We use apcu with currently 8 active webservers. In our case, memcache seemed to be much slower than apcu. However, I heard also reports claiming there to be not much difference if done right. So I am not sure about the perf. difference. However, I guess apcu will always be somewhat faster due to the non-existen networking overheaad, as you also point out.
It is certainly possible to use it on a cluster, however you have to know what you are doing. E.g. you might need to reload the cache if reloading ctrl structures or language or similar an all webservers. It is easy to run into terrible errors after updates, since you will not see those errors when making short tests to give green light after the update to close the maintenance window, since the webserver you will be working on will have no issue, but all other will etc. etc. etc. So I am not sure I can recommend it, but it can work if done right.
Further note, that you can select what parts of ILIAS you wish to cache in the setup (select client -> Tab "Global Cache).
I am not sure if there is much benefit in storing session data in the some caching accessed by network. Given you have a fast database, all needed is a query using an index to access your session of the session table, which might be very fast since there is a proper index at work and the session table does not grow out of bounds due to regular cleaning up. However, I did not make any measurements, so I could be proven wrong. IMO Global cache is most effective to save data from queries which take some time to run and does not change regularly.
It is certainly possible to use it on a cluster, however you have to know what you are doing. E.g. you might need to reload the cache if reloading ctrl structures or language or similar an all webservers. It is easy to run into terrible errors after updates, since you will not see those errors when making short tests to give green light after the update to close the maintenance window, since the webserver you will be working on will have no issue, but all other will etc. etc. etc. So I am not sure I can recommend it, but it can work if done right.
Further note, that you can select what parts of ILIAS you wish to cache in the setup (select client -> Tab "Global Cache).
I am not sure if there is much benefit in storing session data in the some caching accessed by network. Given you have a fast database, all needed is a query using an index to access your session of the session table, which might be very fast since there is a proper index at work and the session table does not grow out of bounds due to regular cleaning up. However, I did not make any measurements, so I could be proven wrong. IMO Global cache is most effective to save data from queries which take some time to run and does not change regularly.