We have many applications. How can we select a application for analisys. We need to select the applications in which the tuning result in higher gain. Which metric or set of metrics can be used for this choice (to pre-select the applications)?
You will first want to deploy a few "known good" applications to establish that you can deploy reliably and with good visibility. You need to show stakeholders that deploying the monitoring does not affect the application, before they will accept your recommendations. Once you are confident that you can use APM with a "known good" application, then you are in a position to go against a problematic application.
To select candidates for tuning you need only look at the operational incidents.
Selecting good candidate applications is fundamental to APM success and being able to show value with your skills or service capability. Simply take the risk out of the activity by working first with well-known applications that have a reasonable pre-production test environment, so that you can practice using the tool and interpreting the information. Follow the Application Audit process to learn what you can do with the APM tools. Be able to easily identify KPIs for the application and you have the beginnings of being able to identify performance bottlenecks and prove the benefit of configuration changes.
If you cannot show what you can do for a healthy application - you simply cannot expect much success with a problematic application!
If you want to broadly survey a large number of applications, consider using the Application Survey questionnaire. This is a doc that you ask the stakeholder to fill out and lets you organize the applications in terms of monitoring approach that would be needed, as well as sizing the impact of that deployment. More information lets you make better choices - so don;t be afraid to ask - and NEVER assume!
And if you really want to master APM, then check out APM best practices, in paper or ebook.
Thank you Mike for your very rich and very useful response !