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Type:
Question
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Resolution: Won't Fix
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Priority:
Critical - P2
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None
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Affects Version/s: 3.6.5
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Component/s: MapReduce
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Labels:None
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;