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Chapter: Distributed and Cloud Computing: From Parallel Processing to the Internet of Things : Computer Clusters for Scalable Parallel Computing

Clustering for Massive Parallelism

1 Cluster Development Trends 2 Design Objectives of Computer Clusters 3 Fundamental Cluster Design Issues 4 Analysis of the Top 500 Supercomputers



A computer cluster is a collection of interconnected stand-alone computers which can work together collectively and cooperatively as a single integrated computing resource pool. Clustering explores massive parallelism at the job level and achieves high availability (HA) through stand-alone opera-tions. The benefits of computer clusters and massively parallel processors (MPPs) include scalable performance, HA, fault tolerance, modular growth, and use of commodity components. These fea-tures can sustain the generation changes experienced in hardware, software, and network components. Cluster computing became popular in the mid-1990s as traditional mainframes and vector supercomputers were proven to be less cost-effective in many high-performance computing (HPC) applications.


Of the Top 500 supercomputers reported in 2010, 85 percent were computer clusters or MPPs built with homogeneous nodes. Computer clusters have laid the foundation for todays supercomputers, computational grids, and Internet clouds built over data centers. We have come a long way toward becoming addicted to computers. According to a recent IDC prediction, the HPC market will increase from $8.5 billion in 2010 to $10.5 billion by 2013. A majority of the Top 500 super-computers are used for HPC applications in science and engineering. Meanwhile, the use of high-throughput computing (HTC) clusters of servers is growing rapidly in business and web services applications.


1. Cluster Development Trends


Support for clustering of computers has moved from interconnecting high-end mainframe computers to building clusters with massive numbers of x86 engines. Computer clustering started with the linking of large mainframe computers such as the IBM Sysplex and the SGI Origin 3000. Originally, this was motivated by a demand for cooperative group computing and to provide higher availability in critical enterprise applications. Subsequently, the clustering trend moved toward the networking of many minicomputers, such as DECs VMS cluster, in which multiple VAXes were interconnected to share the same set of disk/tape controllers. Tandems Himalaya was designed as a business cluster for fault-tolerant online transaction processing (OLTP) applications.


In the early 1990s, the next move was to build UNIX-based workstation clusters represented by the Berkeley NOW (Network of Workstations) and IBM SP2 AIX-based server cluster. Beyond 2000, we see the trend moving to the clustering of RISC or x86 PC engines. Clustered products now appear as integrated systems, software tools, availability infrastructure, and operating system extensions. This clustering trend matches the downsizing trend in the computer industry. Supporting clusters of smaller nodes will increase sales by allowing modular incremental growth in cluster con-figurations. From IBM, DEC, Sun, and SGI to Compaq and Dell, the computer industry has lever-aged clustering of low-cost servers or x86 desktops for their cost-effectiveness, scalability, and HA features.


1.1 Milestone Cluster Systems


Clustering has been a hot research challenge in computer architecture. Fast communication, job scheduling, SSI, and HA are active areas in cluster research. Table 2.1 lists some milestone cluster research projects and commercial cluster products. Details of these old clusters can be found in [14]. These milestone projects have pioneered clustering hardware and middleware development over the past two decades. Each cluster project listed has developed some unique features. Modern clusters are headed toward HPC clusters as studied in Section 2.5.


The NOW project addresses a whole spectrum of cluster computing issues, including architecture, software support for web servers, single system image, I/O and file system, efficient communication, and enhanced availability. The Rice University TreadMarks is a good example of software-implemented shared-memory cluster of workstations. The memory sharing is implemented with a user-space runtime library. This was a research cluster built over Sun Solaris workstations. Some cluster OS functions were developed, but were never marketed successfully.

A Unix cluster of SMP servers running VMS/OS with extensions, mainly used in high-availability applications. An AIX server cluster built with Power2 nodes and Omega network and supported by IBM Loadleveler and MPI extensions. A scalable and fault-tolerant cluster for OLTP and database processing built with non-stop operating system support. The Google search engine was built at Google using commodity components. MOSIX is a distributed operating systems for use in Linux clusters, multi-clusters, grids, and the clouds, originally developed by Hebrew Univer-sity in 1999.



2. Design Objectives of Computer Clusters


Clusters have been classified in various ways in the literature. We classify clusters using six orthogonal attributes: scalability, packaging, control, homogeneity, programmability, and security.


2.1 Scalability


Clustering of computers is based on the concept of modular growth. To scale a cluster from hundreds of uniprocessor nodes to a supercluster with 10,000 multicore nodes is a nontrivial task. The scalabil-ity could be limited by a number of factors, such as the multicore chip technology, cluster topology, packaging method, power consumption, and cooling scheme applied. The purpose is to achieve scal-able performance constrained by the aforementioned factors. We have to also consider other limiting factors such as the memory wall, disk I/O bottlenecks, and latency tolerance, among others.


2.2 Packaging


Cluster nodes can be packaged in a compact or a slack fashion. In a compact cluster, the nodes are closely packaged in one or more racks sitting in a room, and the nodes are not attached to peripherals (monitors, keyboards, mice, etc.). In a slack cluster, the nodes are attached to their usual peripherals (i.e., they are complete SMPs, workstations, and PCs), and they may be located in different rooms, different buildings, or even remote regions. Packaging directly affects communication wire length, and thus the selection of interconnection technology used. While a compact cluster can uti-lize a high-bandwidth, low-latency communication network that is often proprietary, nodes of a slack cluster are normally connected through standard LANs or WANs.


2.3 Control


A cluster can be either controlled or managed in a centralized or decentralized fashion. A compact cluster normally has centralized control, while a slack cluster can be controlled either way. In a centralized cluster, all the nodes are owned, controlled, managed, and administered by a central opera-tor. In a decentralized cluster, the nodes have individual owners. For instance, consider a cluster comprising an interconnected set of desktop workstations in a department, where each workstation is individually owned by an employee. The owner can reconfigure, upgrade, or even shut down the workstation at any time. This lack of a single point of control makes system administration of such a cluster very difficult. It also calls for special techniques for process scheduling, workload migra-tion, checkpointing, accounting, and other similar tasks.


2.4 Homogeneity

A homogeneous cluster uses nodes from the same platform, that is, the same processor architecture and the same operating system; often, the nodes are from the same vendors. A heterogeneous cluster uses nodes of different platforms. Interoperability is an important issue in heterogeneous clusters. For instance, process migration is often needed for load balancing or availability. In a homogeneous cluster, a binary process image can migrate to another node and continue execution. This is not feasible in a heterogeneous cluster, as the binary code will not be executable when the process migrates to a node of a different platform.


2.5 Security


Intracluster communication can be either exposed or enclosed. In an exposed cluster, the communication paths among the nodes are exposed to the outside world. An outside machine can access the communication paths, and thus individual nodes, using standard protocols (e.g., TCP/IP). Such exposed clusters are easy to implement, but have several disadvantages:


    Being exposed, intracluster communication is not secure, unless the communication subsystem performs additional work to ensure privacy and security.


    Outside communications may disrupt intracluster communications in an unpredictable fashion. For instance, heavy BBS traffic may disrupt production jobs.


    Standard communication protocols tend to have high overhead.


In an enclosed cluster, intracluster communication is shielded from the outside world, which alleviates the aforementioned problems. A disadvantage is that there is currently no standard for efficient, enclosed intracluster communication. Consequently, most commercial or academic clusters realize fast communications through one-of-a-kind protocols.


2.6 Dedicated versus Enterprise Clusters


A dedicated cluster is typically installed in a deskside rack in a central computer room. It is homo-geneously configured with the same type of computer nodes and managed by a single administrator group like a frontend host. Dedicated clusters are used as substitutes for traditional mainframes or supercomputers. A dedicated cluster is installed, used, and administered as a single machine. Many users can log in to the cluster to execute both interactive and batch jobs. The cluster offers much enhanced throughput, as well as reduced response time.


An enterprise cluster is mainly used to utilize idle resources in the nodes. Each node is usually a full-fledged SMP, workstation, or PC, with all the necessary peripherals attached. The nodes are typically geographically distributed, and are not necessarily in the same room or even in the same building. The nodes are individually owned by multiple owners. The cluster administrator has only limited control over the nodes, as a node can be turned off at any time by its owner. The owners local jobs have higher priority than enterprise jobs. The cluster is often configured with heterogeneous computer nodes. The nodes are often connected through a low-cost Ethernet network. Most data centers are structured with clusters of low-cost servers. Virtual clusters play a crucial role in upgrading data centers.


3. Fundamental Cluster Design Issues

In this section, we will classify various cluster and MPP families. Then we will identify the major design issues of clustered and MPP systems. Both physical and virtual clusters are covered. These systems are often found in computational grids, national laboratories, business data centers, supercomputer sites, and virtualized cloud platforms. A good understanding of how clusters and MPPs work collectively will pave the way toward understanding the ins and outs of large-scale grids and Internet clouds in subsequent chapters. Several issues must be considered in developing and using a cluster. Although much work has been done in this regard, this is still an active research and development area.


3.1 Scalable Performance


This refers to the fact that scaling of resources (cluster nodes, memory capacity, I/O bandwidth, etc.) leads to a proportional increase in performance. Of course, both scale-up and scale-down cap-abilities are needed, depending on application demand or cost-effectiveness considerations. Cluster-ing is driven by scalability. One should not ignore this factor in all applications of cluster or MPP computing systems.


3.2 Single-System Image (SSI)


A set of workstations connected by an Ethernet network is not necessarily a cluster. A cluster is a single system. For example, suppose a workstation has a 300 Mflops/second processor, 512 MB of memory, and a 4 GB disk and can support 50 active users and 1,000 processes. By clustering 100 such workstations, can we get a single system that is equivalent to one huge workstation, or a megastation, that has a 30 Gflops/second processor, 50 GB of memory, and a 400 GB disk and can support 5,000 active users and 100,000 processes? This is an appealing goal, but it is very difficult to achieve. SSI techniques are aimed at achieving this goal.


3.3 Availability Support


Clusters can provide cost-effective HA capability with lots of redundancy in processors, memory, disks, I/O devices, networks, and operating system images. However, to realize this potential, avail-ability techniques are required. We will illustrate these techniques later in the book, when we dis-cuss how DEC clusters (Section 10.4) and the IBM SP2 (Section 10.3) attempt to achieve HA.


3.4 Cluster Job Management


Clusters try to achieve high system utilization from traditional workstations or PC nodes that are normally not highly utilized. Job management software is required to provide batching, load balancing, parallel processing, and other functionality. We will study cluster job management systems in Section 3.4. Special software tools are needed to manage multiple jobs simultaneously.


3.5 Internode Communication


Because of their higher node complexity, cluster nodes cannot be packaged as compactly as MPP nodes. The internode physical wire lengths are longer in a cluster than in an MPP. This is true even for centralized clusters. A long wire implies greater interconnect network latency. But more impor-tantly, longer wires have more problems in terms of reliability, clock skew, and cross talking. These problems call for reliable and secure communication protocols, which increase overhead. Clusters often use commodity networks (e.g., Ethernet) with standard protocols such as TCP/IP.


3.6 Fault Tolerance and Recovery

Clusters of machines can be designed to eliminate all single points of failure. Through redundancy, a cluster can tolerate faulty conditions up to a certain extent. Heartbeat mechanisms can be installed to monitor the running condition of all nodes. In case of a node failure, critical jobs running on the failing nodes can be saved by failing over to the surviving node machines. Rollback recovery schemes restore the computing results through periodic checkpointing.


3.7 Cluster Family Classification


Based on application demand, computer clusters are divided into three classes:


    Compute clusters These are clusters designed mainly for collective computation over a single large job. A good example is a cluster dedicated to numerical simulation of weather conditions. The compute clusters do not handle many I/O operations, such as database services. When a single compute job requires frequent communication among the cluster nodes, the cluster must share a dedicated network, and thus the nodes are mostly homogeneous and tightly coupled. This type of clusters is also known as a Beowulf cluster.


When the nodes require internode communication over a small number of heavy-duty nodes, they are essentially known as a computational grid. Tightly coupled compute clusters are designed for supercomputing applications. Compute clusters apply middleware such as a message-passing interface (MPI) or Parallel Virtual Machine (PVM) to port programs to a wide variety of clusters.


    High-Availability clusters HA (high-availability) clusters are designed to be fault-tolerant and achieve HA of services. HA clusters operate with many redundant nodes to sustain faults or failures. The simplest HA cluster has only two nodes that can fail over to each other. Of course, high redundancy provides higher availability. HA clusters should be designed to avoid all single points of failure. Many commercial HA clusters are available for various operating systems.


    Load-balancing clusters These clusters shoot for higher resource utilization through load balancing among all participating nodes in the cluster. All nodes share the workload or function as a single virtual machine (VM). Requests initiated from the user are distributed to all node computers to form a cluster. This results in a balanced workload among different machines, and thus higher resource utilization or higher performance. Middleware is needed to achieve dynamic load balancing by job or process migration among all the cluster nodes.


4. Analysis of the Top 500 Supercomputers


Every six months, the worlds Top 500 supercomputers are evaluated by running the Linpack Benchmark program over very large data sets. The ranking varies from year to year, similar to a competition. In this section, we will analyze the historical share in architecture, speed, operating systems, countries, and applications over time. In addition, we will compare the top five fastest systems in 2010.


4.1 Architectural Evolution


It is interesting to observe in Figure 2.1 the architectural evolution of the Top 500 supercomputers over the years. In 1993, 250 systems assumed the SMP architecture and these SMP systems all disappeared in June of 2002. Most SMPs are built with shared memory and shared I/O devices. There were 120 MPP systems built in 1993, MPPs reached the peak mof 350 systems in mod-200, and dropped to less than 100 systems in 2010. The single instruction, multiple data (SIMD) machines dis-appeared in 1997. The cluster architecture appeared in a few systems in 1999. The cluster systems are now populated in the Top-500 list with more than 400 systems as the dominating architecture class.


In 2010, the Top 500 architecture is dominated by clusters (420 systems) and MPPs (80 systems). The basic distinction between these two classes lies in the components they use to build the systems. Clusters are often built with commodity hardware, software, and network components that are commercially available. MPPs are built with custom-designed compute nodes, boards, modules, and cabi-nets that are interconnected by special packaging. MPPs demand high bandwidth, low latency, better power efficiency, and high reliability. Cost-wise, clusters are affordable by allowing modular growth with scaling capability. The fact that MPPs appear in a much smaller quantity is due to their high cost. Typically, only a few MPP-based supercomputers are installed in each country.


4.2 Speed Improvement over Time


Figure 2.2 plots the measured performance of the Top 500 fastest computers from 1993 to 2010. The y-axis is scaled by the sustained speed performance in terms of Gflops, Tflops, and Pflops. The middle curve plots the performance of the fastest computers recorded over 17 years; peak per-formance increases from 58.7 Gflops to 2.566 Pflops. The bottom curve corresponds to the speed of the 500th computer, which increased from 0.42 Gflops in 1993 to 31.1 Tflops in 2010. The top curve plots the speed sum of all 500 computers over the same period. In 2010, the total speed sum of 43.7 Pflops was achieved by all 500 computers, collectively. It is interesting to observe that the total speed sum increases almost linearly with time.


4.3 Operating System Trends in the Top 500


The five most popular operating systems have more than a 10 percent share among the Top 500 computers, according to data released by TOP500.org (www.top500.org/stats/list/36/os) in November 2010. According to the data, 410 supercomputers are using Linux with a total processor count exceeding 4.5 million. This constitutes 82 percent of the systems adopting Linux. The IBM AIX/ OS is in second place with 17 systems (a 3.4 percent share) and more than 94,288 processors.

Third place is represented by the combined use of the SLEs10 with the SGI ProPack5, with 15 sys-tems (3 percent) over 135,200 processors. Fourth place goes to the CNK/SLES9 used by 14 systems (2.8 percent) over 1.13 million processors. Finally, the CNL/OS was used in 10 systems (2 percent) over 178,577 processors. The remaining 34 systems applied 13 other operating systems with a total share of only 6.8 percent. In conclusion, the Linux OS dominates the systems in the Top 500 list.


4.4 The Top Five Systems in 2010


In Table 2.2, we summarize the key architecture features and sustained Linpack Benchmark performance of the top five supercomputers reported in November 2010. The Tianhe-1A was ranked as the fastest MPP in late 2010. This system was built with 86,386 Intel Xeon CPUs and NVIDIA GPUs by the National University of Defense Technology in China. We will present in Section 2.5 some of the top winners: namely the Tianhe-1A, Cray Jaguar, Nebulae, and IBM Roadrunner that were ranked among the top systems from 2008 to 2010. All the top five machines in Table 2.3 have achieved a speed higher than 1 Pflops. The sustained speed, Rmax, in Pflops is measured from the execution of the Linpack Benchmark program corresponding to a maximum matrix size.

The system efficiency reflects the ratio of the sustained speed to the peak speed, Rpeak, when all computing elements are fully utilized in the system. Among the top five systems, the two U.S.-built systems, the Jaguar and the Hopper, have the highest efficiency, more than 75 percent. The two sys-tems built in China, the Tianhe-1A and the Nebulae, and Japans TSUBAME 2.0, are all low in efficiency. In other words, these systems should still have room for improvement in the future. The average power consumption in these 5 systems is 3.22 MW. This implies that excessive power con-sumption may post the limit to build even faster supercomputers in the future. These top systems all emphasize massive parallelism using up to 250,000 processor cores per system.

4.5 Country Share and Application Share


In the 2010 Top-500 list, there were 274 supercomputing systems installed in the US, 41 systems in China, and 103 systems in Japan, France, UK, and Germany. The remaining countries have 82 systems. This country share roughly reflects the countries economic growth over the years. Countries compete in the Top 500 race every six months. Major increases of supercomputer applications are in the areas of database, research, finance, and information services.


4.6 Power versus Performance in the Top Five in 2010


In Figure 2.3, the top five supercomputers are ranked by their speed (Gflops) on the left side and by their power consumption (MW per system) on the right side. The Tiahhe-1A scored the highest with a 2.57 Pflops speed and 4.01 MW power consumption. In second place, the Jaguar consumes the highest power of 6.9 MW. In fourth place, the TSUBAME system consumes the least power, 1.5 MW, and has a speed performance that almost matches that of the Nebulae sys-tem. One can define a performance/power ratio to see the trade-off between these two metrics. There is also a Top 500 Green contest that ranks the supercomputers by their power efficiency. This chart shows that all systems using the hybrid CPU/GPU architecture consume much less power.


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