  In our previous work we introduced a so-called Amdahl blade microserver that
combines a low-power Atom processor, with a GPU and an SSD to provide a
balanced and energy-efficient system. Our preliminary results suggested that
the sequential I/O of Amdahl blades can be ten times higher than that a cluster
of conventional servers with comparable power consumption. In this paper we
investigate the performance and energy efficiency of Amdahl blades running
Hadoop. Our results show that Amdahl blades are 7.7 times and 3.4 times as
energy-efficient as the Open Cloud Consortium cluster for a data-intensive and
a compute-intensive application, respectively. The Hadoop Distributed
Filesystem has relatively poor performance on Amdahl blades because both disk
and network I/O are CPU-heavy operations on Atom processors. We demonstrate
three effective techniques to reduce CPU consumption and improve performance.
However, even with these improvements, the Atom processor is still the system's
bottleneck. We revisit Amdahl's law, and estimate that Amdahl blades need four
Atom cores to be well balanced for Hadoop tasks.
