For purposes of generation merging, it is recommended that you set the ulimit -v and -m parameters to unlimited.
An unlimited setting for the -v option sets no limit on the maximum amount of virtual memory available to a process, and for the -m option sets no limit on the maximum resident set size. These unlimited settings can help prevent problems when the Dgraph is merging the generation files.
ERROR 04/03/13 05:24:35.668 UTC (1364966675668) DGRAPH {dgraph} BackgroundMergeTask: exception thrown: Can't parse generation file Endeca.generation.v2-v446.dat, caused by I/O Exception: While mapping file, caused by mmap failure (file=/data/mdx/workspace/test_indexes/generations/Endeca.generation.v2-v446.dat offset=0 length=28728057528): Cannot allocate memory
In this case, the problem was caused because the Dgraph could not allocate enough virtual memory for its merging task.