Where I can find useful Tcodes to Check System Performance?

It's really useful to check the system general status about the RAM, amount of user, sessions, process running in a period of time, could you recommend some Tcodes to check that king of things in a SAP system?

Thank you in advance.

The following are really useful Tcodes to check the system in order to detect performance issues and other stats for analysis.

SM51 - Check Application Servers

Here you can see which services or work processes are configured in each instance.

SM50 - Monitoring Work Processes for Individual Instances

Displays all running, waiting, stopped and PRIV processes related to a particular instance. Under this step we check all the processes; the process status should always be waiting or running. If any process is having a status other than waiting or running we need to check that particular process and report accordingly.

SM66 - Monitoring System-wide Work Processes

By checking the work process load using the global work process overview, we can quickly investigate the potential cause of a system performance problem.

AL08 and SM04 - Monitor Application User

This transaction displays all the users of active instances.

SM13 - Monitoring Update Processes

If there are no long pending updates records or no updates are going on then this queue will be empty as shown in the below screenshot.

SM12 - Monitoring Lock Entries

SAP provides a locking mechanism to prevent other users from changing the record that you are working on. In some situations, locks are not released. This could happen if the users are cut off i.e. due to network problem before they are able to release the lock.1

SM21 - Monitoring System Log

We can use the log to pinpoint and rectify errors occurring in the system and its environment.
We check the log for the previous day with the following selection/option

ST02 - Tune Summary

Idle CPU utilization rate must be 60-65%, if it exceeds the value then we must start checking at least below things:

Run OS level commands – top and check which processes are taking most resources.
Go to SM50 or SM66. Check for any long running jobs or any long update queries being run.
Go to SM12 and check lock entries
Go to SM13 and check Update active status.
Check for the errors in SM21.

ST22 - ABAP Dumps

Here we check the dumps occurred in a period of time, you can also filter it by username.

SP01 - Spool Request Monitoring

Record only those requests which are terminated with problems.

SM37 - Monitoring Batch Jobs

For Monitoring background jobs

SM58 - Transactional RFC Administration

We need to select the display period for which we want to view the tRFCs and then select '*' in the username field to view all the calls which have not be executed correctly or waiting in the queue.

SMQ1 - QRFC Administration (Outbound Queue-SMQ1)

We should specify the client name over here and see if there any outgoing qRFCs in waiting or error state.

SMQ2 - QRFC Administration (Inbound Queue-SMQ2)

We should specify the client name over here and see if there any outgoing qRFCs in waiting or error state.

DB02 - Database Administration

Check important database settings and status

DB12 - Database Backup logs

We could determine when the last successful backup of the system was. We can review the previous day's backups and see if everything was fine or not.