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opt: use gcd to reduce memory overhead #52

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opt: use gcd to reduce memory overhead #52

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21 changes: 19 additions & 2 deletions pingora-load-balancing/src/selection/weighted.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,16 @@ pub struct Weighted<H = FnvHasher> {
algorithm: H,
}

fn gcd(mut a: usize, mut b: usize) -> usize {
let mut r;
while b != 0 {
r = a % b;
a = b;
b = r;
}
a
}

impl<H: SelectionAlgorithm> BackendSelection for Weighted<H> {
type Iter = WeightedIterator<H>;

Expand All @@ -38,9 +48,16 @@ impl<H: SelectionAlgorithm> BackendSelection for Weighted<H> {
"support up to 2^16 backends"
);
let backends = Vec::from_iter(backends.iter().cloned()).into_boxed_slice();
let mut weighted = Vec::with_capacity(backends.len());
let mut g = 0;
let mut total = 0;
// use gcd to reduce the memory overhead
for (_, b) in backends.iter().enumerate() {
g = gcd(g, b.weight);
total += b.weight;
}
let mut weighted = Vec::with_capacity(total / g);
for (index, b) in backends.iter().enumerate() {
for _ in 0..b.weight {
for _ in 0..(b.weight / g) {
weighted.push(index as u16);
}
}
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