[louai]

A characteristic of intrusive data structures is generally good cache friendliness. In this context "intrusive" means that the data structure is embedded with the data that's being stores.

In C terms as in this case that means that the pointers required of individual nodes aren't allocated in separate structs and are instead embedded in one struct that also includes the payload.

This means that the cache behavior is improved, as a given node is stored in a single location. Once you access a node the associated data is already in the cache, instead of having to be fetched via a separate pointer dereference.