What Your Jvm Has Been Trying To Tell You

IBM Software Group

What your JVM has been trying to tell you…

A look at available PD options within the IBM JVM

May 2007 | John Pape, IBM WebSphere SWAT © 2008 IBM Corporation

IBM Java Technology

Agenda

JVM Overview
JVM Components
Problem Scenarios
Summary
References

2 © 2007 IBM Corporation

IBM Java Technology

JVM Overview


IBM Java Technology

JVM Overview
 JVM = A Java Virtual Machine (JVM) is a virtual machine that
interprets and executes Java bytecode. (compiled Java
programs)
 It is a program/process like any other.
 Virtual Machine = software that creates a virtualized
environment between the computer platform and its operating
system, so that the end user can operate software on an
abstract machine.
The JVM allows a developer to write Java code and compile
it, one time, and then deploy that bytecode to any JVM on
any platform and have it run basically the same way.
 The JVM abstracts OS level functionality and programming
interfaces from the Java developer.

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JVM Overview cont…
Java Java
App App
Java
App 3rd party libraries

Java language libraries (I.e java.*, javax.*)

Java Virtual Machine
Abstraction

Operating System APIs

Operating System Kernel


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JVM Components


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Components of the JVM
Object Request
Broker (ORB)
Java Class Java Class
Libraries Extensions
Java and JNI code
Native Code

Core Interface (CI) Execution Management (XM) Native Libraries

Lock (LK)
Classloader Data
(CL) Conversion (dc)
Storage Just In Time
Execution Diagnostics (ST) Compiler (JIT)
Engine(XE) (dg)

HPI (Hardware Platform Interface)

Operating System Platform

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Core Interface
 This subcomponent encapsulates all interaction with the
user, external programs, and operating environment. It is
responsible for initiation of the JVM.
Provides presentation (but not execution) of all external APIs
(for example, JNI, JVMDI, JVMPI)
 Processes command-line input
 Provides internal APIs to enable other sub-components to
interact with the console
 Holds routines for interacting with the console; nominally,
standard in, out, and err
 Provides support for issuing formatted messages that are
suitable for NLS
 Holds routines for accessing the system properties


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Execution Engine

 This subcomponent provides all methods of executing Java
byte codes, both compiled and interpretive.
 Executes the byte code (in whatever form)
 Calls native method routines
 Contains and defines byte code compiler (JIT) interfaces
 Provides support for math functions that the byte code
requires
 Provides support for raising Java exceptions


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Execution Management

This subcomponent provides process control and
management of multiple execution engines. Is
initiated by the core interface. It provides:
Threading facilities
Runtime configuration; setting and inquiry
Support for raising internal exceptions
End JVM processing
Support for the resolution and loading of native
methods


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Diagnostics

 This subcomponent provides all diagnostic and debug
services and facilities. It is also responsible for providing
methods for raising events.
 Support for issuing events
 Implementation of debug APIs v Trace facilities
 Reliability, availability, and serviceability (RAS) facilities
 First failure data capture (FFDC) facilities


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Class Loader

 This subcomponent provides all support functions to Java
classes, except the execution.
 Loading classes
 Resolution of classes
 Verification of classes
 Initialization of classes
 Methods for interrogation of class abilities
 Implementation of reflection APIs


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Data Conversion

 This subcomponent provides support for converting data
between various formats.
 UTF Translation
 String conversion
 Support for primitive types


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Lock

 This subcomponent provides locking and synchronization
services.
 Maintains Java lock monitors
 Manages thread locking in the JVM
 Provides report on deadlocks in Java thread dumps
 Provides report on monitor pool in Java thread dumps


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Storage

 This subcomponent encapsulates all support for storage
services.
 Facilities to create, manage, and destroy discrete units of
storage
 Specific allocation strategies
 The Java object store (garbage collectable heap)


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Hardware Platform Interface

 This subcomponent consists of a set of well-defined functions
that provide low-level facilities and services in a platform-
neutral way.
 The HPI is an external interface that is defined by Sun.


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Problem
Scenarios


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Problem Scenarios

 My application is running  What does my memory
slow when running a certain footprint look like for my
code path, why? application?

 My application has very  Which parts of my application
consume the most CPU time?
erratic response times;
sometimes it’s great, others  Which parts of my application
it’s terrible, why? are taking up the most
memory?
 I am getting out of memory
errors on my application due  Is my application performance
to heap fragmentation, how as result of synchronization
can I tell what objects are problems in my threads?
pinned and dosed?  I need to know the exact
activities of the JVM when a
certain method is executed.

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My application is running slow when executing a certain
code path, why?
 This problem can be approached by setting a JVM method trace.
– Example:
– Database calls are slow when called from my stateless session EJB. The database class is
com.mycorp.db.DatabaseWrapper and the EJB is com.mycorp.ejb.session.LogicBean.
– The following trace could be enabled on the JVM (the –D parameters are delimited by spaces, there is
no line break)
– -Dibm.dg.trc.maximal=mt –
Dibm.dg.trc.methods=com/mycorp/db/DatabaseWrapper.*(),com/mycorp/ejb/session/LogicBean.*
() –Dibm.dg.trc.output=/tmp/jvm.trc
– JVM tracing incurs overhead and can produce large quantities of data, very rapidly. It is best to test the
code path with a single request to minimize any concerns with disk space usage.
– After taking the trace, it cannot be read until formatted. To do this you must execute the TraceFormat
tool contained in the IBM JDK.
– java com.ibm.jvm.format.TraceFormat <trace file> -indent
– -indent is used to provide helpful formatting of the nested levels of method tracing
– Timestamps printed in the JVM trace are in UTC format. This means you must translate the
times into your time zone.
 The net result of this tracing is an EXACT code path taken in the application. It may
be beneficial to add some other classes or packages to the trace to obtain a clear
overall picture (e.g. include some WebSphere Resource Adapter classes to see
server activity around the application calls)


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My application has very erratic response times; sometimes
it’s great, others it’s terrible, why?
 This problem can be approached in 2 ways:
– 1. JVM method trace
– 2. Enable and analyze verbose garbage collection (GC) output
 Since JVM method tracing has been demonstrated already, we’ll focus on
verbose GC analysis
– To enable verbose GC:
– Add –verbosegc or –verbose:gc to JVM arguments
– WebSphere has a check box to tick off, other servers/products
may have similar methods on enablement
– Use –Xverbosegclog:/path/to/desired/gcfile.txt to specify a file to
output verbose GC data to.
– By default, verbose GC outputs to native_stderr.log. It is also possible
to create rolling, generational verbose GC logs. Consult the IBM JVM
Diagnostic Guide for your version of JDK for details.


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I am getting out of memory errors on my application due to
heap fragmentation, how can I tell what objects are pinned
and dosed?
 Pinned Object = Objects on the heap that are permanently immobile until
explicitly unpinned by JNI. Moving a pinned object would cause a null pointer
in the native code referencing it.
 Dosed Object = Objects on the heap that are temporarily immobile.

 Pinned and Dosed objects cannot move and thus cannot be compacted
thereby reducing the overall amount of contiguous free space in memory.
 Useful traces
– Add -Dibm.dg.trc.print=st_verify
• Displays the # of pinned/dosed objects on the heap
– Add -Dibm.dg.trc.print=st_compact_verbose
• Displays the pinned/dosed objects on the heap during GC
compaction
 Knowing the layout of the objects on the heap can help you troubleshoot
OutOfMemoryErrors as well.

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What does my memory footprint look like for my
application?
 Heapdumps are the primary means  Heapdumps on JDK 1.4.1 SR1 and
of viewing memory heap contents. later explicitly do a GC before dumping,
this assures that only live objects are in
 To enable signal-based heapdumps the heapdump.
– that is heapdumps that produced
on a kill -3 signal:  Heapdumps are representations of
memory, so if the JVM has a large
– Add an environment entry called heap size, expect a large dump.
IBM_HEAPDUMP with a value of true
– Heapdumps can also be called from Java  Heapdumps can be produced in binary
code (.phd) format or in text (.txt) format.
– E.g. com.ibm.jvm.Dump.HeapDump();

 Heapdumps will be produced when a  The Sun HotSpot JVMs handle
heapdump generation differently and
JVM exhausts its Java heap and have different parameters to invoke.
throws an OutOfMemoryException. Consult with the references section of
– To disable this behavior: add environment this presentation for more information.
entry IBM_HEAPDUMP_OUTOFMEMORY
with a value of false (same can be done for
javacores/javadumps –
IBM_JAVADUMP_OUTOFMEMORY=false)
– On older JVM’s you may need to set this
value in order to produce heapdumps on an
OutOfMemoryException


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Which parts of my application consume the most CPU
time?
 Using the HPROF profiler, included with the JDK, you can determine which
methods are consuming the most CPU time.

 To invoke HPROF:
– Add –Xrunhprof:<name>=<value> / where <name> and <value> are
name/value pairs of HPROF parameters
– To obtain CPU calculations:
– -Xrunhprof:cpu=samples
– To obtain greater detail (with a performance trade-off)
– -Xrunhprof:cpu=timings
 EPROF can be executed on SUN HotSpot JVM's
– -Xeprof
 Running HPROF can result in an unstable JVM which can crash
unexpectedly, use with caution (not for production environments)


IBM Java Technology

Which parts of my application are taking up the most
memory?
 The HPROF profiler can assist here once again
– To obtain data on memory allocations by method:
– Add –Xrunhprof:heap=sites
 HPROF will provide a sorted list of sites with the most heavily
allocated objects at the top.
 This data will show you were the “hot spots” are in the code, that is,
the places in the code path that is responsible for the generation of
new objects.
 Useful for determining what part of the application is contributing the
most to the overall memory footprint.
 Can be used to stem potential memory leaks way before they
become an issue.


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Is my application performance a result of synchronization
problems in my threads?
HPROF can be used here once again
– To collect thread and synchronization data from the
JVM:
– Add –Xrunhprof:monitor=y,thread=y
This setup will provide data shows how much time
threads are waiting to access resources that are
already locked (resource contention)
It also provides a list of active monitors in the JVM,
this info can be useful to determine the presence of
deadlocks.


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I need to know the exact activities of the JVM when a
certain method is executed.
 In this case, you need to trigger a dump of some kind on a specific
trigger.
 The JVM can be setup to trigger several different types of dumps on
many conditions:
– Example : trigger java dump on uncaught
ArrayIndexOutOfBoundsException
– -Xdump:java:events=uncaught,filter=*ArrayIndexOutOfBoundsException*
 Types of dumps that can be produced
– Java dump / Java core / thread dump
– Heap dump
– Core dump / System dump
– Snap trace
– Stack dump (JDK 5 SR10 and onwards)


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Thanks!


What Your Jvm Has Been Trying To Tell You

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