[Boost-users] [intrusive] linear hashing

Hi, Can anyone explain the performance characteristics of linear hashing. I read the Wikipedia article and I'm still not sure. The article mentions growing the hash table incrementally by 1, but the instrusive  unordered_set doesn't grow/rehash unless you tell it to? When does it make sense to use the linear hashing option? Why is it not on by default? Is it because it slows down the address lookup into the table. Is the ideal bucket size still 1 per expected element. Thanks -ephi