DB::Put() to estimate write batch data size needed and pre-allocate buffer

Summary:
In one of CPU profiles, we see some CPU costs of string::reserve() inside Batch.Put(). This patch should be able to reduce some of the costs by allocating sufficient buffer before hand.

Since it is a trivial percentage of CPU costs, I didn't find a way to show the improvement in one of the benchmarks. I'll deploy it to same application and do the same CPU profiling to make sure those CPU costs are reduced.

Test Plan: make all check

Reviewers: haobo, kailiu, igor

Reviewed By: haobo

CC: leveldb, nkg-

Differential Revision: https://reviews.facebook.net/D15135

db/db_impl.cc

@@ -3789,7 +3789,10 @@ Status DBImpl::GetDbIdentity(std::string& identity) {
 // Default implementations of convenience methods that subclasses of DB
 // can call if they wish
 Status DB::Put(const WriteOptions& opt, const Slice& key, const Slice& value) {
-  WriteBatch batch;
+  // Pre-allocate size of write batch conservatively.
+  // 8 bytes are taken by header, 4 bytes for count, 1 byte for type,
+  // and we allocate 11 extra bytes for key length, as well as value length.
+  WriteBatch batch(key.size() + value.size() + 24);
   batch.Put(key, value);
   return Write(opt, &batch);
 }

db/write_batch.cc

@@ -35,7 +35,8 @@ namespace rocksdb {
 // WriteBatch header has an 8-byte sequence number followed by a 4-byte count.
 static const size_t kHeader = 12;
 
-WriteBatch::WriteBatch() {
+WriteBatch::WriteBatch(size_t reserved_bytes) {
+  rep_.reserve((reserved_bytes > kHeader) ? reserved_bytes : kHeader);
   Clear();
 }
 

include/rocksdb/write_batch.h

@@ -35,7 +35,7 @@ struct SliceParts;
 
 class WriteBatch {
  public:
-  WriteBatch();
+  explicit WriteBatch(size_t reserved_bytes = 0);
   ~WriteBatch();
 
   // Store the mapping "key->value" in the database.