Implement snapshot consistency under the write load

core/dash.h

@@ -298,6 +298,17 @@ class DashTable : public detail::DashTableBase {
     return const_bucket_iterator{it.owner_, it.seg_id_, it.bucket_id_, 0};
   }
 
+  // Capture Version Change. Runs cb(it) on every bucket! (not entry) in the table whose version
+  // would potentially change upon insertion of 'k'.
+  // In practice traversal is limited to a single segment. The operation is read-only and
+  // simulates insertion process. 'cb' must accept const_iterator. Note: the interface a bit hacky
+  // since the iterator may point to non-existing slot. In practice it only can be sent to
+  // TraverseBucket function.
+  // The function returns any bucket that is touched by the insertion flow. In practice, we
+  // could tighten estimates by taking the new version into account.
+  // Not sure how important this is though.
+  template <typename U, typename Cb> void CVCUponInsert(const U& key, Cb&& cb) const;
+
   void Clear();
 
  private:
@@ -349,6 +360,32 @@ DashTable<_Key, _Value, Policy>::~DashTable() {
   });
 }
 
+template <typename _Key, typename _Value, typename Policy>
+template <typename U, typename Cb>
+void DashTable<_Key, _Value, Policy>::CVCUponInsert(const U& key, Cb&& cb) const {
+  uint64_t key_hash = DoHash(key);
+  uint32_t seg_id = SegmentId(key_hash);
+  assert(seg_id < segment_.size());
+  const SegmentType* target = segment_[seg_id];
+
+  uint8_t bids[2];
+  unsigned num_touched = target->CVCOnInsert(key_hash, bids);
+  if (num_touched > 0) {
+    for (unsigned i = 0; i < num_touched; ++i) {
+      cb(const_iterator{this, seg_id, bids[i], 0});
+    }
+    return;
+  }
+
+  static_assert(kPhysicalBucketNum < 0xFF, "");
+
+  // Segment is full, we need to return the whole segment, because it can be split
+  // and its entries can be reshuffled into different buckets.
+  for (uint8_t i = 0; i < kPhysicalBucketNum; ++i) {
+    cb(const_iterator{this, seg_id, i, 0});
+  }
+}
+
 template <typename _Key, typename _Value, typename Policy>
 void DashTable<_Key, _Value, Policy>::Clear() {
   auto cb = [this](SegmentType* seg) {

server/snapshot.cc

@@ -33,9 +33,26 @@ SliceSnapshot::~SliceSnapshot() {
 
 void SliceSnapshot::Start(DbSlice* slice) {
   DCHECK(!fb_.joinable());
+  db_slice_ = slice;
 
-  snapshot_version_ =
+  auto on_change = [this, slice](DbIndex db_index, const DbSlice::ChangeReq& req) {
-      slice->RegisterOnChange([this](DbIndex index, const DbSlice::ChangeReq& req) {});
+    PrimeTable* table = slice->GetTables(db_index).first;
+
+    if (const MainIterator* it = get_if<MainIterator>(&req)) {
+      if (it->GetVersion() < snapshot_version_) {
+        side_saved_ += SerializePhysicalBucket(table, *it);
+      }
+    } else {
+      string_view key = get<string_view>(req);
+      table->CVCUponInsert(key, [this, table](PrimeTable::const_iterator it) {
+        if (it.MinVersion() < snapshot_version_) {
+          side_saved_ += SerializePhysicalBucket(table, it);
+        }
+      });
+    }
+  };
+
+  snapshot_version_ = slice->RegisterOnChange(move(on_change));
   VLOG(1) << "DbSaver::Start - saving entries with version less than " << snapshot_version_;
   sfile_.reset(new io::StringFile);
 
@@ -53,7 +70,7 @@ void SliceSnapshot::Join() {
 
 static_assert(sizeof(PrimeTable::const_iterator) == 16);
 
-void SliceSnapshot::SerializeCb(MainIterator it) {
+void SliceSnapshot::SerializeSingleEntry(MainIterator it) {
   error_code ec;
 
   string tmp;
@@ -163,23 +180,28 @@ bool SliceSnapshot::SaveCb(MainIterator it) {
   }
 
   physical_mask_.set(it.bucket_id());
-
+  SerializePhysicalBucket(prime_table_, it);
-  // Both traversals below execute atomically.
-  // traverse physical bucket and write into string file.
-  prime_table_->TraverseBucket(it, [this](auto entry_it) { this->SerializeCb(move(entry_it)); });
-
-  // Theoretically we could merge version_cb into the traversal above but then would would need
-  // to give up on DCHECK.
-  auto version_cb = [this](MainIterator entry_it) {
-    DCHECK_LE(entry_it.GetVersion(), snapshot_version_);
-    DVLOG(3) << "Bumping up version " << entry_it.bucket_id() << ":" << entry_it.slot_id();
-
-    entry_it.SetVersion(snapshot_version_);
-  };
-
-  prime_table_->TraverseBucket(it, version_cb);
 
   return false;
 }
 
+unsigned SliceSnapshot::SerializePhysicalBucket(PrimeTable* table, PrimeTable::const_iterator it) {
+  // Both traversals below execute atomically.
+  // traverse physical bucket and write into string file.
+  unsigned result = 0;
+  table->TraverseBucket(it, [&](auto entry_it) {
+    ++result;
+    SerializeSingleEntry(move(entry_it));
+  });
+
+  table->TraverseBucket(it, [this](MainIterator entry_it) {
+    DCHECK_LE(entry_it.GetVersion(), snapshot_version_);
+    DVLOG(3) << "Bumping up version " << entry_it.bucket_id() << ":" << entry_it.slot_id();
+
+    entry_it.SetVersion(snapshot_version_);
+  });
+
+  return result;
+}
+
 }  // namespace dfly

server/snapshot.h

@@ -33,9 +33,12 @@ class SliceSnapshot {
  private:
   void FiberFunc();
   bool FlushSfile(bool force);
-  void SerializeCb(MainIterator it);
+  void SerializeSingleEntry(MainIterator it);
   bool SaveCb(MainIterator it);
 
+  // Returns number of entries serialized.
+  unsigned SerializePhysicalBucket(PrimeTable* table, PrimeTable::const_iterator it);
+
   ::boost::fibers::fiber fb_;
 
   enum {PHYSICAL_LEN = 128};
@@ -47,9 +50,10 @@ class SliceSnapshot {
   // version upper bound for entries that should be saved (not included).
   uint64_t snapshot_version_ = 0;
   PrimeTable* prime_table_;
+  DbSlice* db_slice_ = nullptr;
   StringChannel* dest_;
 
-  size_t serialized_ = 0, skipped_ = 0;
+  size_t serialized_ = 0, skipped_ = 0, side_saved_ = 0;
 };
 
 }  // namespace dfly