SERVER-21459 reserve bytes in ReplyBuilder buffer to avoid excessive reallocing

1 files changed, 7 insertions, 0 deletions

diff --git a/src/mongo/db/commands/getmore_cmd.cpp b/src/mongo/db/commands/getmore_cmd.cpp
index 4ba445d1825..882215570f0 100644
--- a/src/mongo/db/commands/getmore_cmd.cpp
+++ b/src/mongo/db/commands/getmore_cmd.cpp
@@ -107,6 +107,13 @@ public:
         return LogicalOp::opGetMore;
     }
 
+    std::size_t reserveBytesForReply() const override {
+        // The extra 1K is an artifact of how we construct batches. We consider a batch to be full
+        // when it exceeds the goal batch size. In the case that we are just below the limit and
+        // then read a large document, the extra 1K helps prevent a final realloc+memcpy.
+        return FindCommon::kMaxBytesToReturnToClientAtOnce + 1024u;
+    }
+
     /**
      * A getMore command increments the getMore counter, not the command counter.
      */