INFO: PERFORM.TXT: Performance Tuning Tips for Visual Basic and Access

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PERFORM.TXT

Release Notes for Microsoft (R) Visual Basic (R) Professional Edition This document contains performance tuning tips for Microsoft Visual Basic for Windows version 3.0 and Microsoft Access (TM) Relational Database Systems for Windows version 1.1.

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Contents

Part    Description
----    -----------
 1      Running Multiple Data Access Applications
 2      Manipulating Secured Microsoft Access Databases
 3      Tuning [ISAM] Entries in VB.INI or <APPNAME>.INI
 4      Using Transactions to Maximize Data Throughput
 5      Minimizing Keyset Overhead When Working with Large Recordsets
 6      Performance Tips for Visual Basic Data Access
				

Part 1: Running Multiple Data Access Applications

Visual Basic and Microsoft Access both use the same database engine to perform their database-related operations. When these applications are run at the same time on the same machine, it is important to coordinate use of the database engine, since it is only initialized by the first program accessing a data access operation. Re-initialization does not take place until all programs using the database engine are ended and another started.

Proper database initialization is especially important when accessing external databases like dBASE, FoxPro, Paradox, or Btrieve. All of these require special notations in the initialization file.

Note that data access applications can take the form of one or more instances of:

• Microsoft Access.
• A Microsoft Access application.
• Visual Basic at design time.
• Visual Basic at run time.
• A Visual Basic application.

For example, you might have a copy of Microsoft Access running with an .EXE file created with Visual Basic. On the other hand, you might have two or more dissimilar applications running -- both of which need to access the database engine. You will need to make sure that the initialization files are set up to deal with each of these situations.

To ensure that all applications using the database engine function as intended, you must ensure that all initialization parameters pertaining to external databases are identified in various .INI files under section headings (such as [Installable ISAM], [Paradox Isam], [Btrieve ISAM]) and are copied into each of the .INI files pertaining to each data access application that can potentially be running at the same time. The list below shows where each program looks for its initialization information:

Program .INI file

All of these .INI files are located in your Windows directory. During development, your Visual Basic application defaults to VB.INI unless your application uses the SetDataAccessOption statement to indicate a specific .INI file location. Once you create an executable program with Visual Basic, the initialization file will default to <APPNAME>.INI unless you use SetDataAccessOption.

If you want to coordinate operations between an instance of a Visual Basic application (or .EXE), you will want to force Visual Basic to indicate the same initialization file that a second instance of your program or an instance of Microsoft Access will use. This way, regardless of which application starts (and initializes) the database engine, both applications will be using the same initialization parameters.

Part 2: Manipulating Secured Microsoft Access Databases

For Visual Basic to manipulate secured Microsoft Access databases, you must provide Visual Basic with the location of the SYSTEM.MDA file associated with that Microsoft Access database, a valid user name, and password. This can be accomplished in three steps:

1. Use the SetDataAccessOptions statement to point to a valid .INI file.
2. Include a path to the SystemDB with a valid VB.INI or <appname>.INI entry to locate the file. For example:

   [Options]
   SystemDB=C:\ACCESS\SYSTEM.MDA

3. Set the user name and password (if other than "admin" with no password) with the SetDefaultWorkspace statement.

Part 3: Tuning [ISAM] Entries in VB.INI or <APPNAME>.INI

You can enhance the database access performance of Visual Basic by:

1. Adding or changing entries in the VB.INI or <APPNAME>.INI initialization file.
2. Using transactions.

Visual Basic automatically provides default internal settings for most common database operations. However, advanced users may want to tune these settings to provide maximum performance for a particular system configuration or application.

Setup automatically installs <APPNAME>.INI in your Windows directory. These [ISAM] entries determine the sizes of data page and read-ahead caches in memory, the amount of time data is held in a page cache, and the number of times Visual Basic will retry a lock operation.

WARNING

Determining the best settings for your system configuration or application can be time-consuming and difficult, usually involving much trial and error. In addition, settings that seem optimal for one situation may not be optimal for others. Casual users should not try to edit these entries.

Visual Basic automatically includes a PageTimeout entry in the [ISAM] section of the VB.INI or <APPNAME>.INI file. This entry sets the amount of time Visual Basic holds a data page in memory. For additional performance tuning, you can add MaxBufferSize, ReadAheadPages, LockRetry, and CommitLockRetry entries to this section.

Visual Basic reads these initialization settings at startup time. They can be changed while Visual Basic is running, but the changes won't take effect until you restart Visual Basic or your application. As with all .INI settings that affect the database engine, these settings are fixed once the engine is initialized just before the first data access operation.

PageTimeout Entry (Shared Data Only)

The PageTimeout entry sets the amount of time, in tenths of a second, that Visual Basic holds a data page in a memory "page cache" if the database has been opened for shared access. Visual Basic reads data in 2K blocks of records, or "pages."

For example, when Visual Basic reads a data page, it places the data in the page cache. If Visual Basic receives another read request for the same data page during the timeout period, it reads the data directly from the page cache rather than from disk.

Note

Access Basic ignores the PageTimeout setting unless your code allows background processing by periodically calling the DoEvents statement or function.

PageTimeOut Settings

Maximum: 2147483647 (max Long integer)
Minimum: 0
Default: 300

If you remove the PageTimeout entry, Visual Basic uses a default PageTimeout setting of 5. For example:

MaxBufferSize Entry

The MaxBufferSize entry sets the amount of memory, in kilobytes, reserved for use as a page cache. Visual Basic reads data in 2K pages, placing the data in the page cache. Once the data is placed in the cache, Visual Basic can use it wherever it is needed -- in tables, queries, forms, or reports.

When Visual Basic receives a read request, it first checks the data pages in the page cache. If the page isn't in the cache, Visual Basic reads the data page from disk and then places it in the page cache. Visual Basic uses physical memory and if necessary virtual memory to create the cache. All pages stay in the cache until it is full and pages need to be flushed to make room for new reads.

MaxBufferSize Settings

Maximum: 4096
Minimum: 18
Default: If there is no MaxBufferSize entry in your VB.INI or <APPNAME>.INI

Note

Because Visual Basic reads data in 2-kilobyte pages, it always uses an even MaxBufferSize setting. If you type an odd number, Visual Basic uses a MaxBufferSize setting of one less than the number. For example:

ReadAheadPages Entry

The ReadAheadPages entry sets the size, in data pages, of a "read-ahead" cache used by Visual Basic for sequential page reads. A sequential page read occurs when Visual Basic detects that data in a current read request is on a data page adjacent on physical disk to the data page of the previous request. Visual Basic uses the "read-ahead" cache only when it detects that a sequential read is taking place.

1. If Visual Basic detects a sequential page read, it reads the requested page plus the next (N-1) pages in that direction, where N is the ReadAheadPages setting, placing the data pages in the read-ahead cache.
2. If Visual Basic then detects a sequential read, it can make the next N reads directly from the read-ahead cache.

The read-ahead cache increases the speed of sequential reads, especially for reading data stored on a network. It increases record updates per second (throughput) on a network by sending a few large packets rather than many small packets over the network. If possible, Visual Basic places the read-ahead cache in the first 640K of memory in order to benefit from the ability of Windows to read from and write to conventional memory. If the read-ahead cache can't be placed in conventional memory, Visual Basic places it in high memory. Placing the cache in high memory is less efficient than placing it in conventional memory because Windows must copy all reads and writes to its own buffer before completing the memory operation.

ReadAheadPages Settings

Maximum: 31
Minimum: 0
Default: If there is no ReadAheadPages entry in your VB.INI or
<APPNAME>.IN
file, Visual Basic uses a default setting of 8. For example:

Note

Visual Basic creates a separate read-ahead cache for each database open on your computer. Each library database has its own read-ahead cache.

LockRetry Entry

The LockRetry entry sets the number of times Visual Basic retries a page- locking operation before it reports an error. For example, if a user tries to lock a data page that is already locked by another user, the attempt will fail. Visual Basic will try to lock the page N more times, where N is the LockRetry setting. If the attempt to lock the page still fails on the Nth retry, Visual Basic reports an error.

LockRetry Settings

Maximum: 2147483647 (max Long integer)
Minimum: 0
Default: If there is no LockRetry entry in your VB.INI or <APPNAME>.INI
file, Visual Basic uses a default setting of 20. For example:

CommitLockRetry Entry

The CommitLockRetry entry is used with the LockRetry entry to set the number of retries that Visual Basic attempts when a user tries to lock a record on a data page already locked by a transaction. If a user tries to lock a data page that is already locked by a transaction, Visual Basic will try to lock the page N more times, where N is the product of the LockRetry setting and the CommitLockRetry setting. For example, if the LockRetry setting is 5 and the CommitLockRetry is 6, Visual Basic will try to lock the page 30 more times.

CommitLockRetry Settings

Maximum: 2147483647 (max Long integer)
Minimum: 0
Default: If there is no CommitLockRetry entry in your VB.INI or
<APPNAME>.INI file, Visual Basic uses a default setting of 20.
For example:

         CommitLockRetry = 6   ; Assuming a LockRetry setting of 6,
                               ; this example causes Visual Basic to
                               ; retry a page locked by a transaction 36
                               ; times.
				

Part 4: Using Transactions to Maximize Data Throughput

In a multi-user environment, you can further tune the performance of Visual Basic by using transactions for operations that update data. A transaction is a series of operations that must execute as a whole or not at all. You mark the beginning of a transaction with the BeginTrans statement. You use the Rollback or CommitTrans statement to end a transaction.

You can usually increase the record updates per second (throughput) of an application by placing operations that update data within an Access Basic transaction.

Tip

Because Visual Basic locks data pages used in a transaction until the transaction ends, using transactions will prevent access to those data pages while the transaction is pending. If you use transactions, try to find a balance between data throughput and data access.

Part 5: Minimizing Keyset Overhead When Working with Large Recordsets

When a query selects a large number of records from the database, Visual Basic only fetches the first row of that Dynaset or Snapshot and places the key to re-fetch that row in memory. Once a record is fetched or visited, it becomes a member of the recordset. As you "visit" additional rows of the recordset, the keys are stored in workstation memory (in a temporary table), and in the case of Snapshots, so is the data. If you move back to previously fetched rows, Visual Basic re-fetches the rows using the old key fetched from the temporary key table.

• If the database record is no longer there, you get a trappable error.
• If the record has changed, the new information is fetched from the database.

As you move further and further into the recordset, more and more memory is taken up storing the keys. Eventually, Visual Basic will begin saving the keyset on disk. If this happens, space is used on disk in the directory specified by your \TEMP environment variable. Generally, you won't see a performance degradation until Visual Basic has to swap the keyset temporary table to disk. If you run out of disk space because Visual Basic has exhausted the space in your \TEMP directory, you will get a trappable error.

Moving to the end of the Dynaset or Snapshot does at least two things: First, it forces Visual Basic to visit all of the records in your recordset. Hence, all keys will be saved on the workstation. If this is a few hundred rows, this may not take long or take up more space than the workstation can handle.

However, for larger recordsets, a MoveLast operation may be far more than the workstation can save. When working with Snapshots, not only are the keys fetched, but the data for all records is also brought into local memory. Generally, you should avoid operations that fetch more rows than your user or workstation can deal with. Operations that must touch each record in a recordset may best be performed with an action query that consumes less system resources. In any case, your performance will not be severely degraded, either as you move forward until you have to swap, or hardly at all if you move backwards in the recordset -- even to the first record.

Note

The Dynaset or Snapshot membership is not set until the record is actually fetched for the first time. Since this can take from seconds to days depending on how fast you fetch the records (moving down through the recordset with MoveNext or with MoveLast), no Dynaset or Snapshot is really a frozen subset of the data at a point in time. The only way to ensure that no changes are made while the recordset is built is to get exclusive access to the table or database before fetching -- which essentially locks out all other users until the recordset (or database) is closed.

Part 6: Performance tips for Visual Basic Data Access

The following tips are suggested for operations involving more than just a few records to increase the overall performance of your system.

1. When working with large recordsets (Dynasets or Snapshots), do not use the MoveLast method unless absolutely necessary.
   
   Moving to the end of a recordset requires Visual Basic to load all keys for the recordset into memory. In the case of Snapshots, not only are keys loaded into memory, but the data is also brought into workstation memory. If temporary memory space is exhausted, Visual Basic may be forced to swap this temporary cache to disk. In this case, Visual Basic will use space as addressed by the \TEMP environment variable. Once this space is exhausted, your application will trigger a trappable error.
2. When you want to access external tables fast, attach the table to your database instead of using the IN clause in a SQL statement or addressing the table directly.
   
   When Visual Basic needs to access your external table, all linkage information is resolved when the database is opened and does not have to be re-established and initialized each time the data is accessed (for example, with non-attached tables.
3. For reasonably small recordsets, especially where you do not intend to write to the recordset, use Snapshots instead of Dynasets.
   
   If possible, set the READONLY flag on the data control or DB_READONLY option when opening databases. This will permit Visual Basic to bypass significant logic to handle multi-user read-write access to your tables.
4. In cases where you are working with external ODBC databases, you will achieve maximum possible speed if you use SQL Passthrough instead of attaching or direct access that involves the Visual Basic database engine.