Pipeline Parallelism

Overview of Pipeline Parallelism

Pipeline parallelism occurs when the execution of Task A and Task B have interdependencies. For example, a SAS DATA step might be followed by a PROC SORT of the data set that is created by the DATA step. PROC SORT is dependent on the execution of the DATA step, because the output of the DATA step is the input needed by PROC SORT. However, the execution of the two steps can be overlapped, and the DATA step can pipe its output into PROC SORT. The piping feature of MP CONNECT provides pipeline parallelism.

Piping enables you to overlap the execution of SAS DATA steps and some SAS procedures. This is accomplished by starting one SAS session to run one DATA step or SAS procedure and piping its output through a TCP/IP socket as input into another SAS session that is running another DATA step or SAS procedure. This pipeline can be extended to include multiple steps and can be extended between different physical computers. Piping improves performance not only because it enables overlapped task execution, but also because intermediate I/O is directed to a TCP/IP pipe instead of written to disk by one task and then read from disk by the next task.

Piping is implemented by using a LIBNAME statement to identify a port to be used for the pipe. For details about using the LIBNAME statement to implement piping, see LIBNAME Statement, SASESOCK Engine . For an example of piping, see Example 6: Using MP CONNECT with Piping .

Limitation of Pipeline Parallelism

A limitation of piping is that it supports single-pass, sequential data processing. Because piping stores data for reading and writing in TCP/IP ports instead of disks, the data is never permanently stored. Instead, after the data is read from a port, the data is removed entirely from that port and the data cannot be read again. If your data requires multiple passes for processing, piping cannot be used.

Here are some examples of SAS procedures and statements that process single-pass, sequential data:

DATA step
SORT procedure
SUMMARY procedure
GANTT procedure
PRINT procedure
COPY procedure
CONTENTS procedure

Considerations for Piping

The benefit of piping should be weighed against the cost of potential CPU or I/O bottlenecks. If execution time for a SAS procedure or statement is relatively short, piping is probably counterproductive.
Ensure that each SAS procedure or statement is reading from and writing to the appropriate port.

For example, a single SAS procedure cannot have multiple writes to the same pipe simultaneously or multiple reads from the same pipe simultaneously. You might minimize port access collisions on the same computer by reserving a range of ports in the SERVICES file. To completely eliminate the potential for port collisions, request a dynamically allocated port instead of selecting an explicit port for use. For details, see LIBNAME Statement .
Ensure that the port that the output is written to is on the same computer that the asynchronous process is running on. However, a SAS procedure that is reading from that port can be running on another computer.
Ensure that the task that reads the data does not complete before the task that writes the data. For example, if one process uses a DATA step that is writing observations to a pipe and PROC PRINT is running in another task that is reading observations from the pipe, PROC PRINT must not complete before the DATA step is complete. This problem might occur if the DATA step is producing a large number of observations, but PROC PRINT is printing only the first few observations that are specified by the OBS= option. This would result in the reading task closing the pipe after the first few observations had been printed, which would cause an error for the DATA step, which would continue to try to write to the pipe that had been closed.

Note: Although the task that is writing generates an error and will not complete, the task that is reading will complete successfully. You could ignore the error in the writing task if the completion of this task is not required (as is the case with the DATA step and PROC PRINT example in this item).
Be aware of the timing of each task's use of the pipe. If the task that is reading from the pipe opens the pipe to read and there is a delay before the task that is writing actually begins to write to the pipe, the reading task might timeout and close the pipe prematurely. This could happen if the writing task has other steps to execute before the DATA step or SAS procedure that is actually writing to the pipe.

Use the TIMEOUT= option in the LIBNAME statement to increase the timeout value for the task that is reading. Increasing the value for the TIMEOUT= option causes the reading task to wait longer for the writing task to begin writing to the pipe. This will allow the initial steps in the writing task to complete and the DATA step or SAS procedure to begin writing to the pipe before the reading task timeout expires. For an example, see Example 7: Preventing Pipes from Closing Prematurely .