Data-Intensive Text Processing with MapReduce. Jimmy Lin

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СКАЧАТЬ user data creates large-data problems, many of which are suitable for MapReduce. To give two concrete examples: a social-networking site analyzes connections in the enormous globe-spanning graph of friendships to recommend new connections. An online email service analyzes messages and user behavior to optimize ad selection and placement. These are all large-data problems that have been tackled with MapReduce.⁹

      Another important facet of cloud computing is what’s more precisely known as utility computing [31; 129]. As the name implies, the idea behind utility computing is to treat computing resource as a metered service, like electricity or natural gas. The idea harkens back to the days of time-sharing machines, and in truth isn’t very different from this antiquated form of computing. Under this model, a “cloud user” can dynamically provision any amount of computing resources from a “cloud provider” on demand and only pay for what is consumed. In practical terms, the user is paying for access to virtual machine instances that run a standard operating system such as Linux. Virtualization technology (e.g., [15]) is used by the cloud provider to allocate available physical resources and enforce isolation between multiple users who may be sharing the same hardware. Once one or more virtual machine instances have been provisioned, the user has full control over the resources and can use them for arbitrary computation. Virtual machines that are no longer needed are destroyed, thereby freeing up physical resources that can be redirected to other users. Resource consumption is measured in some equivalent of machine-hours and users are charged in increments thereof.

      Both users and providers benefit in the utility computing model. Users are freed from upfront capital investments necessary to build datacenters and substantial reoccurring costs in maintaining them. They also gain the important property of elasticity—as demand for computing resources grow, for example, from an unpredicted spike in customers, more resources can be seamlessly allocated from the cloud without an interruption in service. As demand falls, provisioned resources can be released. Prior to the advent of utility computing, coping with unexpected spikes in demand was fraught with challenges: under-provision and run the risk of service interruptions, or over-provision and tie up precious capital in idle machines that are depreciating.

      From the utility provider point of view, this business also makes sense because large datacenters benefit from economies of scale and can be run more efficiently than smaller datacenters. In the same way that insurance works by aggregating risk and redistributing it, utility providers aggregate the computing demands for a large number of users. Although demand may fluctuate significantly for each user, overall trends in aggregate demand should be smooth and predictable, which allows the cloud provider to adjust capacity over time with less risk of either offering too much (resulting in inefficient use of capital) or too little (resulting in unsatisfied customers). In the world of utility computing, Amazon Web Services currently leads the way and remains the dominant player, but a number of other cloud providers populate a market that is becoming increasingly crowded. Most systems are based on proprietary infrastructure, but there is at least one, Eucalyptus [111], that is available open source. Increased competition will benefit cloud users, but what direct relevance does this have for MapReduce? The connection is quite simple: processing large amounts of data with MapReduce requires access to clusters with sufficient capacity. However, not everyone with large-data problems can afford to purchase and maintain clusters. This is where utility computing comes in: clusters of sufficient size can be provisioned only when the need arises, and users pay only as much as is required to solve their problems. This lowers the barrier to entry for data-intensive processing and makes MapReduce much more accessible.

      A generalization of the utility computing concept is “everything as a service”, which is itself a new take on the age-old idea of outsourcing. A cloud provider offering customers access to virtual machine instances is said to be offering infrastructure as a service, or IaaS for short. However, this may be too low level for many users. Enter platform as a service (PaaS), which is a rebranding of what used to be called hosted services in the “pre-cloud” era. Platform is used generically to refer to any set of well-defined services on top of which users can build applications, deploy content, etc. This class of services is best exemplified by Google App Engine, which provides the backend datastore and API for anyone to build highly scalable web applications. Google maintains the infrastructure, freeing the user from having to backup, upgrade, patch, or otherwise maintain basic services such as the storage layer or the programming environment. At an even higher level, cloud providers can offer software as a service (SaaS), as exemplified by Salesforce, a leader in customer relationship management (CRM) software. Other examples include outsourcing an entire organization’s email to a third party, which is commonplace today.

      What does this proliferation of services have to do with MapReduce? No doubt that “everything as a service” is driven by desires for greater business efficiencies, but scale and elasticity play important roles as well. The cloud allows seamless expansion of operations without the need for careful planning and supports scales that may otherwise be difficult or cost-prohibitive for an organization to achieve. Cloud services, just like MapReduce, represent the search for an appropriate level of abstraction and beneficial divisions of labor. IaaS is an abstraction over raw physical hardware—an organization might lack the capital, expertise, or interest in running datacenters, and therefore pays a cloud provider to do so on its behalf. The argument applies similarly to PaaS and SaaS. In the same vein, the MapReduce programming model is a powerful abstraction that separates the what from the how of data-intensive processing.

1.2 BIG IDEAS

      Tackling large-data problems requires a distinct approach that sometimes runs counter to traditional models of computing. In this section, we discuss a number of “big ideas” behind MapReduce. To be fair, all of these ideas have been discussed in the computer science literature for some time (some for decades), and MapReduce is certainly not the first to adopt these ideas. Nevertheless, the engineers at Google deserve tremendous credit for pulling these various threads together and demonstrating the power of these ideas on a scale previously unheard of.

      Scale “out”, not “up”. For data-intensive workloads, a large number of commodity low-end servers (i.e., the scaling “out” approach) is preferred over a small number of high-end servers (i.e., the scaling “up” approach). The latter approach of purchasing symmetric multi-processing (SMP) machines with a large number of processor sockets (dozens, even hundreds) and a large amount of shared memory (hundreds or even thousands of gigabytes) is not cost effective, since the costs of such machines do not scale linearly (i.e., a machine with twice as many processors is often significantly more than twice as expensive). On the other hand, the low-end server market overlaps with the high-volume desktop computing market, which has the effect of keeping prices low due to competition, interchangeable components, and economies of scale.

      Barroso and Hölzle’s recent treatise of what they dubbed “warehouse-scale computers” [18] contains a thoughtful analysis of the two approaches. The Transaction Processing Council (TPC) is a neutral, non-profit organization whose mission is to establish objective database benchmarks. Benchmark data submitted to that organization are probably the closest one can get to a fair “applesto-apples” comparison of cost and performance for specific, well-defined relational processing applications. Based on TPC-C benchmark results from late 2007, a low-end server platform is about four times more cost efficient than a high-end shared memory platform from the same vendor. Excluding storage costs, the price/performance advantage of the low-end server increases to about a factor of twelve.

      What if we take into account the fact that communication between nodes in a high-end SMP machine is orders of magnitude faster than communication between nodes in a commodity network-based cluster? Since workloads today are beyond the capability of any single machine (no matter how powerful), the comparison is more accurately between a smaller cluster of high-end machines and a larger cluster of low-end machines (network communication is unavoidable in both cases). Barroso and Hölzle model these two approaches under workloads that demand more or less communication, and conclude that a cluster of low-end servers approaches the performance of the equivalent СКАЧАТЬ