How can I completely purge docs.deleted for an index?

I'v done cleaning index with following command:

curl -XPOST 'http://localhost:9200/my-indices-*/_forcemerge?only_expunge_deletes=true&max_num_segments=1&flush=true'

But there is still numbers of docs.deleted docs left in index.
How can I completely delete these deleted documents in index ?

You need to set index.merge.policy.expunge_deletes_allowed to 0 (it defaults to 10%), but force merge, even with only_expunge_deletes, is a costly operation and insisting on 0% deletes just makes it more so!

Thanks for your reply.
I just need index takes as few storeage space as possible.
So I can ignore the cost ..