RE: AS/400 and Java

[Danny Oentung <DANNY@xxxxxxxxxxxxxxxx>]

i've read a lot of benchmark that ibmers say that they've achieved like
Volano, SPECjbb2000, and the result is good but i am not sure that is it
perform the same with all the iseries server
http://www.spec.org/osg/jbb2000/results/jbb2000.html
. anyway it is good to hear things all people's experience in running java
app. in iseries.

maybe this could help (ps:if you have read this or a newer version, just
ignore it :)
AS/400 Java Performance Tuning (for V4R2) Tips (or go to
:http://www-919.ibm.com/developer/java/faq/perffaq.html#1)


AS/400 Java Performance Tuning (for V4R2) Tips
Introduction
You asked for it, and now it is here. The AS/400 Java Performance Tips. Keep
watching this page as it evolves. If you have any questions, or would like
to share a tip, use the "Feedback" button at the top of this page.

Table of Contents
Top Tips


Load the latest CUM package and PTFs
Use CRTJVAPGM on .zip and .jar and .class files
Use the native JDBC driver or ToolBox record I/O for local AS/400 data
access
Leave GCHMAX as default; Adjust GCHINL and GCHPTY as necessary; Ignore
GCHFRQ
Minimize the use of String Objects
Minimize object creation
Minimize synchronized methods
Use Prepared Statements
AS/400 Java Tips


Load the latest CUM package and PTFs
Use CRTJVAPGM on .class files
Use CRTJVAPGM on .zip and .jar files

Java Language Performance Tips


Minimize synchronized methods
Use the -O javac option
Minimize object creation
Minimize the use of String objects
Leverage variable scoping
Minimize use of exceptions
Minimize JVM start up
Use static final when creating constants

AS/400 Database Access Tips


Use the native JDBC driver
Use Prepared Statements
Store character data in DB2 as Unicode
Store numeric data in DB2 as float
Use ToolBox record I/O
Check ToolBox for existence of a Java program object
Close statement objects before recreating similar statements
Use getField() rather than getFields() in Toolbox recordaccess
Use get...(int columnIndex) rather than get...(String columnName)

Garbage Collection Tips


Leave GCHMAX as default
Adjust GCHINL as necessary
Adjust GCHPTY as necessary
Ignore GCHFRQ
Monitor GC Heap faulting
Introduction | Table of Contents | The Good Stuff !!


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The Good Stuff
AS/400 Java Tips


Load the latest CUM package and PTFs

To be sure that you have the best performing code, be sure to load the
latest CUM packages and PTFs for all products that you are using.
Information on the AS/400 JVM can be found at the Developer Kit for Java Web
Site. Information on the IBM Toolbox for Java can be found at the Toolbox
Web Site.


Use CRTJVAPGM on .class files

Java .class files should be converted into direct execution (machine
instruction) Java program objects through the CRTJVAPGM command. Use this
command before running any Java programs. The Program object is permanent
and will be re-used once it is created. Use the DSPJVAPGM command to
determine if the Java Program Object exists. Optimization levels 10,20,30
and 40 are supported on the CRTJVAPGM command

Relative Performance:
Results will vary by application. For computation and call intensive
applications the relative gains can be dramatic. Here are the relative
performance gains for Towers of Hanoi.

      Relative time
Optimization level 40       1.00
Optimization level 30       1.73
Optimization level 20       2.02
Optimization level 10       2.65
Interpretive       12.95


Comparisons based on v4r2. JDK 1.1.4.


Use CRTJVAPGM on .zip and .jar files

CRTJVAPGM must be used on .zip and .jar files. If the CRTJVAPGM was not run,
the JAVA/RUNJVA command will implicitly create program objects for each
class file in a .zip or .jar. Unlike a .class file however, these program
objects are not saved. The implicit creation of program objects will
significantly impact JVM startup or class loading. The implicit creation
will occur every time you start the JVM. To determine if your .zip/.jar file
has a permanent program object use the DSPJVAPGM command.

NOTE 1: Packaging multiple classes in one .zip or .jar file may also improve
class loading time and code optimization.

NOTE 2: If using the IBM Toolbox for Java classes on v4r2, you will need to
perform CRTJVAPGM on jt400.zip and jt400.jar.

Java Language Performance Tips


Minimize synchronized methods

Synchronized method calls take at least 10 times more processing than a
non-synchronized method call. Synchronized methods are only necessary if you
have multiple threads sharing the same object. If you share objects between
threads, keep the duration of the synchronized method low to improve
parallel execution.


Use the -O javac option

The javac compiler may in-line methods if the -O option is selected. In
order for a method to be in-lined it must be a final method (static, final,
or protected). Final methods can not be subclassed, which is the key to
allowing in-lining. Method in-lining is not supported in the AS/400 JVM
other than through the -O option.


Minimize object creation

Object creation can occur implicitly within the Java api's that you use as
well as within your program. Object creation and the resulting garbage
collection can typically take 15% of a server transaction workload. To
minimize this cost you can reuse objects by implementing a "reset" method
that clears the local variables in the object. The code fragment
if objx = null then
        objx = new x();
else
        objx.reset();

can provide significant performance improvements.


Minimize the use of String objects

String objects in Java are immutable. This means that you can not change
(append, etc.) to a string object without creating a new object. Object
creation is expensive and can occur multiple times for each String object
you are using. To minimize the use of String objects you should use either
StringBuffer or Char[]. StringBuffer may also be a problem since the
StringBuffer classes use synchronized method calls. An array of characters
(char[]) can be used to simulate fixed length strings. This is recommended
for applications which make heavy use of string data.

Relative Performance:
The following table shows the relative performance difference when using
String, StringBuffer, or char[]. The test case concatenates two strings. For
the char[] case. the concatentation reduces to simple array assignment, thus
avoiding the creation of objects and the synchronization overhead associated
with StringBuffer. In the following table the String "World Wide" was
concatenated to the string "Wait". For the Char[] test there were simply
four char[] assigments for the characters 'W' 'A' 'I' 'T'.

      Relative time
char[]       1
StringBuffer       35
String       363


Comparisons based on Optimization level 40, v4r2. JDK 1.1.4.


Leverage variable scoping

Java supports multiple techniques for accessing variables. One typical
technique is to write an "accessor" method. Local variables are the fastest.


Relative performance:
Here are five comparisons on variable access time and their relative
performance. A local variable is the fastest and is given a relative
performance of 1.       Relative time
Local variable :       1.0
Instance variable:       2.0
Accessor method in-lined:       2.0
Accessor method:       5.9
Synchronized accessor method:       99.6


Comparisons based on Optimization level 40, v4r2. JDK 1.1.4.


Minimize use of exceptions

In v4r2 Java exceptions use the full ILE exception handling mechanism. This
is a relatively expensive operation. Significant improvements are planned in
the next release. As with any JVM however, exceptions should be used for
error conditions that occur infrequently.


Minimize JVM start up

The JAVA/RUNJVA commands create a new batch immediate Job to run the JVM.
Limit this operation to relatively long running Java programs. If you need
to invoke Java frequently from non-Java programs consider passing messages
through an AS/400 Data Queue. The ToolBox Data Queue classes may be used to
implement "hot" JVM's.


Use static final when creating constants

When data is invariant, declare it as static final. For example here are two
array initializations:
        class test1 {
              int myarray[] = { 1,2,3,4,5,6,7,8,9,10,
                                2,3,4,5,6,7,8,9,10,11,
                                3,4,5,6,7,8,9,10,11,12,
                                4,5,6,7,8,9,10,11,12,13,
                                5,6,7,8,9,10,11,12,13,14 };
              {

        class test2 {
              static final int myarray2[] = { 1,2,3,4,5,6,7,8,9,10,
                                              2,3,4,5,6,7,8,9,10,11,
                                              3,4,5,6,7,8,9,10,11,12,
                                              4,5,6,7,8,9,10,11,12,13,
              }                               5,6,7,8,9,10,11,12,13,14 };




Relative Performance:
When 10,000 objects of type test1 and test2 were created the relative time
for test1 was about 2.3x longer than test2. The test saved each array object
to avoid any garbage collection.

Comparisons based on Optimization level 40, v4r2. JDK 1.1.4.

AS/400 Database Access Tips


Use the native JDBC driver

There are two AS/400 JDBC drivers that may be used to access local data. The
Toolbox driver is located at URL "jdbc:as400:system-name" where system-name
is the AS/400 tcp/ip system name. The native JDBC driver is located at URL
"jdbc:db2:system-name" where the system-name is the Data Base name. The
native AS/400 JDBC driver uses an internal shared memory condition variable
to communicate with the SQL/CLI Server Job. The ToolBox JDBC driver assumes
that the data is remote and uses a socket connection into the client access
ODBC driver. The native JDBC driver is faster when you are accessing local
data.


Use Prepared Statements

The JDBC prepareStatement should be used for repeatable executeQuery or
executeUpdate methods. If prepareStatement is not used, the execute
statement will cause an implicit prepareStatement every time the execute
statement is run.


Store character data in DB2 as Unicode

The AS/400 JVM stores string data internally as 2 byte Unicode. If you are
reading or writing large amounts of string data and the data is stored in
EBCDIC, the data will be converted on every database access. You can avoid
this conversion by storing the data in DB2 as 2 byte Unicode. Use the SQL
graphic type with CCID 13488 or the DDS grahic type with CCID 13488.


Store numeric data in DB2 as float

Decimal data can not be represented in Java as a primitive type. Decimal
data is converted to the abstract class Java.lang.BigDecimal on every
database read or write. This conversion can be avoided by storing numberic
data in the database as float or double. Alternatively, you may considering
converting the BigDecimal type to float for heavy calculations while leaving
the Database in decimal form. Note that rounding differences may be
introduced with the use of float or double.


Use ToolBox record I/O

The IBM Toolbox for Java provides native record level access classes. These
classes are specific to the AS/400 platform. They provide a significant
performance gain over the the use of JDBC access.


Check ToolBox for existence of a Java program object

The jt400.jar file contains the IBM Toolbox for Java product. After
installation, this .jar file does not have a Java program object. Use the
CRTJVAPGM at optimization level 40 to create the program object. See tip #3.
Use the CRTJVAPGM command during low system activity as it will take some
time. Use the DSPJVAPGM command to see if the program object already exists.



Close statement objects before recreating similar statements

The AS/400 attempts to reuse SQL cursors by constructing a cursor cache.
Normally a cursor is created for each unique query as part of the
executeQuery or executeUpdate methods. Cursors are available for reuse if
they are not actively attached to a statement object. If you use the close()
method on a statement, it makes the associated cursor eligible for reuse.
This reuse can save a large amount of processing and is similar to sharing
open files in RPG. A statement object may be eligible for garbage collection
if you no longer hold a reference to that object. Garbage collection will
implicitly close the statement, thus making the cursor elibible for reuse.
Garbage collection points vary by application. It is better to close() the
statement than to assume that cursor reuse will occur implicitly.


Use getField() rather than getFields() in Toolbox recordaccess

The ToolBox record class supports retrieving individual fields through
getField() or all of the fields through getFields(). On a file with 9 packed
decimal fields, 10 character fields, and one date field the second loop
takes about 2 times longer than the first loop. Since most applications do
not need all of the fields in a record. It is better to use getField on only
the fields used by the application.



        partialKey[0] = new BigDecimal("-2");
        partialKey[1] = new BigDecimal("-5");


        //
        for (int i = 0; i < RowCntArg; i++) {
           partialKey[0] = ((BigDecimal)partialKey[0]).add(EIGHT);

            ...
            tablexdata = file.read(partialKey); // reads record no
conversion
            ...
        {

        Object[] fieldobj = null;
        for (int i = 0; i < RowCntArg; i++) {
           partialKey[0] = ((BigDecimal)partialKey[0]).add(EIGHT);
            ...

            tablexdata = file.read(partialKey);
            ...
            fieldobj = tablexdata.getFields(); // Will convert every field
to a Java type
            ...
         {


Use get...(int columnIndex) rather than get...(String columnName)

Depending on the number of columns in your result set,a getxx field
operation using the columnIndex is about 2 times faster than a getxx field
operation using the columnName. There is an application consideration
however when using getxx columnIndex in that your application becomes more
dependent on the order of the columns in the result set. Note that on some
JDBC drivers, the retrieval of columns by index is limited to ever
increasing index values. This is not a restriction on AS/400 JDBC drivers.

Garbage Collection


Leave GCHMAX as default

The GCHMAX parameter on the JAVA/RUNJVA command specifies the maximum amount
of storage that you want to allocate to the garbage collection heap. In
general the default value (set to the largest possible value) should be
used. The system does not allocate the storage until it is needed. A large
value should not impact performance. If this maximum is reached, the JVM
will stop all threads and attempt to synchronously collect objects. Setting
this value to a small number may force unnecessary synchronous collections.


Adjust GCHINL as necessary

The GCHINL parameter on the JAVA/RUNJVA command specifies the amount of
initial storage that you want to allocate to the garbage collection heap.
This parameter indirectly affects the frequency of the asynchronous garbage
collection processing. Each time the total allocation for new objects
reaches this value, asynchronous collection is started. This parameter is
application dependent and may require some adjustment.


Adjust GCHPTY as necessary

This parameter affects the relative priority of the garbage collection
thread. A higher priority value will reduce the amount of time spent in the
asynchronous portion of the collection. You should not adjust this value
unless the application is memory constrained.


Ignore GCHFRQ

This parameter is not used. It has no effect on performance.


Monitor GC Heap faulting

Java objects are maintained in the JVM heap. Excessive page faults may occur
if the memory pool for your JVM is too small. These faults will be reported
as non-database page faults on the WRKSYSSTS command display. Typically the
storage pool for your JVM is *INTERACT. Fault rates between 20 and 30 per
second are acceptable. Higher rates should be reduced by increasing memory
pool size.


Introduction | Table of Contents | The Good Stuff !!





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