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chore: add Dash::Prefetch function #4476

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merged 2 commits into from
Jan 19, 2025
Merged

chore: add Dash::Prefetch function #4476

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13472b5
chore: add Dash::Prefetch function
romange Jan 19, 2025
34344d7
chore: fixes
romange Jan 19, 2025
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11 changes: 11 additions & 0 deletions src/core/dash.h
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Original file line number Diff line number Diff line change
Expand Up @@ -149,6 +149,9 @@ class DashTable : public detail::DashTableBase {
template <typename U> const_iterator Find(U&& key) const;
template <typename U> iterator Find(U&& key);

// Prefetches the memory where the key would resize into the cache.
template <typename U> void Prefetch(U&& key) const;

// Find first entry with given key hash that evaulates to true on pred.
// Pred accepts either (const key&) or (const key&, const value&)
template <typename Pred> iterator FindFirst(uint64_t key_hash, Pred&& pred);
Expand Down Expand Up @@ -699,6 +702,14 @@ auto DashTable<_Key, _Value, Policy>::Find(U&& key) -> iterator {
return FindFirst(DoHash(key), EqPred(key));
}

template <typename _Key, typename _Value, typename Policy>
template <typename U>
void DashTable<_Key, _Value, Policy>::Prefetch(U&& key) const {
uint64_t key_hash = DoHash(key);
uint32_t seg_id = SegmentId(key_hash);
segment_[seg_id]->Prefetch(key_hash);
}

template <typename _Key, typename _Value, typename Policy>
template <typename Pred>
auto DashTable<_Key, _Value, Policy>::FindFirst(uint64_t key_hash, Pred&& pred) -> iterator {
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10 changes: 10 additions & 0 deletions src/core/dash_internal.h
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Expand Up @@ -502,6 +502,7 @@ template <typename _Key, typename _Value, typename Policy = DefaultSegmentPolicy

// Find item with given key hash and truthy predicate
template <typename Pred> Iterator FindIt(Hash_t key_hash, Pred&& pred) const;
void Prefetch(Hash_t key_hash) const;

// Returns valid iterator if succeeded or invalid if not (it's full).
// Requires: key should be not present in the segment.
Expand Down Expand Up @@ -1188,6 +1189,15 @@ auto Segment<Key, Value, Policy>::FindIt(Hash_t key_hash, Pred&& pred) const ->
return Iterator{};
}

template <typename Key, typename Value, typename Policy>
void Segment<Key, Value, Policy>::Prefetch(Hash_t key_hash) const {
uint8_t bidx = BucketIndex(key_hash);
const Bucket& target = bucket_[bidx];

// Prefetch the home bucket that might hold the key with high probability.
__builtin_prefetch(&target, 0, 1);
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what is the bucket size? if the size of the bucket is more than the cache line maybe it will be better to load the whole bucket

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the bucket size is dependent on the key/value size but the bucket metadata is located at the beginning. we use the metadata/fingerprints to do the initial scan.

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@BorysTheDev BorysTheDev Jan 19, 2025 •
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Could you clarify what metadata do you want to read, because to my mind it's more than 64 bytes

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SlotBitmap<NUM_SLOTS> slotb_;  // allocation bitmap + pointer bitmap + counter

  /*only use the first 14 bytes, can be accelerated by
    SSE instruction,0-13 for finger, 14-17 for overflowed*/
  FpArray finger_arr_;
  StashFpArray stash_arr_;

  uint8_t stash_busy_ = 0;  // kStashFpLen+1 bits are used
  uint8_t stash_pos_ = 0;   // 4x2 bits for pointing to stash bucket.

BucketBase<NUM_SLOTS, NUM_OVR>::CompareFP is the function that usually hits a cold memory

}

template <typename Key, typename Value, typename Policy>
template <typename Cb>
void Segment<Key, Value, Policy>::TraverseAll(Cb&& cb) const {
Expand Down
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