[SERVER-35590] GlobalWrite during MapReduce: outType: reduce nonAtomic: true - MongoDB Jira

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I check the source code in the file mr.cpp and I can see

that a Map-Reduce with nonAtomic: true and outType: reduce can generate a Global Lock.

In the comment in the code is written: "This must be global because we may write across different databases."

My question is how can a map reduce will write across different databases, if it is needed to specified the output database of the map reduce?

This Global locks has a lot of influence in the performance of another operations that are I am running (make them slow).

Code (mr.cpp):

if (_config.outputOptions.outType == Config::REDUCE) {

 // reduce: apply reduce op on new result and existing one

 BSONList values;

 const auto count = collectionCount(opCtx, _config.tempNamespace, callerHoldsGlobalLock);

 stdx::lock_guard<Client> lk(*opCtx->getClient());

 curOp->setMessage_inlock(

 "m/r: reduce post processing", "M/R Reduce Post Processing Progress", count);

 unique_ptr<DBClientCursor> cursor = _db.query(_config.tempNamespace.ns(), BSONObj());

 while (cursor->more()) {

 // This must be global because we may write across different databases.

 {color:#f6c342}Lock::GlobalWrite lock(opCtx);{color}

 BSONObj temp = cursor->nextSafe();

 BSONObj old;