Two performance improvements in BlockBuilder (#9039)

Summary:
Primarily, this change reserves space in the std::string for building
the next block once a block is finished, using `block_size` as
reservation size. Note: also tried reusing same std::string in the
common "unbuffered" path but that showed no benefit or regression.

Secondarily, this slightly reduces the work in resetting `restarts_`.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9039

Test Plan:
TEST_TMPDIR=/dev/shm/rocksdb1 ./db_bench -benchmarks=fillseq -memtablerep=vector -allow_concurrent_memtable_write=false -num=50000000
Compiled with DEBUG_LEVEL=0
Test vs. control runs simulaneous for better accuracy, units = ops/sec

Run 1, Primary change only: 292697 vs. 280267 (+4.4%)
Run 2, Primary change only: 288763 vs. 279621 (+3.3%)
Run 1, Secondary change only: 260065 vs. 254232 (+2.3%)
Run 2, Secondary change only: 275925 vs. 272248 (+1.4%)
Run 1, Both changes: 284890 vs. 270372 (+5.3%)
Run 2, Both changes: 263511 vs. 258188 (+2.0%)

Reviewed By: zhichao-cao

Differential Revision: D31701253

Pulled By: pdillinger

fbshipit-source-id: 7e40810afbb98e6b6446955e77bda59e69b19ffd

table/block_based/block_based_table_builder.cc

@@ -1023,6 +1023,7 @@ void BlockBasedTableBuilder::WriteBlock(BlockBuilder* block,
                                         BlockType block_type) {
   block->Finish();
   std::string raw_block_contents;
+  raw_block_contents.reserve(rep_->table_options.block_size);
   block->SwapAndReset(raw_block_contents);
   if (rep_->state == Rep::State::kBuffered) {
     assert(block_type == BlockType::kData);

table/block_based/block_builder.cc

@@ -50,7 +50,7 @@ BlockBuilder::BlockBuilder(
     : block_restart_interval_(block_restart_interval),
       use_delta_encoding_(use_delta_encoding),
       use_value_delta_encoding_(use_value_delta_encoding),
-      restarts_(),
+      restarts_(1, 0),  // First restart point is at offset 0
       counter_(0),
       finished_(false) {
   switch (index_type) {
@@ -64,14 +64,13 @@ BlockBuilder::BlockBuilder(
       assert(0);
   }
   assert(block_restart_interval_ >= 1);
-  restarts_.push_back(0);  // First restart point is at offset 0
   estimate_ = sizeof(uint32_t) + sizeof(uint32_t);
 }
 
 void BlockBuilder::Reset() {
   buffer_.clear();
-  restarts_.clear();
-  restarts_.push_back(0);  // First restart point is at offset 0
+  restarts_.resize(1);  // First restart point is at offset 0
+  assert(restarts_[0] == 0);
   estimate_ = sizeof(uint32_t) + sizeof(uint32_t);
   counter_ = 0;
   finished_ = false;