distributed supercomputer can't handle a centralized task like a real supercomputer.
Yes, exactly. You don't have a shared memory in distributed computing which creates a lot of problems to begin with. You need to virtualize and manage the distribued memory as if it's a single gigantic memory (Source : Single system image - Wikipedia, the free encyclopedia You use frameworks like Hazelcast to create that "illusion". Such systems has a lot of downsides compared to a supercomputer. For starters, they use internet to communiate. Internet connection is never reliable. Even a few seconds of offline time can ruin the entire calculation. They also use data duplication for fail-safe purposes which creates 20-25% waste of memory by itself.
But let's say memory is cheap. What about CPU power? Processes need to communicate when they are running. Normally in supercomputers processes communicate via closed-network communication and it's very very fast. Tianhe-2A uses Infiniband (Source : InfiniBand - Wikipedia, the free encyclopedia which is an industry standard. For managing the network communication between CPU's Tianhe-2A uses FeiTeng processeor (source:FeiTeng (processor) - Wikipedia, the free encyclopedia FeiTeng is used for smart network management and load balancing.
However there was a huge Japanese success with K computer. As you may now K computer still leads graph-500 list which is a specific test for data intensive computing. Japanese engineers tried to strike a balance between single centered approach and distributed computing. They programmed it with Computer Clustering Architecture (source : http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=5993668). That architecture creates a distributed memory, which has a scalability advantage. However unlike peer-to-peer systems or grid computing inter-memory communication is managed via a closed network with MPI protocol (source : Message passing in computer clusters - Wikipedia, the free encyclopedia K computer uses Tofu as an MPI (source : http://www.fujitsu.com/downloads/TC/sc10/programming-on-k-computer.pdf) which is a modified version of MPI for K-Computer specifically and a 6 dimensional Torus Interconnect (source : http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=5331902) for network topology. As far as I know 6D Torus interconnect is a unique system. Cray uses 3D Torus interconnect. You can't even visualise 6D Torus interconnect. I don't know how Japanese engineers did it. Probably they've used another supercomputer to design 6D Torus interconnect scheme. Brilliant.
Now all of those creates extra overhead for K-Computer. However it is very easily scalable and cost effective. It also forces the programmer to think about data distribution while programming. Programmers have much more flexibility in such systems. I guess that's why K-Computer is very good in data intensive computing. Because with a poorly written code distributed memory architecture turns into a nightmare. @Nihonjin1051, Japanese people always manage to amaze me. This is a good system.