Branch mispredictions in KV transaction fusing & reordering that is built on top of the std::map and rocksdb::WriteBatch::Iterate

perf-histogram.0.txt

-   22,54%     0,06%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] BlueStore::_kv_sync_thread
   - 22,48% BlueStore::_kv_sync_thread
      + 14,55% RocksDBStore::submit_transaction_sync
      - 7,12% fuse_and_reorder_transactions
         + 5,71% rocksdb::WriteBatch::Iterate
         + 0,69% rocksdb::WriteBatch::Put
+   22,50%     0,00%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] BlueStore::KVSyncThread::entry
+   22,50%     0,00%  bstore_kv_sync  libpthread-2.23.so          [.] start_thread
+   22,38%     0,00%  bstore_kv_sync  libc-2.23.so                [.] __clone
+   15,39%     0,08%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::WriteBatch::Iterate
+   14,55%     0,01%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] RocksDBStore::submit_transaction_sync
+   14,51%     0,02%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] RocksDBStore::submit_common
+   14,49%     0,02%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::DBImpl::Write
+   14,47%     0,06%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::DBImpl::WriteImpl
+    9,70%     0,01%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::WriteBatchInternal::InsertInto
+    9,55%     0,06%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::MemTable::Add
+    9,18%     1,23%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::InlineSkipList<rocksdb::MemTableRep::KeyComparator const&>::Insert<false>
+    9,09%     0,02%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::MemTableInserter::PutCF
+    7,12%     0,18%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] fuse_and_reorder_transactions
+    6,54%     0,21%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::MemTable::KeyComparator::operator()
+    6,42%     0,16%  bstore_kv_sync  libfio_ceph_objectstore.so  [.] rocksdb::InternalKeyComparator::Compare
+    6,41%     6,36%  bstore_kv_sync  libc-2.23.so                [.] __memcmp_sse4_1