Download The Heart Of The Matter

The Java Virtual Machine
Mike Brunt
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What is the JVM?
Main JVM Suppliers
ColdFusion and the JVM
Java J2EE – Java EE Servlet Containers
Where everything sits-fits
Tuning the JVM
Metrics Logging
Using utilities to aid in tuning the JVM
References
Summary
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Sun Microsystems created Java as a high-level
programming language which should run on as
many operating systems and hardware variations
as possible. The goal of “write once run
anywhere” meant that a predictable encapsulated
operating system, no matter what the underlying
hardware and software, was required.
The Java Virtual Machine (JVM or JRE) is a multithreaded environment abstracting the actual
computer operating system from the application
code.
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These are the main suppliers of Java Virtual
Machines, there are others.
Sun Hotspot JVM
IBM JVM
BEA JRockit
MAC OSRuntime for OSX
Oracle JVM aka JServer – OJVM
Microsoft JVM
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Since ColdFusion MX 6.0-6.1, CF has been a
Java Application which needs what is known
as a Servlet Container to run on. The most
popular Servlet Container for ColdFusion is
JRun.
In the Enterprise edition of ColdFusion, the
actual Servlet Container to be used can be
selected during the install. In non-Enterprise
installs JRun is automatically used. The main
point being that there is always a Servlet
Container.
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This is a list 0f Java Servlet Containers which are
certified as fully J2EE – Java EE compliant by Sun
Microsystems.
Geronimo from Apache
JBoss from Red Hat
JRun from Adobe
Oracle OC4J
Sun Java System Application Server (Sun One).
WebLogic Server from BEA
WebSphere Server from IBM
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This is a simple diagram showing where the Operating
System, JVM, Servlet Container and Java Application Server sit
in relativity.
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Standard-Server
Windows Install
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Enterprise
Windows Install
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This is a simple diagram showing where the Operating
System, JVM, Servlet Container and Java Application Server sit
in relativity.
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Garbage Collectors
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Garbage Collectors (different arguments to jvm.config)
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-XX:+UseParallelGC Use parallel garbage collection, this is typically the default
collector for ColdFusion installations. This collector is also referred to as the throughput
collector. It uses a parallel version of the young generation collector. The old (tenured)
generation is still cleaned with the default collector. Note This option can not be used in
conjunction with -XX:+UseConcMarkSweepGC.
-XX:+UseConcMarkSweepGC Use concurrent garbage collection. This collector is also
referred to as the concurrent low pause collector. It collects garbage in the old (tenured)
generation concurrently to executing the application. Note that this option can not be
used in conjunction with -XX:+UseParallelGC. Instead you may combine it with XX:+UseParNewGC
-XX:+UseParNewGC Instructs the VM to use a parallel collector for the young
generation. This option can be used in conjunction with -XX:+UseConcMarkSweepGC.
* As a note I have found that using the ConcMarkSweepGC along with ParNewGC can
cause longer Full GC’s, I tend to use ParNewGC on it’s own.
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Full Garbage Collections – Minimize
Sun JVM’s use a generational garbage collector with two
young generations and an old generation. New objects are
always allocated in the young generation part of the heap.
Every time the young generation is full a minor garbage
collection takes place. Minor garbage collections don't take
very long.
Objects which survived some minor collections, because they
are still referenced from the application, are promoted to the
older generation. Usually the older generation is full at some
point too, and needs to be cleaned. This leads to a major
garbage collection or Full Garbage (Full GC) collection that
removes garbage from both the old and the younger
generations. Full GC’s are “stop-the-world” events and need
to occur only when absolutely necessary.
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Full Garbage Collections – Minimize
One of the first methods I always want to employ when
troubleshooting is to ascertain how many Full GC’s are taking
place. To do so I add these arguments to the jvm.config file,
which is located in one of these directories depending on the
installation:
Enterprise – {drive-volume}\jrun4\bin
 Standard/Server - {drive-volume}\CFusionMX7\runtime\bin
-XX:+PrintGCDetails -XX:+PrintGCTimeStamps
-XX:+PrintHeapAtGC -verbose:gc -Xloggc:yourname.log*
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*This will send Garbage collection output to the {std}-out.log and will create a
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log of all GC activity with whatever name you gave yourname in either..
Enterprise – {drive-volume}\jrun4\bin
Standard/Server - {drive-volume}\CFusionMX7\runtime\bin
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Full Garbage Collections – Minimize
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Here is an example of the Verbose Garbage Collection log output…
par new generation
total 97088K, used 0K [0x10010000, 0x171d0000, 0x171d0000)
eden space 77696K,
0% used [0x10010000, 0x10010000, 0x14bf0000)
from space 19392K,
0% used [0x14bf0000, 0x14bf0000, 0x15ee0000)
to
0% used [0x15ee0000, 0x15ee0000, 0x171d0000)
space 19392K,
tenured generation
total 932096K, used 11109K [0x171d0000, 0x50010000, 0x50010000)
the space 932096K,
1% used [0x171d0000, 0x17ca95b8, 0x17ca9600, 0x50010000)
compacting perm gen
total 26624K, used 26606K [0x50010000, 0x51a10000, 0x58010000)
the space 26624K,
99% used [0x50010000, 0x51a0b8a0, 0x51a0ba00, 0x51a10000)
} , 0.1523967 secs]
3691.704: [Full GC
{Heap before GC invocations=8:
Heap
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This Full GC was almost certainly unnecessary because at the time it ran only 1109k out
of 932096k of the tenured (old) generation had been used. I also note the timestamp
(3691.704) of every Full GC in the log. If I see a regular pattern it almost always means
something is calling a Full GC perhaps using system.gc()
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Full Garbage Collections – Minimize – How?
One pattern I have seen over and over with ColdFusion is that
Full GC’s are very often occurring at regular 60 second
intervals. Although I have never been able to pin this down
fully, I suspect it is due to a Java process called RMI(Remote
Method Invocation). So then what do we do to control these
unnecessary “stop-the-world” Full GC’s? There are two
alternatives…
1/ Disable Full GC’s using the -XX:+DisableExplicitGC jvm.config
argument. I am not totally happy doing this and in many cases it will not
work
2/ Run Full GC’s at longer regular intervals. Applying these arguments
instead of -XX:+DisableExplicitGC will cause Full GC’s every 10 minutes
(600,000 ms).
-Dsun.rmi.dgc.client.gcInterval=600000
-Dsun.rmi.dgc.server.gcInterval=600000
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I have found that enabling metrics looking is invaluable in
troubleshooting ColdFusion and JRun application issues.
Metrics logging will show the status of the CF-JRun instance
at pre-selected time intervals in terms of number of threads
in use and available memory. In order to enable metrics
logging we need to edit the jrun.xml file, here…
Enterprise install
{drive-volume}\JRun4\servers\{servername}\SERVER-INF
Server-Standard install
{drive-volume}
\CFusionMX7\runtime\servers\{servername}\SERVER-INF
Setting detail is on the next slide
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Here is how to enable metrics logging in the jrun.xml file
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Firstly uncomment the service class…
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Then set “metricsEnabled” to true, we will leave other settings to default.
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Lastly I like to spilt the logs out to make reading them easier by this change
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Sun JVM Versions up to 1.5 (5.0)
http://java.sun.com/j2se/codenames.html
GC Viewer Tool (Sun JVM up to 1.4x)
http://www.javaperformancetuning.com/tools/gcviewer/index.shtml
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I have found that the only way to really
determine the correct garbage collector to
use for your applications and what the correct
arguments are to pass to the JVM, via the
jvm.config file, to effectively manage garbage
collections is to first determine how many
Full GC’s are occurring and control that if
there are too many. Then load-test the
application whilst tuning the arguments
passed to the JVM, via the jvm.config file.
Mike Brunt
[email protected]