I have installed Cassandra (from DataStax) and Riak in my computer. I want to benchmark them with varieties workload and record size. I am using YCSB tool.
Do I need to use any public datacentre/cloud environment before I benchmark, or the Databases are already running in some cloud environments?
The short answer, I believe if I follow your question, is no.
It is quite possible to benchmark the database on your local computer without having to set up environments in the "cloud". If you have the databases correctly setup on your local machine and point YCSB to them properly your should be able to run the tests.
There are of course some additional considerations in benchmarking like: