The reason to reduce default amount of memory allocated for Ignite from 80% to 20% was numerous complaints about severe performance degradation. The investigations showed that was due to remaining 20% of RAM often was not enough for the operating system and other typical enterprise utilities to operate efficiently thus killing the server performance. 20% was considered a safe default amount and you can always configure amount of available memory <https://apacheignite.readme.io/v2.1/docs/memory-configuration#section-memory-policies> for your specific setup.
The memory configuration <https://apacheignite.readme.io/v2.1/docs/memory-configuration#section-memory-policies> is applicable only for off-heap memory. If you enabled on-heap caching, it is strongly recommended to configure an eviction policy <https://apacheignite.readme.io/v2.1/docs/evictions#section-entry-based-eviction-for-on-heap-cache> to limit the on-heap memory usage. Otherwise you can go out of memory.
