Limitation for DirSync

Does anyone knows limitations of QMAD for the DirSync? I see that the cached memory runs against a 2 GB border (what is the maximum for addressable  memory) and then after a whiel the eror message "out of memory" will be raised. After this, the process seems to run, but it will not synchronize objects anymore.

There is somewhere a critical number of objects listed - 1 Mio. But the problem I see already reaching less than 100K objects on the InitialSync. Any hints?