I was having a swelling under molar gum since 6 months. Week ago I go to dentist he said it is abscess n make opening fr root canal. Nw swelling half?

It is normal. This may be due to a previous infection. If the area was swollen before the root canal treatment it may take a while to heal. Take care.
Question ? Really don't see a question, just that you waited six months to start root canal therapy, and now that you did, the swelling has subsided.