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//! Per-engine memory budget tracking.
use std::sync::atomic::{AtomicUsize, Ordering};
/// A memory budget for a single engine.
///
/// Tracks current allocation against a configurable limit using atomic
/// counters (safe to read from any thread — metrics exporter, governor, etc.).
#[derive(Debug)]
pub struct Budget {
/// Hard limit in bytes. Allocations beyond this are rejected.
limit: AtomicUsize,
/// Current allocated bytes.
allocated: AtomicUsize,
/// Peak allocated bytes (high-water mark).
peak: AtomicUsize,
/// Number of times an allocation was rejected due to budget exhaustion.
rejection_count: AtomicUsize,
}
impl Budget {
/// Create a new budget with the given limit.
pub fn new(limit: usize) -> Self {
Self {
limit: AtomicUsize::new(limit),
allocated: AtomicUsize::new(0),
peak: AtomicUsize::new(0),
rejection_count: AtomicUsize::new(0),
}
}
/// Try to reserve `size` bytes from this budget.
///
/// Returns `true` if the reservation succeeded, `false` if it would
/// exceed the limit.
pub fn try_reserve(&self, size: usize) -> bool {
let limit = self.limit.load(Ordering::Relaxed);
// CAS loop to atomically check and increment.
loop {
let current = self.allocated.load(Ordering::Relaxed);
if current + size > limit {
self.rejection_count.fetch_add(1, Ordering::Relaxed);
return false;
}
match self.allocated.compare_exchange_weak(
current,
current + size,
Ordering::AcqRel,
Ordering::Relaxed,
) {
Ok(_) => {
// Update peak if necessary.
let new_allocated = current + size;
let mut peak = self.peak.load(Ordering::Relaxed);
while new_allocated > peak {
match self.peak.compare_exchange_weak(
peak,
new_allocated,
Ordering::Relaxed,
Ordering::Relaxed,
) {
Ok(_) => break,
Err(actual) => peak = actual,
}
}
return true;
}
Err(_) => continue, // Retry CAS.
}
}
}
/// Release `size` bytes back to the budget.
///
/// Saturates to zero if `size` exceeds the current allocation (which can
/// happen when data is replayed from WAL without a matching reservation).
pub fn release(&self, size: usize) {
loop {
let current = self.allocated.load(Ordering::Acquire);
let new_val = current.saturating_sub(size);
match self.allocated.compare_exchange_weak(
current,
new_val,
Ordering::Release,
Ordering::Relaxed,
) {
Ok(_) => {
if size > current {
tracing::warn!(
released = size,
allocated = current,
"memory release exceeds allocation (WAL replay or accounting drift)"
);
}
return;
}
Err(_) => continue,
}
}
}
/// Current allocated bytes.
pub fn allocated(&self) -> usize {
self.allocated.load(Ordering::Relaxed)
}
/// Hard limit in bytes.
pub fn limit(&self) -> usize {
self.limit.load(Ordering::Relaxed)
}
/// Remaining bytes available.
pub fn available(&self) -> usize {
let limit = self.limit();
let allocated = self.allocated();
limit.saturating_sub(allocated)
}
/// Utilization as a percentage (0-100).
pub fn utilization_percent(&self) -> u8 {
let limit = self.limit();
if limit == 0 {
return 100;
}
let allocated = self.allocated();
((allocated * 100) / limit).min(100) as u8
}
/// Peak allocation (high-water mark).
pub fn peak(&self) -> usize {
self.peak.load(Ordering::Relaxed)
}
/// Number of rejected allocation attempts.
pub fn rejections(&self) -> usize {
self.rejection_count.load(Ordering::Relaxed)
}
/// Update the limit dynamically (for rebalancing).
///
/// The new limit must be >= current allocation. If it's less, the limit
/// is set to the current allocation (no immediate eviction).
pub fn set_limit(&self, new_limit: usize) {
let allocated = self.allocated();
let effective = new_limit.max(allocated);
self.limit.store(effective, Ordering::Release);
}
/// Reset all counters (for testing).
#[cfg(test)]
pub fn reset(&self) {
self.allocated.store(0, Ordering::Relaxed);
self.peak.store(0, Ordering::Relaxed);
self.rejection_count.store(0, Ordering::Relaxed);
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn reserve_within_limit() {
let budget = Budget::new(1024);
assert!(budget.try_reserve(512));
assert_eq!(budget.allocated(), 512);
assert_eq!(budget.available(), 512);
assert_eq!(budget.utilization_percent(), 50);
}
#[test]
fn reserve_at_limit() {
let budget = Budget::new(1024);
assert!(budget.try_reserve(1024));
assert!(!budget.try_reserve(1));
assert_eq!(budget.rejections(), 1);
}
#[test]
fn reserve_exceeds_limit() {
let budget = Budget::new(100);
assert!(!budget.try_reserve(101));
assert_eq!(budget.allocated(), 0);
assert_eq!(budget.rejections(), 1);
}
#[test]
fn release_frees_capacity() {
let budget = Budget::new(1024);
assert!(budget.try_reserve(512));
assert!(budget.try_reserve(512));
assert!(!budget.try_reserve(1));
budget.release(256);
assert!(budget.try_reserve(256));
}
#[test]
fn peak_tracks_high_water_mark() {
let budget = Budget::new(1024);
budget.try_reserve(800);
budget.release(500);
budget.try_reserve(100);
assert_eq!(budget.peak(), 800);
assert_eq!(budget.allocated(), 400);
}
#[test]
fn dynamic_limit_adjustment() {
let budget = Budget::new(1024);
budget.try_reserve(600);
// Increase limit.
budget.set_limit(2048);
assert_eq!(budget.limit(), 2048);
assert!(budget.try_reserve(1000));
// Decrease limit — but not below current allocation.
budget.set_limit(100);
assert_eq!(budget.limit(), 1600); // max(100, 1600 allocated)
}
#[test]
fn concurrent_reserves() {
use std::sync::Arc;
use std::thread;
let budget = Arc::new(Budget::new(10_000));
let mut handles = Vec::new();
for _ in 0..10 {
let b = Arc::clone(&budget);
handles.push(thread::spawn(move || {
let mut reserved = 0;
for _ in 0..100 {
if b.try_reserve(10) {
reserved += 10;
}
}
reserved
}));
}
let total_reserved: usize = handles.into_iter().map(|h| h.join().unwrap()).sum();
// All 1000 reservations of 10 bytes should succeed (10 * 100 * 10 = 10000).
assert_eq!(total_reserved, 10_000);
assert_eq!(budget.allocated(), 10_000);
}
}