Memory Management

The amount of memory on a machine is the physical memory. The amount of memory that can be used by an application can be larger, because the operating system can provide virtual memory. Virtual memory makes the machine appear to have more memory available than there actually is, by sharing physical memory between applications when they need it and by using disk space as memory.

When memory is not used and other applications need to allocate memory, the operating system pages out the memory that is not currently needed to support the other applications. When the paged-out memory is needed again, some other memory needs to be paged out. Paging means to write some of the contents of memory onto a disk.

Paging does affect performance, but some amount of paging is acceptable. Using virtual memory enables you to access tables that exceed the amount of physical memory on the machine. So long as the time to write pages to the disk and read them from the disk is short, the server performance is good.

One advantage of SASHDAT tables that are read from HDFS is that the server performs the most efficient paging of memory.

When you load a table to memory with the SAS LASR Analytic Server engine, the server allocates physical memory to store the rows of data. This applies to both distributed and non-distributed servers.

When a distributed server loads a table from HDFS to memory with the LASR procedure, the server defers reading the rows of data into physical memory. You can direct the server to perform an aggressive memory allocation scheme at load time with the READAHEAD option for the PROC LASR statement.

Physical memory is used when the server performs analytic operations such as summarizing a table. The amount of memory that a particular operation requires typically depends on the cardinality of the data. In most cases, the cardinality of the data is not known until the analysis is requested. When the server performs in-memory analytics, the following characteristics affect the amount of physical memory that is used:

Some requests, especially with high-cardinality variables, can generate large result sets. To enable interactive near-real-time work with high cardinality problems, the server allocates memory for data structures that speed performance. The following list identifies some of these uses:

The following list identifies some of the options that SAS provides for managing memory:

By default, whenever the amount of physical memory in use rises above 75% of the total memory available on a node of a distributed server, adding tables (including temporary ones), appending rows, or any other operation that consumes memory for storing data fails.

If the machine has already crossed the threshold, your requests to add data are immediately rejected. If you attempt to add a table and the server crosses the threshold as the data is added, the server removes the table you attempted to add and frees the memory. Similarly, if you attempt to append rows and the server crosses the threshold during the request, the entire append request fails. The table remains as it was before the append was attempted.

You can specify the threshold when you start a server with the TABLEMEM= option in the PROC LASR statement or alter it for a running server with the SERVERPARM statement in the VASMP procedure. By default, TABLEMEM=75 (%).

Be aware that the TABLEMEM= option does not specify the percentage of memory that can be filled with tables. The memory consumption is measured across all processes of a machine.

A separate memory setting can be applied to processes that extract data from a server on a worker node. SAS High-Performance Analytics procedures can do this. If you set the EXTERNALMEM= option in the PROC LASR statement or through the SERVERPARM statement in the VASMP procedure, then you are specifying the threshold of total memory (expressed as a percentage) at which the server stops sending data to the high-performance analytics procedure.