Using Compute Services

Scalability with MP CONNECT

Scalability reduces the time-to-solution for your critical tasks. Scalability can be accomplished by performing two or more tasks in parallel (independent parallelism) or overlapping two or more tasks (pipeline parallelism). Scalability requires two things: 1) that some part(s) of your application can be overlapped or performed in parallel, and 2) that you have hardware that is capable of multiprocessing. All applications are not scalable, and not all hardware configurations are capable of providing scalability.

To decide whether an application can be scaled, consider the following questions:

Does the time that is required to run a job exceed the batch window of time that you have available?
Does the time that is required to run a job allow enough time so you can make appropriate decisions after you get the information from the application? The applications that are the best candidates for scalability generally take hours, days, or maybe even weeks to execute.
Can the application (or some part of it) be segmented into sub-tasks that are independent and can be run in parallel? It might be worthwhile to duplicate some data in order to achieve this independence.
Does the application contain dependent steps that could benefit from piping?

Hardware that is capable of multiprocessing would include an SMP computer or multiple computers on a network with each computer containing one or more processors. In addition to the number of processors, it is important to have multiple I/O channels. This is inherent to multiple computers on a network. For an SMP computer, this can be accomplished with RAID arrays that enable you to stripe or spread your data across multiple physical disks. Even for a single threaded application, this can improve I/O performance, because the operating system is able to read data from multiple drives simultaneously and synchronize the result for the application.

Parallel Threads and Parallel Processes

SAS 9 has the capability to leverage the available hardware resources to both scale up and scale out your applications. SAS provides scalability in two ways:

parallel SAS processes
parallel threads within a SAS process

Parallel Processes

A SAS process consists of many pieces, including execution units, data structures, and resources. A process corresponds to an operating environment process. A process has a largely private address space. It is scheduled by the operating environment, and its resources are managed by the operating environment at the lowest level. Multiple SAS processes use multiple processors on an SMP computer, but they can also be run on multiple remote single or multiprocessor computers on a network. When running multiple SAS processes on an SMP computer, SAS does not schedule a specific process to a specific processor; scheduling is controlled by the operating environment. MP CONNECT provides the ability to run multiple SAS processes.

Parallel Threads

A process consists of one or more threads. A thread is also scheduled by the operating environment, but the running process might influence the behavior of threads by using synchronization techniques. All threads in a process share an address space and must cooperatively share the resources of the process. Multiple threads use multiple processors on an SMP computer but cannot be executed across computers. When running multiple threads within a SAS process, SAS does not schedule a specific thread to a specific processor; scheduling is controlled by the operating environment.

Scaling Up

Scaling up means to increase the number of processors, disk drives, and I/O channels on a single server computer. Scaling up also means to leverage the multiple processors, disk drives, and I/O channels on a single server computer.

Scaling Out

Scaling out means adding more hardware, not bigger hardware. Scaling out also means to exploit network resources to run parts of an application. When you scale out, the size and speed of an individual computer does not limit the total capacity of the network.

Multiple Threads and Multiple Processors

Beginning in SAS 9, multiple threads are used to scale up and make use of multiple processors in SMP hardware. Multithreading has been incorporated into SAS 9 (and later), including many SAS servers, several performance-critical SAS procedures, and many SAS engines. Multithreading is used for both computing-intensive parts as well as I/O-intensive parts in order to process data quickly and reduce the total execution time.

Multiple SAS processes (MP CONNECT) are used to both scale up and scale out. By running multiple processes on an SMP computer, the operating environment can schedule the processes on different processors to use all the hardware resources on the computer. In addition, by running multiple SAS processes across the computers that are available on a network, you can use idle processors and put multiple, slow, inexpensive computers to work in parallel on a job and turn them into a valuable, powerful, inexpensive computing resource.

Multithreading and multiple SAS processes (MP CONNECT) are not mutually exclusive. For some applications, the greatest gains in performance result from applying a solution that incorporates multiple threads and multiple processes. Provided you have the hardware resources to support it, you can use MP CONNECT to run multiple SAS processes and each process can use multithreading. When running multiple processes by using multiple threads on an SMP computer, it might be necessary to set SAS system options in each of the SAS processes to tune the amount of threading that is performed by each process. Tuning threading behavior avoids the sum of the processes and threads from overloading your system. When using multiple remote computers with each SAS process running on a physically separate computer, it might be better to let the threading within the process fully use the individual computers.

Successfully scaled performance is not obtained by installing more and faster processors or more and faster I/O devices. Scalability involves making choices about investing in SMP hardware, upgrading I/O configurations, using networked computers, reorganizing your data, and modifying your application. True scalability results from choosing scalable hardware and the appropriate software that is specifically designed to leverage it. The extent of the original problem that can be processed in parallel determines the amount of scalability that is achievable from the software solution.

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