When you are determining whether to add additional engines, the major factors to consider are the:
Load on existing engines
Contention for resources such as locks on tables, disks, and cache spinlocks
If the load on existing engines is more than 80 percent, adding an engine should improve response time, unless contention for resources is high or the additional engine causes contention.
Before configuring more engines, use sp_sysmon to establish a baseline. Look at the sp_sysmon output for the following sections in Chapter 39, “Monitoring Performance with sp_sysmon.”
In particular, study the lines or sections in the output that may reveal points of contention:
After increasing the number of engines, run sp_sysmon again under similar load conditions, and check the “Engine Busy Utilization” section in the report along with the possible points of contention listed above.