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

1 files changed, 5 insertions, 0 deletions

diff --git a/src/mongo/db/commands/mr.cpp b/src/mongo/db/commands/mr.cpp
index 1d9a52067fa..2d050c924f1 100644
--- a/src/mongo/db/commands/mr.cpp
+++ b/src/mongo/db/commands/mr.cpp
@@ -55,6 +55,7 @@
 #include "mongo/db/operation_context_impl.h"
 #include "mongo/db/query/get_executor.h"
 #include "mongo/db/query/query_planner.h"
+#include "mongo/db/query/find_common.h"
 #include "mongo/db/range_preserver.h"
 #include "mongo/db/repl/replication_coordinator_global.h"
 #include "mongo/db/s/collection_metadata.h"
@@ -1251,6 +1252,10 @@ public:
         return true;
     }
 
+    std::size_t reserveBytesForReply() const override {
+        return FindCommon::kInitReplyBufferSize;
+    }
+
     virtual void help(stringstream& help) const {
         help << "Run a map/reduce operation on the server.\n";
         help << "Note this is used for aggregation, not querying, in MongoDB.\n";