Answer
There are two ways to determine optimum number of WP's (DIA,BTC,UPD..).
1. Use AL12 -> Statistics -> Cursor Cache -> All servers
You will see the total requests for each WP's. If your DIA WP's all have high request, then you have to add additional DIA WP's.
But if you have DIA WP's which have less requests among others, it means you are not using this WP's usually. So you can decrease your WP number to open free space in buffer.
2. Use SM50 and display CPU Time via CTRL+SHIFT+F6.
Here you will see the total CPU time of WP's. With the same logic, if you have WP's whose CPU Time is less than 0:10 then you can remove these WP's via INST_PROFILE.
But if all of your WP's CPU Time is greater than 0:50,
you have to increase number of WP's.
Notes
Each work process use 150 MB space and each dialog process can cater the 5 - 7 user's and you can make the optimum utilization of work process thru the operation modes i.e., keep the dialog process more during the peak hours and change the some of the dialog process into background work processes during the off-peak hours.. This helps your system performance.
Eventually, it depends on your system capacity and also
how much users you have. You should have more DIA processes in day and
more BTC in night. In day users will execute report and do their daily
business related tasks that is why you should have more DIA. In night users
may schedule background jobs to get daily/weekly reports. If you have 20
WPs in that you can have 11 DIA, 4 BTC, 1 SPO, 2 updates, 1 ENQ, 1 UP2
etc.
Additional Notes
The number of work processes is determined by two factors:
1. Usage of the system
2. Available hardware.
I mean in order to sustain the performance of a system with large number of users you may need greater number of work processes but at the same time you cannot increase the total number of work processes to any number of your choice since it depends of your hardware specifically memory. Every WP consumes memory on your application server as well as DB server.
Coming back to work process distribution there are few useful recommendations that SAP makes.
1. The number of dialog process should be at least equal or more than the combined number of other work processes i.e. equal to or more than 50% of total work processes during day time.
2. Number of dialog processes in day time should be more in day time while the number of background processes should be greater at night. This you can control through operation mode switch.
3. Also configuration of number of background work processes with job class A needs to be done carefully. At times of high load the number can be decreased.
4. The other work processes also need to be configured as per the requirement.
