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A buffer in computing terms is a temporary memory area which helps to transfer data between programs. Because buffers reside in memory, the transfer of information is much faster than accessing it from physical disk storage. For this reason, buffers play a very important role in the overall system performance. The SAP R/3 buffers reduce the physical database accesses by holding the data most frequently used and making it available to the processes of an instance. Once the data is available in the buffers, the workload is sensibly reduced because the work processes do not have to repeatedly access the database to get the same information. There is, of course, both a locking and a synchronization mechanism to prevent both the modification to buffered data from different application systems, as well as for making available the buffered information to other application servers. The R/3 system has several types of buffers for holding dictionary data, programs, screens, and even company specific data, such as the factory calendar which usually remains unchanged during normal system operation. The SAP buffer monitors let you analyze the performance of the buffer sizes to get an overview of whether the configured values are good enough or need some further tuning and adjustments. Each SAP instance has its own buffers. So, in case an application server has more than one instance, each has its own buffer areas. Sometimes, the buffers are known as client caches because they are located in the 416
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Buffer Performance Monitors application servers (that is the client from the point of view of the database server). The SAP buffers can be either located in memory areas which are local to individual work processes or can be located in shared memory areas accessible by all the work processes of an instance. To meet some operating system restrictions on the number of allowed shared memory allocation per process, SAP has grouped together some of the buffers in shared memory segments known as pools. While displaying SAP instance profile parameters you can see the size values assigned to several R/3 buffer pools. These parameters have the syntax ipc / shm_psize_<nn> where nn is the pool number, for example, ipc / shm_psize_40. Buffer Types The R/3 system includes seven groups of buffers in the system shared memory. These groups are as follows: Repository buffers. These buffers contain the active table and field definitions (metadata) of the ABAP dictionary. These buffers are also known as dictionary buffers or nametab buffers (NTAB buffers). When a table or field definition is activated in the R/3 system, an entry is made in these buffers. The repository buffers include four buffers in shared memory: TTAB buffer, containing the table definitions, which are held on table DDNTT FTAB buffer, containing the field descriptions corresponding to table DDNTF IREC buffer, containing the initial record layout which is initialized depending on the field type SNTAB buffer, containing a brief summary of the TTAB and FTAB buffers Table buffers. Table buffers contain table entries. Whether tables are buffered or not and the way tables are buffered can be set using the ABAP dictionary technical settings utility. There are two table buffers: TABLP buffer is the partial table buffer, also known as single key buffer, which stores single record table entries. TABL buffer is the generic table buffer, or generic key buffer, which stores a range of table entries, that is, a group of records with their field values. Sometimes the generic table buffer can contain all the record entries for a table. This is called full buffering. Program buffer. Contain the generated and executable versions of the ABAP programs. This buffer is also known as the PXA buffer (program execution area) or ABAP buffer. This buffer uses the tables D010L (ABAP loads), D010T (texts), and D010Y (symbol table) for storing its contents. Note Tables D010L, D010T, D010Y, DDNTT, and DDNTF, as well as some other repository related tables, are only found on the physical database and do not have corresponding ABAP dictionary definitions. GUI buffers. These types of buffers contain the R/3 graphical elements such as screens, menus, pushbuttons, icons, and so forth. There are two GUI buffers: Presentation buffer contains the generated R/3 screens (dynpro loads). This buffer is also known as the screen buffer. Menu buffer, or CUA buffer, stores graphical objects for the presentation interface, such as menus, icons, and push buttons. SAP roll and paging buffers. These buffers contain part of the roll and paging areas for the SAP instances. There are additional roll and paging areas located on the roll and page files on disk. The roll 417
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Buffer Performance Monitors area is used for storing user contexts when the user process is roll out of a work process. The paging area is used for storing larger data such as internal tables. Calendar Buffer. The R/3 calendar buffer contains all the defined factory and holiday calendars. These calendars are kept in the tables TFACS and THOCS. SAP Cursor Cache. This is a special type of buffer which holds reusable SQL statements. A good cursor cache ratio significantly improves performance since it reduces the process of parsing SQL statements. This buffer is database dependent and should not be changed without SAP guidance since it can affect other system areas. All the buffers can be adjusted with their respective parameters in the instance profiles. However, you should be very careful when doing so, since it affects the overall sizing of the pools, the needed operating system swap space, and the available physical memory. Working with the Buffer Monitor The buffer monitor utilities include an extensive list of functions for analyzing and tuning the size and quality of the SAP buffers. To call the buffer monitor, from the initial performance menu, select Setup / Buffers Buffers or, alternatively, enter transaction code ST02 in the command field. Figure 11 20 shows the initial buffer monitor screen, also known as the tune summary overview. The information displayed on the tune summary screen includes only the buffer and memory usage information for the instance where you are currently logged on.
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Figure 11 20: Tune summary screen for buffer monitoring. The buffer overview screen has four parts displaying specific information. These parts are buffers, SAP memory, and call statistics. To see them all, you probably have to use the R/3 window scrolling functions. For each of the parts, there are different assigned columns with the related performance and statistical information. To see the cursor cache, select the option Goto Current local data SAP cursor cache ID cache. Buffers. In the first column of the display you can see the names for each of the buffer types. For each of them, the following columns are available: Hit Ratios. A hit occurs when a SAP object is accessed from a buffer. A hit fails when the objects have to be accessed directly from the database. The hit ratios indicate the percentage of hits. This is a quality indicator. As it approaches 100 percent the better the buffer since the 418
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Buffer Performance Monitors performance is increased with buffer accesses. Allocated Size. This column contains in KB the size allocated to buffers. It differs a little bit from the available buffer size, since a small part of the buffer size is used for management purposes. Freespace. Contains in KB the available free space in the buffer. The percentage is a good indicator for right sizing this buffer. Dir size entries. Contains the number of directory entries for the buffer Sometimes, even when the buffer has free space, objects cannot be loaded because there are no more free directories. The directory contains the pointers to the location of the buffered objects. Free directories. Shows the number and percentage of free space of directory entries for the buffer. Swaps. This column indicates the number of swaps, i.e., the number of times an object has to be taken out of a buffer for making room for another object. Swapping occurs when a buffer doesn't have enough free space or free directory entries. Database Accesses. This column shows the number of database accesses indicating the number of times when objects could not be read from the buffers and had to be accessed by directly accessing the database. When critical situations occur in any of the buffer columns, the system displays the figures in red. SAP Memory. This part of the display includes information about the SAP memory areas used by the system such as the rolling and paging areas and the extended and heap memory. Columns to the right contain information about the current and maximum use and the sizes allocated in physical memory and in disk files. You can get additional details about any of the entries by double clicking on the line. Call Statistics. Under this section, the system displays access statistics for the data, which can reside either in buffers or in the physical database. The screen displays the different types of table accesses (select single, select, insert, update, delete), together with the hit ratios, ABAP processor calls, and database statistic calls for each type of access. You can notice that no hit ratios exist for the Update, Insert, and Delete calls, since they always have to be passed to the database system. SAP Cursor Cache. The R/3 system cursor cache stores parsed SQL statements such as the SELECT statements which can be reused avoiding the SQL parsing preparation processing. The SAP cursor cache has a fixed size and cannot be tuned. From the tune summary screen, you can access additional buffer performance information. By clicking on the History button you can display the buffer history and analyze whether the sizes have been correct by comparing several days activity. You can further analyze individual buffers, memory, and table call statistics from the Detail analysis menu. On the buffer overview, click on the Detail analysis button on the application toolbar. Regarding memory configuration, SAP recommends allocating more memory to the buffers rather than distributing it to the database. However, when allocating more memory than required, you could waste memory space which can cause excessive system paging on the operating system. From the tune summary screen you can even see the specific buffered objects, such as tables or programs. To see them, double click on the particular buffer. The system displays a detailed screen for the buffer. On this detailed screen, you can select the Buffered Objects button, which will display an alphabetical list of the objects currently contained in the buffers. Figure 11 21 shows an example of the buffered programs.
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