reduce memory usage of cuckoo table builder

Summary:
builder currently buffers all key value pairs as a vector of
pair<string, string>. That is too much due to std::string
overhead. It wasn't able to fit 1B key/values (12bytes total) in 100GB
of ram. Switch to use a plain string to store the key/value sequence and
use only 12GB of ram as a result.

Test Plan: db_bench

Reviewers: igor, sdong, yhchiang

Reviewed By: sdong

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D23763

table/cuckoo_table_builder.cc

@@ -60,6 +60,9 @@ CuckooTableBuilder::CuckooTableBuilder(
       hash_table_size_(use_module_hash ? 0 : 2),
       is_last_level_file_(false),
       has_seen_first_key_(false),
+      key_size_(0),
+      value_size_(0),
+      num_entries_(0),
       ucomp_(user_comparator),
       use_module_hash_(use_module_hash),
       identity_as_first_hash_(identity_as_first_hash),
@@ -72,7 +75,7 @@ CuckooTableBuilder::CuckooTableBuilder(
 }
 
 void CuckooTableBuilder::Add(const Slice& key, const Slice& value) {
-  if (kvs_.size() >= kMaxVectorIdx - 1) {
+  if (num_entries_ >= kMaxVectorIdx - 1) {
     status_ = Status::NotSupported("Number of keys in a file must be < 2^32-1");
     return;
   }
@@ -90,15 +93,18 @@ void CuckooTableBuilder::Add(const Slice& key, const Slice& value) {
     has_seen_first_key_ = true;
     smallest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
     largest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
+    key_size_ = is_last_level_file_ ? ikey.user_key.size() : key.size();
+    value_size_ = value.size();
   }
   // Even if one sequence number is non-zero, then it is not last level.
   assert(!is_last_level_file_ || ikey.sequence == 0);
   if (is_last_level_file_) {
-    kvs_.emplace_back(std::make_pair(
-          ikey.user_key.ToString(), value.ToString()));
+    kvs_.append(ikey.user_key.data(), ikey.user_key.size());
   } else {
-    kvs_.emplace_back(std::make_pair(key.ToString(), value.ToString()));
+    kvs_.append(key.data(), key.size());
   }
+  kvs_.append(value.data(), value.size());
+  ++num_entries_;
 
   // In order to fill the empty buckets in the hash table, we identify a
   // key which is not used so far (unused_user_key). We determine this by
@@ -111,21 +117,32 @@ void CuckooTableBuilder::Add(const Slice& key, const Slice& value) {
     largest_user_key_.assign(ikey.user_key.data(), ikey.user_key.size());
   }
   if (!use_module_hash_) {
-    if (hash_table_size_ < kvs_.size() / max_hash_table_ratio_) {
+    if (hash_table_size_ < num_entries_ / max_hash_table_ratio_) {
       hash_table_size_ *= 2;
     }
   }
 }
 
+Slice CuckooTableBuilder::GetKey(uint64_t idx) const {
+  return Slice(&kvs_[idx * (key_size_ + value_size_)], key_size_);
+}
+
+Slice CuckooTableBuilder::GetUserKey(uint64_t idx) const {
+  return is_last_level_file_ ? GetKey(idx) : ExtractUserKey(GetKey(idx));
+}
+
+Slice CuckooTableBuilder::GetValue(uint64_t idx) const {
+  return Slice(&kvs_[idx * (key_size_ + value_size_) + key_size_], value_size_);
+}
+
 Status CuckooTableBuilder::MakeHashTable(std::vector<CuckooBucket>* buckets) {
   buckets->resize(hash_table_size_ + cuckoo_block_size_ - 1);
   uint64_t make_space_for_key_call_id = 0;
-  for (uint32_t vector_idx = 0; vector_idx < kvs_.size(); vector_idx++) {
+  for (uint32_t vector_idx = 0; vector_idx < num_entries_; vector_idx++) {
     uint64_t bucket_id;
     bool bucket_found = false;
     autovector<uint64_t> hash_vals;
-    Slice user_key = is_last_level_file_ ? kvs_[vector_idx].first :
-      ExtractUserKey(kvs_[vector_idx].first);
+    Slice user_key = GetUserKey(vector_idx);
     for (uint32_t hash_cnt = 0; hash_cnt < num_hash_func_ && !bucket_found;
         ++hash_cnt) {
       uint64_t hash_val = CuckooHash(user_key, hash_cnt, use_module_hash_,
@@ -140,10 +157,8 @@ Status CuckooTableBuilder::MakeHashTable(std::vector<CuckooBucket>* buckets) {
           bucket_found = true;
           break;
         } else {
-          if (ucomp_->Compare(user_key, is_last_level_file_
-                ? Slice(kvs_[(*buckets)[hash_val].vector_idx].first)
-                : ExtractUserKey(
-                  kvs_[(*buckets)[hash_val].vector_idx].first)) == 0) {
+          if (ucomp_->Compare(user_key,
+                GetUserKey((*buckets)[hash_val].vector_idx)) == 0) {
             return Status::NotSupported("Same key is being inserted again.");
           }
           hash_vals.push_back(hash_val);
@@ -183,10 +198,10 @@ Status CuckooTableBuilder::Finish() {
   std::vector<CuckooBucket> buckets;
   Status s;
   std::string unused_bucket;
-  if (!kvs_.empty()) {
+  if (num_entries_ > 0) {
     // Calculate the real hash size if module hash is enabled.
     if (use_module_hash_) {
-      hash_table_size_ = kvs_.size() / max_hash_table_ratio_;
+      hash_table_size_ = num_entries_ / max_hash_table_ratio_;
     }
     s = MakeHashTable(&buckets);
     if (!s.ok()) {
@@ -224,14 +239,13 @@ Status CuckooTableBuilder::Finish() {
       AppendInternalKey(&unused_bucket, ikey);
     }
   }
-  properties_.num_entries = kvs_.size();
-  properties_.fixed_key_len = unused_bucket.size();
-  uint32_t value_length = kvs_.empty() ? 0 : kvs_[0].second.size();
-  uint32_t bucket_size = value_length + properties_.fixed_key_len;
+  properties_.num_entries = num_entries_;
+  properties_.fixed_key_len = key_size_;
   properties_.user_collected_properties[
         CuckooTablePropertyNames::kValueLength].assign(
-        reinterpret_cast<const char*>(&value_length), sizeof(value_length));
+        reinterpret_cast<const char*>(&value_size_), sizeof(value_size_));
 
+  uint64_t bucket_size = key_size_ + value_size_;
   unused_bucket.resize(bucket_size, 'a');
   // Write the table.
   uint32_t num_added = 0;
@@ -240,9 +254,9 @@ Status CuckooTableBuilder::Finish() {
       s = file_->Append(Slice(unused_bucket));
     } else {
       ++num_added;
-      s = file_->Append(kvs_[bucket.vector_idx].first);
+      s = file_->Append(GetKey(bucket.vector_idx));
       if (s.ok()) {
-        s = file_->Append(kvs_[bucket.vector_idx].second);
+        s = file_->Append(GetValue(bucket.vector_idx));
       }
     }
     if (!s.ok()) {
@@ -251,7 +265,7 @@ Status CuckooTableBuilder::Finish() {
   }
   assert(num_added == NumEntries());
   properties_.raw_key_size = num_added * properties_.fixed_key_len;
-  properties_.raw_value_size = num_added * value_length;
+  properties_.raw_value_size = num_added * value_size_;
 
   uint64_t offset = buckets.size() * bucket_size;
   properties_.data_size = offset;
@@ -330,19 +344,18 @@ void CuckooTableBuilder::Abandon() {
 }
 
 uint64_t CuckooTableBuilder::NumEntries() const {
-  return kvs_.size();
+  return num_entries_;
 }
 
 uint64_t CuckooTableBuilder::FileSize() const {
   if (closed_) {
     return file_->GetFileSize();
-  } else if (kvs_.size() == 0) {
+  } else if (num_entries_ == 0) {
     return 0;
   }
 
   if (use_module_hash_) {
-    return (kvs_[0].first.size() + kvs_[0].second.size()) * kvs_.size() /
-           max_hash_table_ratio_;
+    return (key_size_ + value_size_) * num_entries_ / max_hash_table_ratio_;
   } else {
     // Account for buckets being a power of two.
     // As elements are added, file size remains constant for a while and
@@ -350,11 +363,10 @@ uint64_t CuckooTableBuilder::FileSize() const {
     // only after it exceeds the file limit, we account for the extra element
     // being added here.
     uint64_t expected_hash_table_size = hash_table_size_;
-    if (expected_hash_table_size < (kvs_.size() + 1) / max_hash_table_ratio_) {
+    if (expected_hash_table_size < (num_entries_ + 1) / max_hash_table_ratio_) {
       expected_hash_table_size *= 2;
     }
-    return (kvs_[0].first.size() + kvs_[0].second.size()) *
-           expected_hash_table_size - 1;
+    return (key_size_ + value_size_) * expected_hash_table_size - 1;
   }
 }
 
@@ -390,7 +402,7 @@ bool CuckooTableBuilder::MakeSpaceForKey(
   // of the method. We store this number into the nodes that we explore in
   // current method call.
   // It is unlikely for the increment operation to overflow because the maximum
-  // no. of times this will be called is <= max_num_hash_func_ + kvs_.size().
+  // no. of times this will be called is <= max_num_hash_func_ + num_entries_.
   for (uint32_t hash_cnt = 0; hash_cnt < num_hash_func_; ++hash_cnt) {
     uint64_t bucket_id = hash_vals[hash_cnt];
     (*buckets)[bucket_id].make_space_for_key_call_id =
@@ -408,9 +420,7 @@ bool CuckooTableBuilder::MakeSpaceForKey(
     CuckooBucket& curr_bucket = (*buckets)[curr_node.bucket_id];
     for (uint32_t hash_cnt = 0;
         hash_cnt < num_hash_func_ && !null_found; ++hash_cnt) {
-      uint64_t child_bucket_id = CuckooHash(
-          (is_last_level_file_ ? kvs_[curr_bucket.vector_idx].first :
-           ExtractUserKey(Slice(kvs_[curr_bucket.vector_idx].first))),
+      uint64_t child_bucket_id = CuckooHash(GetUserKey(curr_bucket.vector_idx),
           hash_cnt, use_module_hash_, hash_table_size_, identity_as_first_hash_,
           get_slice_hash_);
       // Iterate inside Cuckoo Block.

table/cuckoo_table_builder.h

@@ -75,6 +75,10 @@ class CuckooTableBuilder: public TableBuilder {
       uint64_t* bucket_id);
   Status MakeHashTable(std::vector<CuckooBucket>* buckets);
 
+  inline Slice GetKey(uint64_t idx) const;
+  inline Slice GetUserKey(uint64_t idx) const;
+  inline Slice GetValue(uint64_t idx) const;
+
   uint32_t num_hash_func_;
   WritableFile* file_;
   const double max_hash_table_ratio_;
@@ -83,10 +87,17 @@ class CuckooTableBuilder: public TableBuilder {
   const uint32_t cuckoo_block_size_;
   uint64_t hash_table_size_;
   bool is_last_level_file_;
+  bool has_seen_first_key_;
+  uint64_t key_size_;
+  uint64_t value_size_;
+  // A list of fixed-size key-value pairs concatenating into a string.
+  // Use GetKey(), GetUserKey(), and GetValue() to retrieve a specific
+  // key / value given an index
+  std::string kvs_;
+  // Number of key-value pairs stored in kvs_
+  uint64_t num_entries_;
   Status status_;
-  std::vector<std::pair<std::string, std::string>> kvs_;
   TableProperties properties_;
-  bool has_seen_first_key_;
   const Comparator* ucomp_;
   bool use_module_hash_;
   bool identity_as_first_hash_;