HI @laoshiren,
That's a bit odd since you're not training a classifier here and if you have selected 11G RAM to your VirtualBox (not just what the max could be in the launch settings) I would have thought that would be enough for just assigning taxonomy. But I suppose it's not unheard of either, see here for a similar post. The apparent solution there was:
Use greengenes instead of SILVA — that should help a lot. You can also adjust the
reads-per-batchparameter (e.g. try 2000) to have a longer but lower memory job
You can also use a more powerful machine if you have access to.