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Overview

Jamm provides MemoryMeter, a Java agent for all Java versions to measure actual object memory use including JVM overhead.

Jamm assume that the JVM running the code is an HotSpot JVM. It has not been tested with other type of JVMs.

Building

"mvn package"; optionally, "mvn install"

Setup your toolchains.xml

We want to compile and test using different JVM versions. Configuration option jdkToolchain can be used to supply an alternate toolchain specification. To achieve our goal we need first to setup toolchains.xml. The toolchains.xml file is the configuration file where you set the installation paths of your toolchains. This file should be put in your ${user.home}/.m2 directory. When the maven-toolchains-plugin executes, it looks for the toolchains.xml file, reads it and looks for a toolchain matching the toolchains requirements configured in the plugin. Jamm repo contains a toolchains.example.xml which you can use as a baseline for your own toolchains.xml. You need it to be able to run the tests. Copy toolchains.example.xml to ${user.home}/.m2, rename the file to toolchains.xml. In toolchains.xml, check to update your vendor and jdkHome for JDK8, JDK11 and JDK17 which you have installed on your machine.

Running Tests

The tests can be run with "mvn test". The JvmArgs property can be used to specify the JVM arguments that can be used for running the tests. For example:

mvn test -DjvmArgs="-Xmx64g"
mvn test -DjvmArgs="-Xmx64g -XX:ObjectAlignmentInBytes=16 -XX:-UseCompressedClassPointers"

mvn test runs all tests with JDK8, JDK11, and then with JDK17 To run the tests with only one particular JDK version run:

  • for JDK8:
mvn surefire:test@test-default

or

mvn surefire:test
  • for JDK11:
mvn surefire:test@test-jdk11
  • for JDK17:
mvn surefire:test@test-jdk17

Use

The best way to use MemoryMeter is to start the JVM with "-javaagent:/jamm.jar" in order to use the Instrumentation strategy to guess objects' sizes.

MemoryMeter can be used in your code like this:

MemoryMeter meter = MemoryMeter.builder().build();
meter.measure(object);
meter.measureDeep(object);

If you would like to use MemoryMeter in a web application, make sure that you do NOT put this jar in WEB-INF/lib, as that may cause problems since your code is accessing a MemoryMeter from a different class loader than the one loaded by the -javaagent and won't see it as initialized.

If you want MemoryMeter not to measure some specific classes, you can mark the classes (or interfaces) using the Unmetered annotation.

The Maven coordinates for the latest version of Jamm

  <groupId>com.github.jbellis</groupId>
  <artifactId>jamm</artifactId>
  <version>0.4.0</version>

0.4.0 breaking changes

The 0.4.0 version comes with speed improvements and support java versions up to Java 17 but also some breaking changes at the API level.

  • The MemoryMeter constructor and the static methods used to configure the different options (withGuessing, ignoreOuterClassReference, ignoreKnownSingletons, ignoreNonStrongReferences, enableDebug) have been removed. Instead MemoryMeter instances must be created through a Builder.
  • The omitSharedBufferOverhead option has been removed. Instead a ByteBufferMode can be provided as an argument to measureDeep. The provided mode must match the way the application is creating SLABs for accurate results.
  • The ability to provide a tracker for visited object has been removed.
  • Guess values have been changed to each represent a single strategy. Fallback strategies can be defined through the MemoryMeter.Builder::withGuessing method.
  • MemoryMeter.countChildren has been removed.
  • The MemoryMeter.measure and MemoryMeter.measureDeep now accept null parameters
  • Jamm is not trying anymore to support non Hotspot JVM (e.g. OpenJ9)
  • By default MemoryMeter.measureDeep is now ignoring the space occupied by known singletons such as Class objects, enums, ClassLoaders, AccessControlContexts as well as non-strong references (like weak/soft/phantom references). If you want MemoryMeter to measure them you need to enable those measurements through MemoryMeter.builder().measureKnownSingletons() and MemoryMeter.builder().measureNonStrongReferences().
  • When measuring direct ByteBuffer objects MemoryMeter is ignoring some fields from the Cleaner as it might lead to some incorrect measurements by including references to other Cleaner instances
  • When measuring Thread objects MemoryMeter is ignoring the group field as it references the all the threads from the group
  • The behavior around non-strong references has changed. When non-strong references are ignored (the default) MemoryMeter will ignore all the fields from the Reference class as well as the head field from ReferenceQueue.

Supported Java versions

The 0.4.0 release has been tested with Java 8, 11 and 17 and the following JVM arguments that can affect the memory layout:

  • -Xmx
  • UseCompressedClassPointers
  • ObjectAlignmentInBytes
  • UseCompressedOops
  • RestrictContended
  • EnableContended
  • ContendedPaddingWidth
  • UseEmptySlotsInSupers

The Specification strategy does not work correctly with UseEmptySlotsInSupers disabled for some classes (like direct ByteBuffer) that interleave fields from different classes when they should not.

The ContendedPaddingWidth and EnableContended arguments are broken in Java 17. Changing the padding width has no effect and disabling @Contended (-XX:-EnableContended) has 2 bugs:

  • It does not work for contended annotation on fields
  • It does not work for the ConcurrentHashMap use of the class Contended annotation

Those bugs might caused the Unsafe and Specification strategies to return wrong results when the classes are using @Contended and those JVM arguments are used.

The fine print

Measurement strategies

MemoryMeter can use different strategies to guess the objects sizes. We have tried to ensure that the output of the strategies is the same.

Instrumentation

If the JVM has been started with -javaagent, MemoryMeter will use java.lang.instrument.Instrumentation.getObjectSize to get an estimate of the space required to store the given object. It is the safest strategy.

Instrumentation and specification

This strategy requires java.lang.instrument.Instrumentation as the Instrumentation strategy and will use it to measure non array object. For measuring arrays it will use the Specification strategy way. This strategy tries to combine the best of both strategies the accuracy and speed of Instrumentation for non array object and the speed of Specification for measuring array objects for which all strategy are accurate. For some reason Instrumentation is slower for arrays before Java 17.

Unsafe

MemoryMeter will use Unsafe.objectFieldOffset to guess the object offset. Java 14 introduced records and Java 15 introduced Hidden classes which are used from Java 15 onward for Lambda expressions. Unfortunately, calling Unsafe.objectFieldOffset on the Field of a record or hidden class will result into an UnsupportedOperationException therefore for record and hidden classes the unsafe strategy delegates the measurement to the specification strategy.

Specification

MemoryMeter will guess the object size based on what it knows from the JVM.

Object graph crawling

Default crawling approach

When measureDeep is called by default MemoryMeter will use reflection to crawl the object graph. In order to prevent infinite loops due to cycles in the object graph MemoryMeter tracks visited objects imposing a memory cost of its own.

Java 9 introduced the Java Platform Module System (JPMS) that made illegal reflective access between some modules. This is breaking the ability for Jamm to crawl the object graph. To avoid that problem, if Jamm detects that it cannot use reflection to retrieve field data it will rely on Unsafe to do it. Unfortunately, despite the fact that the code is designed to go around those illegal accesses the JVM might emit some warning for access that only will be illegal in future versions. The Unsafe approach might also fail for some scenarios as Unsafe.objectFieldOffset do not work for records or hidden classes such as lambda expressions. In such cases add-exports or add-opens should be used.

Optimized crawling approach

For your own classes, MemoryMeter provides a way to avoid the use of reflections by having the class implement the Measurable interface. When MemoryMeter encounter a class that implements the Measurable interface it will call the addChildrenTo to let the class adds its fields to the stack of objects that need to be measured instead of using reflection. Therefore avoiding the reflection cost.

Filtering

By default MemoryMeter.measureDeep is ignoring known singletons such as Class objects, enums, ClassLoaders, AccessControlContexts as well as non-strong references (like weak/soft/phantom references). If you want MemoryMeter to measure them you need to enable those measurements through MemoryMeter.builder().measureKnownSingletons() and MemoryMeter.builder().measureNonStrongReferences().

Skipping objects

If you want MemoryMeter not to measure some specific classes or fields, you can mark the classes/interfaces or fields using the @Unmetered annotation.

    public class WithAnnotationField {

        @Unmetered
        private String s;

        public WithAnnotationField(String s) {
            this.s = s;
        }
        ...
    }

    @Unmetered
    private static class WithTypeAnnotation {
        private String s;

        public WithTypeAnnotation(String s) {
            this.s = s;
        }
        ...
    }

For a finer control on which classes and fields should be filtered out it is possible to use the MemoryMeter(MemoryMeterStrategy, FieldAndClassFilter, FieldFilter , boolean, MemoryMeterListener.Factory) constructor.

ByteBuffer measurements

MemoryMeter has 3 ways to measure ByteBuffers: NORMAL, SLAB_ALLOCATION_NO_SLICE and SLAB_ALLOCATION_SLICE.

Normal (default) mode

In this mode MemoryMeter.measureDeep will crawl the object graph and sum its different elements. For a HeapByteBuffer like ByteBuffer.allocate(20) the crawled graph will be:

root [java.nio.HeapByteBuffer] 96 bytes (56 bytes)
  |
  +--hb [byte[]] 40 bytes (40 bytes)

For a DirectByteBuffer like ByteBuffer.allocateDirect(20) the crawled graph will be:

root [java.nio.DirectByteBuffer] 136 bytes (64 bytes)
  |
  +--cleaner [jdk.internal.ref.Cleaner] 72 bytes (40 bytes)
    |
    +--thunk [java.nio.DirectByteBuffer$Deallocator] 32 bytes (32 bytes)

In reality the cleaner field has some extra fields that MemoryMeter is excluding as they might result in an incorrect measurement. Those fields are:

  • queue as it is a dummy queue referenced by all Cleaner instances
  • next and prev as they are used to create a doubly-linked list of live cleaners and therefore refer to other Cleaners instances

If slice, duplicate or asReadOnlyBuffer is used to create a new buffer from a heap buffer, the resulting buffer will have a reference to the original buffer array, whereas if the buffer was a direct buffer, the new buffer would have a direct reference to the original direct buffer:

root [java.nio.DirectByteBuffer] 200 bytes (64 bytes)
  |
  +--att [java.nio.DirectByteBuffer] 136 bytes (64 bytes)
    |
    +--cleaner [jdk.internal.ref.Cleaner] 72 bytes (40 bytes)
      |
      +--thunk [java.nio.DirectByteBuffer$Deallocator] 32 bytes (32 bytes)

In the NORMAL mode those underlying arrays and direct buffer size will always be included in the final total size.

SLAB allocation no slice mode

The goal of this mode is to omit the size of the shared data in slabs when the slabs are allocated through the use of: duplicate().position(x).limit(y). This is done by comparing the number of remaining bytes with the buffer capacity and considering only the remaining bytes for the size when the number of remaining bytes is smaller than the capacity.

SLAB allocation slice mode

The goal of this mode is to omit the size of the shared data in slabs when the slabs are allocated through the use of: duplicate().position(x).limit(y).slice(). This is done by comparing the buffer's capacity with the array'size for heap buffers or with the size of the underlying buffer for direct buffers. If a buffer is considered a slab, only its capacity will considered for the size.

@Contended

@Contended was introduced in Java 8 as sun.misc.Contended but was repackaged in the jdk.internal.vm.annotation package in Java 9. Therefore, in Java 9+ unless -XX:-RestrictContended or --add-exports java.base/jdk.internal.vm.annotation=ALL-UNNAMED are specified MemoryMeter will not have access to the value() method of @Contended and will be unable to retrieve the contention group tags. Making it potentially unable to computes the correct sizes with the Unsafe or Spec strategies. As it also means that only the internal Java classes will use that annotation, MemoryMeter will rely on its knowledge of those internal classes to try to go around that problem.

Moreover as specified in the Supported Java versions section the ContendedPaddingWidth and EnableContended arguments logics are broken in Java 17. Therefore the use of the ContendedPaddingWidth argument or of -XX:-EnableContended might caused the Unsafe and Specification strategies to return wrong results when the classes are using @Contended.

Non-strong references

By default MemoryMeter will ignore non-strong reference objects. It will also ignore all the Reference's fields and the head field of ReferenceQueue. The Reference fields next and discovered are used by ReferenceQueue instances to create a linked list, and taking them into account could lead to measuring a much larger part of the heap than what is usually intended. The queue field is either a singleton ReferenceQueue.NULL or a provided queue that users hold a reference to and therefore should be ignored too. To be consistent, the head field of ReferenceQueue is also ignored to fully decouple queue and references. This means that calling measureDeep on a ReferenceQueue will by default only measure the ReferenceQueue instance and its related objects but not the References that are part of the queue.

If measureNonStrongReferences is set, MemoryMeter will measure referenced objects but also all the fields from Reference objects and ReferenceQueue object.

Debugging

Layout and JVM information

The org.github.jamm.strategies.LogInfoAtStartup system property can be set to true to request Jamm to log at startup in System.out information about the JVM and the detected memory layout.

Example of output:

Jamm starting with: java.version='1.8.0_144', java.vendor='Oracle Corporation', instrumentation=true, unsafe=true, Memory Layout: [objectHeaderSize=12 , arrayHeaderSize=16, objectAlignment=8, referenceSize=4, contendedPaddingWidth=128]

Visited object tree

In order to see the object tree visited when calling MemoryMeter.measureDeep and ensuring that it matches your expectations you can build the MemoryMeter instance using printVisitedTree:

    MemoryMeter meter = MemoryMeter.builder().printVisitedTree().build();

If a problem occurs while crawling the graph, MemoryMeter will not print the graph in the System.out but instead will print in System.err the stack from the object that could not be accessed up to the original input object.

JMH Benchmarks

The Jamm JMH benchmarks can be run using:

    mvn jmh:benchmark