When I configure

 
<async
    enabled="true"
    threadPoolSize="3"/>

the implementation creates three threads names "CoalescedAsyncStore" per cache store.

When adding a large amount of data to the cache (for example indexing a lot of Lucene documents), the application will stop working with a lot of OutOfMemory exceptions.

This is caused by the implementation of org.infinispan.loaders.decorators.AsyncStore that uses an unbounded LinkedBlockingQueue.
A lot of worker threads push a huge amount of modifications into the queues, but the underlying cache store is not able to handle throughput.

As a quick solution change

   private final LinkedBlockingQueue<Modification> changesDeque = new LinkedBlockingQueue<Modification>();

to

   private final LinkedBlockingQueue<Modification> changesDeque = new LinkedBlockingQueue<Modification>(512);

and add a new configuration option to the async element, that specifies the capacity of the Linked Blocking Queue.