From the documentation:
Chapter 14: Adjust Performance Web Agent Cache
By configuring the cache settings, you can manage how this information is stored. The size of the cache is measured by the number of cache entries. The total number of entries in each cache cannot exceed the maximum cache size specified.
Note: You have to restart the Web Server for changes in the Web Agent cache settings to take effect.
(Web Agent Guide r6.x QMR6)
Do we have to restart the Web Agent immediately after the ACO cache configuration modifications?
Yes. You need to restart the Web Agent just after having changed the Web Agent Cache settings, as this ACO parameter affect the behavior of the system process, and thus, you need to restart the process to set its value to the ACO parameter. If not, the process is running with a value and the Web Agent code look for another and you will face errors like:
[07/Nov/2011:03:28:30] [Warning] SiteMinder Agent Resource cache failed to initialize. Failed to attach to the resource cache. Please verify that the LLAWP process is running.