A few months ago,there is the same issue in our live DB.

But I do not want to set MAXDOP and the cost threshold for parallelism.

Because such actions are palliatives, so we must find out all poor efficiency query plans in our database.

You can run these commands.

select top 100 b.text,c.client_net_address,a.last_wait_type,a.*

from sys.dm_exec_requests a cross apply sys.dm_exec_sql_text(a.sql_handle) b

inner join sys.dm_exec_connections c on a.session_id=c.session_id

where a.session_id>50 and a.session_id<>@@spid

--

select top 10 total_worker_time/execution_count as avg_cpu_cost,

plan_handle,execution_count,

(select substring(text,statement_start_offset/2+1,(case when statement_end_offset=-1

then len(convert(nvarchar(max),text))*2

else statement_end_offset

end -statement_start_offset)/2)

from sys.dm_exec_sql_text(sql_handle)) as query_text

from sys.dm_exec_query_stats

order by avg_cpu_cost desc