Do concurrent in-memory databases exist?

I spent days searching the internet but all the databases I found are at most in-memory but not concurrent. By concurrency I mean that they run on multiple threads and can do multiple reads concurrently without read locks. This way a system which uses a lot of complex SQL queries could scale much better with increased number of requests per time interval. The only "solution" to this kind of scaling I found was just running multiple servers with duplicate databases. If they exist, can you give me an example of some in-memory concurrent (relational) database? If not, why do they not exist?