Solr’s default heap size needs to be increased.
The following info is from Shawn Heisey’s post to the Solr mailing list. I copied it here as a note to myself, and in the hopes of helping Solr newcomers.
There are exactly two solutions to the OutOfMemoryError regarding heap
1) Increase the heap size.
2) Decrease the memory requirements.
The default heap size that Solr 5.0 and later starts with is 512MB.
This is a very small heap size. We are aware that the default is very
small — this is intentional, so that the default install is runnable on
virtually any hardware.
Almost all production Solr installs will require increasing the heap
size. If you get a little bit of data in a Solr install and then make
complex query requests, it can easily require more than 512MB of total heap.