Java mission control and java flight recorder

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Java Mission Control and
Java Flight Recorder
Wolfgang Weigend
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Copyright © 2014, Oracle and/or its affiliates. All rights reserved.
Wolfgang Weigend
Sen. Leitender Systemberater
Java Technology and Architecture

The following is intended to outline our general product
direction. It is intended for information purposes only, and
may not be incorporated into any contract. It is not a
commitment to deliver any material, code, or functionality,
and should not be relied upon in making purchasing
Disclaimer
2 Copyright © 2015, Oracle and/or its affiliates. All rights reserved.
and should not be relied upon in making purchasing
decisions. The development, release, and timing of any
features or functionality described for Oracle’s products
remains at the sole discretion of Oracle.

Java Mission Control 5.4

Java Process Browser and JMX Console

• Tracer and Profiler
• Non-intrusive
• Built into the JVM itself
Java Flight Recorder
• On-demand profiling
• After-the-fact capture and analysis
• First released in 7u40

• Captures both JVM and application data
‒ Garbage Collections
‒ Sychronization
‒ Compiler
‒ CPU Usage
Java Flight Recorder - Tracer & Profiler
‒ CPU Usage
‒ Exceptions
‒ I/O
• Sampling-based profiler
‒ Very low overhead
‒ Accurate data

• Typical overhead in benchmarks: 2-3%
• Often not noticeable in typical production environments
• Turn on and off in runtime
Non-Intrusive
• Turn on and off in runtime
• Information already available in the JVM
‒ Zero extra cost

• Core of JFR is inside the JVM
• Can easily interact with other JVM subsystems
Built into the JVM itself
• Optimized C++ code
• Supporting functionality written in Java

• Start from Java Mission Control
‒ Or from the command line
• Easily configure the amount of information to capture
On-Demand Profiling
• Easily configure the amount of information to capture
• For a profile, a higher overhead can be acceptable
• When done, no overhead
• Powerful GUI for analysis

• In its default mode, very low overhead
• Designed to be always-on
• Uses circular buffers to store data
After-the-Fact Analysis
• Uses circular buffers to store data
‒ In-memory or on-disk
• When an SLA breach is detected, dump the current buffers
• Dump will have information leading up to the problem

• Enable
‐‐‐‐XX:+UnlockCommercialFeatures ‐‐‐‐XX:+FlightRecorder
• Start
‐‐‐‐
Configuration
‐‐‐‐XX:StartFlightRecording=filename=<path>,duration=<time>
• Or
Jcmd <pid> JFR.start filename=<path> duration=<time>

Advanced Configuration
Per Recording Session
Max age of data maxage=<time>
Max size to keep maxsize=<size>
Global Settings (-XX:FlightRecorderOptions)
Max stack trace depth stackdepth=<n> (default 64)
Save recording on exit dumponexit=true
Logging loglevel=[ERROR|WARN|INFO|DEBUG|TRACE]
Repository path repository=<path>

• Recordings can specify exactly which information to capture
‒ ~80 events with 3 settings each
• But: two preconfigured settings
Recording Sessions
• But: two preconfigured settings
‒ “default”: provides as much information as possible while keeping
overhead to a minimum
‒ “profile”: has more information, but also higher overhead
• You can configure your own favorites in Mission Control

• Great working approach
• Each session can have its own settings
• Caveat: If there are multiple sessions all of them get the
Many Simultaneous Recording Sessions
• Caveat: If there are multiple sessions all of them get the
union of the enabled events
‒ Example: If event A is enabled in on recording, all recordings will see event A
‒ Example: If event B has two different thresholds, the lower value will apply

Creating Recordings Using Startup Flags
• Documentation of startup flags available in the JDK docs
• The following example starts up a 1 minute recording 20 seconds after
starting the JVM:
-XX:+UnlockCommercialFeatures -XX:+FlightRecorder -
XX:StartFlightRecording=delay=20s,duration=60s,name=MyRecording,filena
Useful for enabling continuous recordings at startup
me=C:TEMPmyrecording.jfr,settings=profile
– The settings parameter takes either the path to, or the name of, a template
– Default templates are located in the jre/lib/jfr folder
– Note: Using the settings parameter will require JDK 7u40 or greater
• To get more information on what is going on, change the log level:
-XX:FlightRecorderOptions=loglevel=info

The Default Recording
• Special short hand to start the JVM with a continuous recording
• Started with -XX:FlightRecorderOptions=defaultrecording=true
• The default recording will have the recording id 0
• Only the default recording can be used with the dumponexit and dumponexitpath
parameters
parameters
• The following example will start up the continuous recording. It will be dumped
when the JVM exits to C:demosdumponexit.jfr.
-XX:+UnlockCommercialFeatures -XX:+FlightRecorder -
XX:FlightRecorderOptions=defaultrecording=true,dumponexit=true,dumponexitpath=C:
demosdumponexit.jfr

Creating Recordings Using JCMD
Usage: jcmd <pid> <command>
Example starting a recording:
jcmd 7060 JFR.start name=MyRecording settings=profile
delay=20s duration=2m filename=c:TEMPmyrecording.jfr
Example checking on recordings:
Useful for controlling JFR from the command line
Example checking on recordings:
jcmd 7060 JFR.check
Example dumping a recording:
jcmd 7060 JFR.dump name=MyRecording
filename=C:TEMPdump.jfr

Flight Recorder Inner Workings
• Extremely low overhead
– Using data already gathered
– High performance recording engine
• Testing!
Focus on performance
• Testing!
• Third party events
– WLS
– Dynamic Monitoring Service DMS custom WLST commands
– JavaFX
– You can add your own events, but this is not yet supported!

• Information gathering
− Instrumentation calls all over the JVM
− Application information via Java API
• Collected in Thread Local buffers
⇢ Global Buffers ⇢Disk
Java Flight Recorder – How is it built?
⇢ Global Buffers ⇢Disk
• Binary, proprietary file format
• Managed via JMX
• Java Flight Recorder
− Start from JMC 5.4 or CLI
• Activate Flight Recorder
− -XX: +UnlockCommercialFeatures
− -XX: +FlightRecorder

Java Flight Recorder – Buffers
• “Circular”
• Designed for low contention

• Header
• Payload
‒ Event specific data
“Everything is an Event”
‒ Event specific data

• Instant
‒ Single point in time
‒ Example: Thread starts
• Duration
Event Types
• Duration
‒ Timing for something
‒ Example: GC
• Requestable
‒ Happens with a specified frequency
‒ Example: CPU Usage every second

• For every event
‒ Name, Path, Description
Event Meta Data
• For every payload item
‒ Name, Type, Description, Content Type

• Describes the semantics of a value
• Used to correctly display the value in the UI
“Content Type”
Content Type Displayed as
Bytes 4 MB
Bytes 4 MB
Percentage 34 %
Address 0x23CDA540
Millis 17 ms
Nanos 4444 ns

Event Definition in Hotspot
<event id="ThreadSleep"
path="java/thread_sleep"
label="Java Thread Sleep“ ...>
<value field="time“
type="MILLIS“
label="Sleep Time"/>
• XML definitions are processed into C++ classes
label="Sleep Time"/>
</event>

Event Emission in Hotspot
JVM_Sleep(int millis){
EventThreadSleep event;
… // actual sleep happens here
event.set_time(millis);
• Now the data will be available in JFR
event.set_time(millis);
event.commit();
}

• Enable/disable event
• Thresholds
‒ Only if duration is longer than X
Filtering Early
‒ Only if duration is longer than X
• Enable/disable stack trace
• Frequency
‒ Sample every X

• Self-contained
‒ Everything needed to parse an event is included in the file
‒ New events instantly viewable in the UI
• Binary, proprietary
• Designed for fast writing
File Format
• Designed for fast writing
• Single file, no dependencies
‒ Example: 5 Minutes of recording, result in a Flight Recorder File with
a size of 957 KB
Header Event Records Event Definitions

• Can’t leak memory
‒ Can’t aggregate information eternally
‒ Can’t keep references that prohibits class unloading
Dynamic Runtime and Long-Running Recordings
• Dynamic Runtime
‒ Classes can come and go
‒ Threads can come and go
• Solutions: Constant Pools, Checkpoints

• If every event contained the class name as a string, we would waste
lots of space
• Solution: Class ID’s
Problem: Many Events Reference Classes

• ID’s need to be part of the file
• Classes can be unloaded at any time
‒ Class may not be around until end of recording
Problem: When do we write the Class IDs?
‒ Class may not be around until end of recording
• Solution: Write Class ID when classes are unloaded

• Many classes are loaded, not all are referenced in events, we want to
save space
• Solution: When a class ID is referenced, the class is also “tagged”
Problem: Size of the Class List
‒ Write only tagged classes in the JFR file

• Over time many classes will be tagged, the size of the class list will
increase
• Solution: Reset the tags each time a class list is written to disk
Problem: Leaking Memory
• We call this a “Checkpoint”
• A recording file may contain many class lists, each one is only valid for
the data immediately preceding it

• The Class List is a special case of a Constant Pool
• Classes
• Methods
Constant Pools
• Methods
• Threads
• Thread Groups
• Stack Traces
• Strings

• At regular intervals, a “checkpoint” is
created in the recording
• Has everything needed to parse
the recording since the last checkpoint
Checkpoints
the recording since the last checkpoint
checkpoint =
events
+ constant pools
+ event meta-data

Analyzing Flight Recordings in JMC
• All tab groups except for the general Events tab group are
preconfigured to show a certain aspect of the recording (sometimes
referred to as static or preconfigured tabs)
• The pre-configured tabs highlights various areas of common
interest, such as code, memory & GC, threads and IO
• General Events tab group - useful for drilling down further and for
rapidly homing in on a set of events with certain properties

Java Flight Recorder Releases
• Resulted from the JRockit and HotSpot JVM Convergence
• Java Mission Control released with the 7u40 JDK
• Java Mission Control 5.4.0 released with 8u20
• Java Mission Control 5.4.0 released with 8u20

• Java Mission Control
http://www.oracle.com/technetwork/java/javaseproducts/mission-control/index.html
• User Guide
http://docs.oracle.com/javacomponents/jmc.htm
More Information
http://docs.oracle.com/javacomponents/jmc.htm
• Forum
https://community.oracle.com/community/java/java_hotspot_virtual_machine/java_mission_control
• JMC Tutorial
http://hirt.se/downloads/oracle/jmc_tutorial.zip
Do Java Flight Recordings and Analyse Flight Recordings

Summary
• Java Flight Recorder provides a central view on the JVM and the Java application
– JVM Events and Java API Events
• Extremely low overhead (<= 2-3%)
– Can keep it always on, dump when necessary
• Tooling for analysing recordings built into the Oracle JDK via Java Mission Control
• Tooling for analysing recordings built into the Oracle JDK via Java Mission Control
• Java APIs available for recording custom information into the Flight Recorder in the
Oracle JDK
• Third party integration giving holistic view of the detailed information recorded by
the Flight Recorder (WebLogic Server, JavaFX)

Thanks to Marcus Hirt for his sources!
Wolfgang.Weigend@oracle.com

Java mission control and java flight recorder

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