Re: perfomance of clear vs swap

I have a call to hash_map::clear() function which takes long time.

someClass::someFunction()
{

     // typedef hash_map<name_id, uint> Mp;
     // Mp p;
     // assuming proper namespace, hash function for name_id obj.

     p.clear();
}

Above p.clear() takes long time, profiling indicates 'number of bucket
of hash table is large'.

Now, just for sake of experiments, I replaced this 'clear' call with
swap. i.e.

someClass::someFunction()
{

     // typedef hash_map<name_id, uint> Mp;
     // Mp p;
     // assuming proper namespace, hash function for name_is obj.

     //p.clear();
     Mp tmp;
     p.swap(tmp);
}

Now runtime drops significantly, 10 fold less.

What's exactly cause this run time reduction?