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//! The lock around the replica (`AppState::doc`), counting how long its takers wait
//! (/admin/stats `document.lockWait`): whether the one document is what the node's
//! work queues behind.
use std::sync::Mutex;
use std::time::{Duration, Instant};
use serde::Serialize;
use tokio::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};
use super::replica::Replica;
use crate::locks::MutexExt;
/// Upper bounds (inclusive, microseconds) of the wait buckets; a last bucket takes the
/// rest.
const WAIT_BUCKETS_US: [u64; 7] = [10, 100, 1_000, 10_000, 100_000, 1_000_000, 10_000_000];
/// `tokio::sync::RwLock<Replica>`, with its waits counted.
pub(crate) struct DocumentLock {
replica: RwLock<Replica>,
waits: Mutex<LockWaits>,
}
/// How long takers of the lock waited for it, by mode, since start.
#[derive(Debug, Clone, Copy, Default, Serialize)]
#[serde(rename_all = "camelCase")]
pub(crate) struct LockWaits {
/// Readers: wait for a writer only.
pub(crate) read: Waits,
/// Writers (applying sync messages, generating them, writes, saves): wait for
/// everyone.
pub(crate) write: Waits,
}
#[derive(Debug, Clone, Copy, Default)]
pub(crate) struct Waits {
/// Per bucket of `WAIT_BUCKETS_US`, then the rest; not cumulative.
buckets: [u64; WAIT_BUCKETS_US.len() + 1],
pub(crate) count: u64,
pub(crate) sum_us: u64,
pub(crate) max_us: u64,
}
impl Waits {
fn record(&mut self, waited: Duration) {
let us = u64::try_from(waited.as_micros()).unwrap_or(u64::MAX);
let bucket = WAIT_BUCKETS_US
.iter()
.position(|&bound| us <= bound)
.unwrap_or(WAIT_BUCKETS_US.len());
self.buckets[bucket] += 1;
self.count += 1;
self.sum_us = self.sum_us.saturating_add(us);
self.max_us = self.max_us.max(us);
}
/// Each bucket's upper bound in microseconds (None for the last) and count.
pub(crate) fn buckets(&self) -> impl Iterator<Item = (Option<u64>, u64)> + '_ {
WAIT_BUCKETS_US
.iter()
.copied()
.map(Some)
.chain([None])
.zip(self.buckets.iter().copied())
}
}
impl Serialize for Waits {
fn serialize<S: serde::Serializer>(&self, serializer: S) -> Result<S::Ok, S::Error> {
#[derive(Serialize)]
#[serde(rename_all = "camelCase")]
struct Bucket {
le_us: Option<u64>,
count: u64,
}
#[derive(Serialize)]
#[serde(rename_all = "camelCase")]
struct Out {
count: u64,
sum_us: u64,
max_us: u64,
/// Not cumulative; the last bucket has no bound.
buckets: Vec<Bucket>,
}
Out {
count: self.count,
sum_us: self.sum_us,
max_us: self.max_us,
buckets: self
.buckets()
.map(|(le_us, count)| Bucket { le_us, count })
.collect(),
}
.serialize(serializer)
}
}
#[derive(Clone, Copy)]
enum Mode {
Read,
Write,
}
impl DocumentLock {
pub(crate) fn new(replica: Replica) -> Self {
Self {
replica: RwLock::new(replica),
waits: Mutex::default(),
}
}
fn waited(&self, mode: Mode, since: Instant) {
let waited = since.elapsed();
let mut waits = self.waits.lock_or_recover();
match mode {
Mode::Read => waits.read.record(waited),
Mode::Write => waits.write.record(waited),
}
}
pub(crate) async fn read(&self) -> RwLockReadGuard<'_, Replica> {
let since = Instant::now();
let guard = self.replica.read().await;
self.waited(Mode::Read, since);
guard
}
pub(crate) async fn write(&self) -> RwLockWriteGuard<'_, Replica> {
let since = Instant::now();
let guard = self.replica.write().await;
self.waited(Mode::Write, since);
guard
}
/// Outside the runtime only (`spawn_blocking`), like `RwLock::blocking_read`.
pub(crate) fn blocking_read(&self) -> RwLockReadGuard<'_, Replica> {
let since = Instant::now();
let guard = self.replica.blocking_read();
self.waited(Mode::Read, since);
guard
}
/// Outside the runtime only (`spawn_blocking`), like `RwLock::blocking_write`.
pub(crate) fn blocking_write(&self) -> RwLockWriteGuard<'_, Replica> {
let since = Instant::now();
let guard = self.replica.blocking_write();
self.waited(Mode::Write, since);
guard
}
pub(crate) fn waits(&self) -> LockWaits {
*self.waits.lock_or_recover()
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn waits_fall_in_their_buckets() {
let mut waits = Waits::default();
waits.record(Duration::from_micros(5));
waits.record(Duration::from_micros(10));
waits.record(Duration::from_millis(2));
waits.record(Duration::from_secs(60));
let buckets: Vec<_> = waits.buckets().collect();
assert_eq!(buckets[0], (Some(10), 2));
assert_eq!(buckets[3], (Some(10_000), 1));
assert_eq!(buckets[7], (None, 1));
assert_eq!(waits.count, 4);
assert_eq!(waits.sum_us, 5 + 10 + 2_000 + 60_000_000);
assert_eq!(waits.max_us, 60_000_000);
}
}
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