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Is 8192 Diffie Hellman overkill?

I've finally ran ssl server test on my server and was presented with

This server supports weak Diffie-Hellman (DH) key exchange parameters. Grade capped to B.

So I tried to generate stronger using openssl dhparam 8196 but it didn't finished even after a day. So, two questions:

1) is it an overkill and is 4096 sufficient for now?

2) can I make openssl dhparam ran on multiple cores?

Thanks