Often we may need some special setup for a special case/cause.
Recently, I had to do a t3d export/import, where the data size was nearly 2GB, and no matter what memory_limit we tried, it always failed.
Finally we tried memory_limit = -1, which basically removes the memory limit, and lets us use the max available memory of the Operating System.
This solution did helped us, and we were able to do the export smoothly.
CAUTION: On a production server try not to do this, if we forget to revert this change, things can get worse!
Instead make a local copy, and then we have a total freedom.