how to avoid OOM while merge index

[James]

I am build the solr index on the hadoop, and at reduce step I run the task that 
merge the indexes, each part of index is about 1G, I have 10 indexes to merge 
them together, I always get the java heap memory exhausted, the heap size is 
about 2G  also. I wonder which part use these so many memory. And how to avoid 
the OOM during the merge process.