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fix: Optimize GetRandomChain() in DenseSet #6033

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Merged
merged 4 commits into from
Nov 11, 2025
Merged

fix: Optimize GetRandomChain() in DenseSet #6033

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9d378f4
fix: Update dense_set
vyavdoshenko Nov 10, 2025
df11a06
Merge branch 'main' into bobik/dense_set_opt
vyavdoshenko Nov 11, 2025
ed064fc
fix: review comments
vyavdoshenko Nov 11, 2025
bed7209
fix: review comments
vyavdoshenko Nov 11, 2025
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23 changes: 22 additions & 1 deletion src/core/dense_set.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -673,7 +673,28 @@ void DenseSet::Delete(DensePtr* prev, DensePtr* ptr) {
}

DenseSet::ChainVectorIterator DenseSet::GetRandomChain() {
size_t offset = absl::Uniform(absl::BitGen{}, 0u, entries_.size());
if (entries_.empty() || size_ == 0) {
return entries_.end();
}

// Use thread-local generator to avoid repeated construction overhead
thread_local absl::BitGen gen;
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@dranikpg dranikpg Nov 10, 2025

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Maybe just using absl::InsecureBitGen is as cheap?

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@vyavdoshenko vyavdoshenko Nov 11, 2025

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Fixed


// Try up to 32 random probes before falling back to linear scan
// Expected probes needed = 1 / utilization
// For 10% util, expected = 10 probes, so 32 gives good probability
constexpr int kMaxProbes = 32;
for (int probe = 0; probe < kMaxProbes; probe++) {
size_t idx = absl::Uniform<size_t>(gen, 0u, entries_.size());
auto it = entries_.begin() + idx;
ExpireIfNeeded(nullptr, &*it);
if (!it->IsEmpty()) {
return it;
}
}

// Fallback or normal path: linear scan from random offset
size_t offset = absl::Uniform<size_t>(gen, 0u, entries_.size());
for (size_t i = offset; i < entries_.size() + offset; i++) {
auto it = entries_.begin() + (i % entries_.size());
ExpireIfNeeded(nullptr, &*it);
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