chore: Implement AddMany method (#3866)

* chore: Implement AddMany method

1. Fix a performance bug in Find2 that made redundant comparisons
2. Provide a method to StringSet that adds several items in a batch
3. Use AddMany inside set_family

Before:
```
BM_Add        4253939 ns      4253713 ns          991
```

After:
```
BM_Add        3482177 ns      3482050 ns         1206
BM_AddMany    3101622 ns      3101507 ns         1360
```

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

* chore: fixes

---------

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/core/dense_set.cc

@@ -549,6 +549,11 @@ void DenseSet::AddUnique(void* obj, bool has_ttl, uint64_t hashcode) {
   ++size_;
 }
 
+void DenseSet::Prefetch(uint64_t hash) {
+  uint32_t bid = BucketId(hash);
+  PREFETCH_READ(&entries_[bid]);
+}
+
 auto DenseSet::Find2(const void* ptr, uint32_t bid, uint32_t cookie)
     -> tuple<size_t, DensePtr*, DensePtr*> {
   DCHECK_LT(bid, entries_.size());
@@ -563,19 +568,23 @@ auto DenseSet::Find2(const void* ptr, uint32_t bid, uint32_t cookie)
   // first look for displaced nodes since this is quicker than iterating a potential long chain
   if (bid > 0) {
     curr = &entries_[bid - 1];
-    ExpireIfNeeded(nullptr, curr);
+    if (curr->IsDisplaced() && curr->GetDisplacedDirection() == -1) {
+      ExpireIfNeeded(nullptr, curr);
 
-    if (Equal(*curr, ptr, cookie)) {
-      return {bid - 1, nullptr, curr};
+      if (Equal(*curr, ptr, cookie)) {
+        return {bid - 1, nullptr, curr};
+      }
     }
   }
 
   if (bid + 1 < entries_.size()) {
     curr = &entries_[bid + 1];
-    ExpireIfNeeded(nullptr, curr);
+    if (curr->IsDisplaced() && curr->GetDisplacedDirection() == 1) {
+      ExpireIfNeeded(nullptr, curr);
 
-    if (Equal(*curr, ptr, cookie)) {
-      return {bid + 1, nullptr, curr};
+      if (Equal(*curr, ptr, cookie)) {
+        return {bid + 1, nullptr, curr};
+      }
     }
   }
 

src/core/dense_set.h

@@ -208,6 +208,7 @@ class DenseSet {
 
  public:
   using MemoryResource = PMR_NS::memory_resource;
+  static constexpr uint32_t kMaxBatchLen = 32;
 
   explicit DenseSet(MemoryResource* mr = PMR_NS::get_default_resource());
   virtual ~DenseSet();
@@ -317,6 +318,8 @@ class DenseSet {
   // Assumes that the object does not exist in the set.
   void AddUnique(void* obj, bool has_ttl, uint64_t hashcode);
 
+  void Prefetch(uint64_t hash);
+
  private:
   DenseSet(const DenseSet&) = delete;
   DenseSet& operator=(DenseSet&) = delete;

src/core/string_set.cc

@@ -51,6 +51,42 @@ bool StringSet::Add(string_view src, uint32_t ttl_sec) {
   return true;
 }
 
+unsigned StringSet::AddMany(absl::Span<std::string_view> span, uint32_t ttl_sec) {
+  uint64_t hash[kMaxBatchLen];
+  string_view* data = span.data();
+  bool has_ttl = ttl_sec != UINT32_MAX;
+  size_t count = span.size();
+  unsigned res = 0;
+
+  if (BucketCount() < count) {
+    Reserve(count);
+  }
+  while (count >= kMaxBatchLen) {
+    for (unsigned i = 0; i < kMaxBatchLen; ++i) {
+      hash[i] = CompactObj::HashCode(data[i]);
+      Prefetch(hash[i]);
+    }
+
+    for (unsigned i = 0; i < kMaxBatchLen; ++i) {
+      void* prev = FindInternal(data + i, hash[i], 1);
+      if (prev == nullptr) {
+        ++res;
+        sds field = MakeSetSds(data[i], ttl_sec);
+        AddUnique(field, has_ttl, hash[i]);
+      }
+    }
+
+    count -= kMaxBatchLen;
+    data += kMaxBatchLen;
+    res += kMaxBatchLen;
+  }
+
+  for (unsigned i = 0; i < count; ++i) {
+    res += Add(data[i], ttl_sec);
+  }
+  return res;
+}
+
 std::optional<std::string> StringSet::Pop() {
   sds str = (sds)PopInternal();
 

src/core/string_set.h

@@ -4,10 +4,11 @@
 
 #pragma once
 
+#include <absl/types/span.h>
+
 #include <cstdint>
 #include <functional>
 #include <optional>
-#include <string>
 #include <string_view>
 
 #include "core/dense_set.h"
@@ -28,6 +29,8 @@ class StringSet : public DenseSet {
   // Returns true if elem was added.
   bool Add(std::string_view s1, uint32_t ttl_sec = UINT32_MAX);
 
+  unsigned AddMany(absl::Span<std::string_view> span, uint32_t ttl_sec);
+
   bool Erase(std::string_view str) {
     return EraseInternal(&str, 1);
   }

src/core/string_set_test.cc

@@ -552,4 +552,34 @@ void BM_Add(benchmark::State& state) {
 }
 BENCHMARK(BM_Add);
 
+void BM_AddMany(benchmark::State& state) {
+  vector<string> strs;
+  mt19937 generator(0);
+  StringSet ss;
+  unsigned elems = 100000;
+  for (size_t i = 0; i < elems; ++i) {
+    string str = random_string(generator, 16);
+    strs.push_back(str);
+  }
+  ss.Reserve(elems);
+  array<string_view, 32> str_views;
+
+  while (state.KeepRunning()) {
+    unsigned offset = 0;
+    while (offset < elems) {
+      unsigned len = min(elems - offset, 32u);
+      for (size_t i = 0; i < len; ++i) {
+        str_views[i] = strs[offset + i];
+      }
+      offset += len;
+      ss.AddMany({str_views.data(), len}, UINT32_MAX);
+    }
+    state.PauseTiming();
+    ss.Clear();
+    ss.Reserve(elems);
+    state.ResumeTiming();
+  }
+}
+BENCHMARK(BM_AddMany);
+
 }  // namespace dfly

src/server/set_family.cc

@@ -93,8 +93,24 @@ struct StringSetWrapper {
 
   unsigned Add(const NewEntries& entries, uint32_t ttl_sec) const {
     unsigned res = 0;
-    for (string_view member : EntriesRange(entries))
-      res += ss->Add(member, ttl_sec);
+    string_view members[StringSet::kMaxBatchLen];
+    size_t entries_len = std::visit([](const auto& e) { return e.size(); }, entries);
+    unsigned len = 0;
+    if (ss->BucketCount() < entries_len) {
+      ss->Reserve(entries_len);
+    }
+    for (string_view member : EntriesRange(entries)) {
+      members[len++] = member;
+      if (len == StringSet::kMaxBatchLen) {
+        res += ss->AddMany({members, StringSet::kMaxBatchLen}, ttl_sec);
+        len = 0;
+      }
+    }
+
+    if (len) {
+      res += ss->AddMany({members, len}, ttl_sec);
+    }
+
     return res;
   }