Reducing memcpy overhead when using Iterators

Use case

In certain scenarios the user may need to Iterate over range of keys and keep them in memory to process them. A simple example could be something like this

ReadOptions ro;
Iterator* iter = db_->NewIterator(ro);

// Get the keys from the DB
std::vector<std::string> db_keys;
for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
	db_keys.push_back(iter->key().ToString());
}

// Process the keys (in this case we simply sort them)
auto key_comparator = [](const std::string& k1, const std::string& k2) {
	if (k1.size() == k2.size()) {
		return k1 < k2;
	}
	return k1.size() < k2.size();
};
std::sort(db_keys.begin(), db_keys.end(), key_comparator);

for (size_t i = 0; i < db_keys.size(); i++) {
	// Use processed keys
}

delete iter;

In this example we simply load keys from the DB into memory, sort them using a comparator that is different from DB comparator and then use the sorted keys.

The Problem

The issue with this approach is in this line

db_keys.push_back(iter->key().ToString());

If our keys are huge the cost of copying the key from RocksDB into our std::vector will be significant and we cannot escape this overhead since iter->key() Slice will be invalid the moment our Iterator move forward when iter->Next() is called.

The Solution

We have introduced a new option for Iterators, ReadOptions::pin_data. When setting this option to true, RocksDB Iterator will pin the data blocks and guarantee that the Slice returned by Iterator::key() will be valid as long as the Iterator is not deleted.

ReadOptions ro;
// Tell RocksDB to keep the key Slice valid as long as
// the Iterator is not deleted
ro.pin_data = true;
Iterator* iter = db_->NewIterator(ro);

// Get the keys from the DB
std::vector<Slice> db_keys;
for (iter->SeekToFirst(); iter->Valid(); iter->Next()) {
	std::string is_key_pinned;
	iter->GetProperty("rocksdb.iterator.is-key-pinned", &is_key_pinned);
	assert(is_key_pinned == "1");

	// iter->key() Slice will be valid as long as
	// the Iterator is not deleted
	db_keys.push_back(iter->key());
}

// Process the keys (in this case we simply sort them)
auto key_comparator = [](const Slice& k1, const Slice& k2) {
	if (k1.size() == k2.size()) {
		return k1.compare(k2) < 0;
	}
	return k1.size() < k2.size();
};

std::sort(db_keys.begin(), db_keys.end(), key_comparator);

for (size_t i = 0; i < db_keys.size(); i++) {
	// Use processed keys
}

delete iter;

After setting ReadOptions::pin_data to true, now we can use Iterator::key() Slice without copying it

db_keys.push_back(iter->key());

Requirements

Right now to use this feature, RocksDB must be created using BlockBased table with BlockBasedTableOptions::use_delta_encoding set to false. To verify that the current key Slice is pinned and will be valid as long as the Iterator is not deleted, We can check rocksdb.iterator.is-key-pinned Iterator property and assert that it's equal to 1

std::string is_key_pinned;
iter->GetProperty("rocksdb.iterator.is-key-pinned", &is_key_pinned);
assert(is_key_pinned == "1");