Sizing JVMs and VM Memory

How to estimate the amount of JVM heap and host memory requirement in complex cases where there are more JVM instances per VM and more apps per JVM and have different usage requirement per app ?

let's see some of the metrics to collect to construct an equation on estimating the sizes.

• list the apps per instance. Ideally there will be multiple apps deployed on one instance of JVM and all these apps are not equally used and each app will have its own characteristics thus have uneven memory requirements i.e. based on req. classes, code logic and constructs etc.,

• And identify the 'typical' transactions.. ideally, in any app, 20% of features are executed 80% of times

• Take a first measurement - server start up heap. Once the server is up completely, get the first point after a full GC (hotspot) or first OC (jrockit)

• Now, ideally the first user access is demanding one as it loads lot of stuff. So, do just logins into each of the app and take a note of OCs/FGCs for each user. So, per app, the numbers gives the amount of heap required for first login

• So, XMS - to make it good enough, it can be sum of (server startup heap + sum of all above deltas )

• Now, exercise the typical 20% transaction as single user per app and do not logout - so these are typical active users on the system - call these deltas as h1, h2 etc . This can be on a warmed up server but best is to have no other logged in users in the system. Each delta can be noted or take the final delta after all different app users are in and executed the typical flows but not logged out.

• XMX i.e. the max amount of memory can be XMS + ( total delta * (%app1 conc. users)   +  ... )

for hotspot jvms or other jvms where there are more heap spaces like young/survivor, perm etc., they can be calculated as proportionate ratio of XMX ..

And, typical host memory usage requirement can be calculated as approx 1.8 times of XMX

If someone can simulate load and want to bare the cost, time and complexity of all setup to do that then it is probably best to get the numbers based on test results.., the benefits in doing so can be not just limited to JVM heap but gives the other resource pools across the layers..