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- Map Reduce Concurrency
Map Reduce Concurrency¶
The map-reduce operation is composed of many tasks, including reads
from the input collection, executions of the map
function,
executions of the reduce
function, writes to a temporary collection
during processing, and writes to the output collection.
During the operation, map-reduce takes the following locks:
- The read phase takes a read lock. It yields every 100 documents.
- The insert into the temporary collection takes a write lock for a single write.
- If the output collection does not exist, the creation of the output collection takes a write lock.
- If the output collection exists, then the output actions (i.e.
merge
,replace
,reduce
) take a write lock. This write lock is global, and blocks all operations on themongod
instance.
Changed in version 2.4: The V8 JavaScript engine, which became the default in 2.4, allows
multiple JavaScript operations to execute at the same time. Prior to
2.4, JavaScript code (i.e. map
, reduce
, finalize
functions) executed in a single thread.
Note
The final write lock during post-processing makes the results appear
atomically. However, output actions merge
and reduce
may
take minutes to process. For the merge
and reduce
, the
nonAtomic
flag is available, which releases the lock between
writing each output
document. See the db.collection.mapReduce()
reference for more
information.