut rand::thread_rng());
// since we want to account for the amount of time the sampling takes
// we keep track of when we should sleep to (rather than just sleeping
// the amount of time from the previous line).
self.desired += Duration::from_nanos(nanos as u64);
// sleep if appropriate, or warn if we are behind in sampling
if self.desired > elapsed {
std::thread::sleep(self.desired - elapsed);
Some(Ok(self.desired - elapsed))
} else {
Some(Err(elapsed - self.desired))
}
}
}
impl Drop for Timer {
fn drop(&mut self) {
#[cfg(windows)]
unsafe {
timeapi::timeEndPeriod(1);
}
}
}