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fix(shm): performance regression in the balancers for large k #129

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Merged
DanielSeemaier merged 1 commit into from
Dec 17, 2025
Merged

fix(shm): performance regression in the balancers for large k #129

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7 changes: 7 additions & 0 deletions kaminpar-shm/refinement/balancer/overload_balancer.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -237,6 +237,10 @@ std::pair<BlockID, float> OverloadBalancer::compute_best_gain(
}
});

if (best_block == kInvalidBlockID) {
best_block = from;
}

return std::make_pair(best_block, compute_relative_gain(best_gain, weight));
}

Expand Down Expand Up @@ -269,6 +273,9 @@ bool OverloadBalancer::add_to_pq(
bool OverloadBalancer::move_node_if_possible(
const NodeID node, const BlockID from, const BlockID to
) {
KASSERT(node < _p_graph->n());
KASSERT(to < _p_graph->k());

if (_p_graph->move(node, from, to, _p_ctx->max_block_weight(to))) {
if (_move_tracker != nullptr) {
_move_tracker(node, from, to);
Expand Down
7 changes: 4 additions & 3 deletions kaminpar-shm/refinement/balancer/overload_balancer.h
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Original file line number Diff line number Diff line change
Expand Up @@ -26,8 +26,9 @@ class OverloadBalancer : public Refiner {
template <typename ConcretizedGraph>
using GainCache = OnTheFlyGainCache<
ConcretizedGraph,
/*iterate_nonadjacent_blocks=*/true,
/*iterate_exact_gains=*/true>;
/*iterate_nonadjacent_blocks=*/false,
/*iterate_exact_gains=*/true,
/*iterate_source_block=*/true>;

public:
using MoveTracker = std::function<void(NodeID, BlockID, BlockID)>;
Expand All @@ -40,7 +41,7 @@ class OverloadBalancer : public Refiner {
OverloadBalancer(const OverloadBalancer &) = delete;

OverloadBalancer &operator=(OverloadBalancer &&) = delete;
OverloadBalancer(OverloadBalancer &&) noexcept = default;
OverloadBalancer(OverloadBalancer &&) noexcept = delete;

[[nodiscard]] std::string name() const final;

Expand Down
5 changes: 3 additions & 2 deletions kaminpar-shm/refinement/balancer/underload_balancer.h
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Expand Up @@ -22,8 +22,9 @@ class UnderloadBalancer : public Refiner {
template <typename ConcretizedGraph>
using GainCache = OnTheFlyGainCache<
ConcretizedGraph,
/*iterate_nonadjacent_blocks=*/true,
/*iterate_exact_gains=*/true>;
/*iterate_nonadjacent_blocks=*/false,
/*iterate_exact_gains=*/true,
/*iterate_source_block=*/true>;

public:
explicit UnderloadBalancer(const Context &ctx);
Expand Down
8 changes: 5 additions & 3 deletions kaminpar-shm/refinement/gains/on_the_fly_gain_cache.h
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Expand Up @@ -21,7 +21,8 @@ template <typename GainCache> class OnTheFlyDeltaGainCache;
template <
typename GraphType,
bool iterate_nonadjacent_blocks = true,
bool iterate_exact_gains = true>
bool iterate_exact_gains = true,
bool iterate_source_block = false>
class OnTheFlyGainCache {
template <typename> friend class OnTheFlyDeltaGainCache;

Expand All @@ -32,6 +33,7 @@ class OnTheFlyGainCache {

constexpr static bool kIteratesNonadjacentBlocks = iterate_nonadjacent_blocks;
constexpr static bool kIteratesExactGains = iterate_exact_gains;
constexpr static bool kIteratesSourceBlock = iterate_source_block;

OnTheFlyGainCache(const Context & /* ctx */, NodeID /* max_n */, const BlockID preallocate_k)
: _rating_map_ets([preallocate_k] {
Expand Down Expand Up @@ -159,13 +161,13 @@ class OnTheFlyGainCache {

if constexpr (kIteratesNonadjacentBlocks) {
for (const BlockID to : p_graph.blocks()) {
if (to != from) {
if (kIteratesSourceBlock || to != from) {
lambda(to, [&] { return map[to] - conn_from; });
}
}
} else {
for (const auto [to, conn_to] : map.entries()) {
if (to != from) {
if (kIteratesSourceBlock || to != from) {
lambda(to, [&, conn_to = conn_to] { return conn_to - conn_from; });
}
}
Expand Down
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