When you are determining whether to add additional engines, consider:
Load on existing engines
Contention for resources, such as locks on tables, disks, and cache spinlocks
Response time
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 Monitoring Performance with sp_sysmon in Performance and Tuning Series: Monitoring Adaptive Server with sp_sysmon. In particular, study the lines or sections in the output that may reveal points of contention:
Logical Lock Contention
Address Lock Contention
ULC Semaphore Requests
Log Semaphore Requests
Page Splits
Lock Summary
Spinlock Contention
I/Os Delayed by
After increasing the number of engines, run sp_sysmon again under similar load conditions, and check the “Engine Busy Utilization” section in the report and the possible points of contention listed above.