JarCastingOverview
Jamm provides MemoryMeter, a Java agent for all Java versions to measure actual object memory use including JVM overhead.
Use
To use MemoryMeter, start the JVM with -javaagent:<path to>/jamm.jar.
You can then use MemoryMeter in your code like this:
MemoryMeter meter = MemoryMeter.builder().build();
meter.measure(object);
meter.measureDeep(object);
See MemoryMeter.Builder for more options.
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.
It is good to reuse existing MemoryMeter instances. Creating new MemoryMeter instances can and will cause significant performance penalties and also unnecessary side effects, since all guess-modes rely on java.lang.ClassValue.
Dependency coordinates for the latest version of Jamm
Gradle:
dependencies {
implementation("com.github.jbellis:jamm:0.4.0")
}
Maven:
<dependency>
<groupId>com.github.jbellis</groupId>
<artifactId>jamm</artifactId>
<version>0.4.0</version>
</dependency>
Building
Build + test:
./gradlew jar test
To run the microbenchmark:
./gradlew microbench
build/microbench <JMH options...>
Public to local Maven repo (in case you need that)
./gradlew publishToMavenLocal
IDEs
You can just open the project in IntelliJ. If you've opened the Maven-ish branch(es) in IntelliJ before, you'll have to remove the .idea folder.
Benchmarks
See Microbench.java
To run the microbenchmarks, execute ./gradlew microbench and run build/microbench from the command line.
Some microbenchmark results (JVM options: -Xms16g -Xmx16g -XX:+UseG1GC -XX:+AlwaysPreTouch):
Java 11.0.8
Benchmark (guess) (nested) (refs) Mode Cnt Score Error Units
Microbench.arrayByteArray ALWAYS_SPEC 100 4 avgt 3 0.012 ± 0.019 us/op
Microbench.arrayByteArray ALWAYS_UNSAFE 100 4 avgt 3 0.012 ± 0.019 us/op
Microbench.arrayByteArray ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.012 ± 0.017 us/op
Microbench.cls1 ALWAYS_SPEC 100 4 avgt 3 0.026 ± 0.004 us/op
Microbench.cls1 ALWAYS_UNSAFE 100 4 avgt 3 0.026 ± 0.005 us/op
Microbench.cls1 ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.026 ± 0.004 us/op
Microbench.cls2 ALWAYS_SPEC 100 4 avgt 3 0.350 ± 0.155 us/op
Microbench.cls2 ALWAYS_UNSAFE 100 4 avgt 3 0.387 ± 0.030 us/op
Microbench.cls2 ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.355 ± 0.040 us/op
Microbench.cls3 ALWAYS_SPEC 100 4 avgt 3 0.761 ± 0.087 us/op
Microbench.cls3 ALWAYS_UNSAFE 100 4 avgt 3 0.733 ± 0.130 us/op
Microbench.cls3 ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.738 ± 0.099 us/op
Microbench.deeplyNested ALWAYS_SPEC 100 4 avgt 3 49.825 ± 1.482 us/op
Microbench.deeplyNested ALWAYS_UNSAFE 100 4 avgt 3 46.260 ± 4.466 us/op
Microbench.deeplyNested ALWAYS_INSTRUMENTATION 100 4 avgt 3 52.301 ± 8.285 us/op
Microbench.justByteArray ALWAYS_SPEC 100 4 avgt 3 0.014 ± 0.005 us/op
Microbench.justByteArray ALWAYS_UNSAFE 100 4 avgt 3 0.014 ± 0.004 us/op
Microbench.justByteArray ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.014 ± 0.006 us/op
Microbench.justByteBuffer ALWAYS_SPEC 100 4 avgt 3 0.056 ± 0.011 us/op
Microbench.justByteBuffer ALWAYS_UNSAFE 100 4 avgt 3 0.058 ± 0.018 us/op
Microbench.justByteBuffer ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.058 ± 0.029 us/op
Microbench.justString ALWAYS_SPEC 100 4 avgt 3 0.059 ± 0.061 us/op
Microbench.justString ALWAYS_UNSAFE 100 4 avgt 3 0.057 ± 0.019 us/op
Microbench.justString ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.056 ± 0.001 us/op
Java 8u265
Benchmark (guess) (nested) (refs) Mode Cnt Score Error Units
Microbench.arrayByteArray ALWAYS_SPEC 100 4 avgt 3 0.011 ± 0.007 us/op
Microbench.arrayByteArray ALWAYS_UNSAFE 100 4 avgt 3 0.011 ± 0.006 us/op
Microbench.arrayByteArray ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.011 ± 0.007 us/op
Microbench.cls1 ALWAYS_SPEC 100 4 avgt 3 0.023 ± 0.009 us/op
Microbench.cls1 ALWAYS_UNSAFE 100 4 avgt 3 0.025 ± 0.039 us/op
Microbench.cls1 ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.024 ± 0.012 us/op
Microbench.cls2 ALWAYS_SPEC 100 4 avgt 3 0.357 ± 0.034 us/op
Microbench.cls2 ALWAYS_UNSAFE 100 4 avgt 3 0.365 ± 0.008 us/op
Microbench.cls2 ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.366 ± 0.100 us/op
Microbench.cls3 ALWAYS_SPEC 100 4 avgt 3 0.773 ± 0.178 us/op
Microbench.cls3 ALWAYS_UNSAFE 100 4 avgt 3 0.754 ± 0.328 us/op
Microbench.cls3 ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.755 ± 0.396 us/op
Microbench.deeplyNested ALWAYS_SPEC 100 4 avgt 3 46.889 ± 18.817 us/op
Microbench.deeplyNested ALWAYS_UNSAFE 100 4 avgt 3 45.166 ± 9.059 us/op
Microbench.deeplyNested ALWAYS_INSTRUMENTATION 100 4 avgt 3 48.041 ± 14.777 us/op
Microbench.justByteArray ALWAYS_SPEC 100 4 avgt 3 0.011 ± 0.008 us/op
Microbench.justByteArray ALWAYS_UNSAFE 100 4 avgt 3 0.011 ± 0.009 us/op
Microbench.justByteArray ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.011 ± 0.007 us/op
Microbench.justByteBuffer ALWAYS_SPEC 100 4 avgt 3 0.053 ± 0.013 us/op
Microbench.justByteBuffer ALWAYS_UNSAFE 100 4 avgt 3 0.053 ± 0.011 us/op
Microbench.justByteBuffer ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.053 ± 0.014 us/op
Microbench.justString ALWAYS_SPEC 100 4 avgt 3 0.055 ± 0.010 us/op
Microbench.justString ALWAYS_UNSAFE 100 4 avgt 3 0.054 ± 0.007 us/op
Microbench.justString ALWAYS_INSTRUMENTATION 100 4 avgt 3 0.054 ± 0.019 us/op
Previous (0.3.x) versions of jamm (w/ 8u265)
Mentioned for comparison reasons! Performance numbers for the recent version above.
Benchmark (guess) (nested) (refs) Mode Cnt Score Error Units
Microbench.cls1 ALWAYS_SPEC 100 4 avgt 3 0.243 ± 0.170 us/op
Microbench.cls1 ALWAYS_UNSAFE 100 4 avgt 3 0.304 ± 0.078 us/op
Microbench.cls1 NEVER 100 4 avgt 3 0.210 ± 0.126 us/op
Microbench.cls2 ALWAYS_SPEC 100 4 avgt 3 2.924 ± 0.272 us/op
Microbench.cls2 ALWAYS_UNSAFE 100 4 avgt 3 3.086 ± 0.419 us/op
Microbench.cls2 NEVER 100 4 avgt 3 2.624 ± 0.131 us/op
Microbench.cls3 ALWAYS_SPEC 100 4 avgt 3 5.919 ± 1.433 us/op
Microbench.cls3 ALWAYS_UNSAFE 100 4 avgt 3 6.031 ± 1.632 us/op
Microbench.cls3 NEVER 100 4 avgt 3 5.329 ± 0.513 us/op
Microbench.deeplyNested ALWAYS_SPEC 100 4 avgt 3 500.569 ± 27.362 us/op
Microbench.deeplyNested ALWAYS_UNSAFE 100 4 avgt 3 495.529 ± 66.984 us/op
Microbench.deeplyNested NEVER 100 4 avgt 3 388.130 ± 83.949 us/op
Microbench.justByteArray ALWAYS_SPEC 100 4 avgt 3 0.012 ± 0.011 us/op
Microbench.justByteArray ALWAYS_UNSAFE 100 4 avgt 3 0.013 ± 0.004 us/op
Microbench.justByteArray NEVER 100 4 avgt 3 0.055 ± 0.010 us/op
Microbench.justByteBuffer ALWAYS_SPEC 100 4 avgt 3 0.942 ± 0.230 us/op
Microbench.justByteBuffer ALWAYS_UNSAFE 100 4 avgt 3 0.935 ± 0.154 us/op
Microbench.justByteBuffer NEVER 100 4 avgt 3 0.689 ± 0.247 us/op
Microbench.justString ALWAYS_SPEC 100 4 avgt 3 0.616 ± 0.165 us/op
Microbench.justString ALWAYS_UNSAFE 100 4 avgt 3 0.684 ± 0.636 us/op
Microbench.justString NEVER 100 4 avgt 3 0.574 ± 0.098 us/op
The fine print
MemoryMeter can use java.lang.instrument.Instrumentation.getObjectSize(), which only claims to provide "approximate" results, but in practice seems to work as expected.
Two more implementations are available as well. One is using sun.misc.Unsafe w/ reflection and one just uses reflection. Both are inaccurate and rely on guesses. It is strongly recommended to not use "unsafe" or "specification" and jamm will emit a warning on stderr:
- The "Unsafe" implementation performs arithmetics on the "cookies" returned by
Unsafe.objectFieldOffset(), althought the Javadoc says:Do not expect to perform any sort of arithmetic on this offset; it is just a cookie which is passed to the unsafe heap memory accessors.The implementation does not always consider Java object layouts in under all circumstances for all JVMs. - The "specification" is just a best-effort guess-timate that can easily produce results that are wrong w/ newer JVMs and GCs and/or newer Java features.
0.4.0 notes
- The method
MemoryMeter.countChildren()has been removed, because there is a) no corresponding implementation from JDK-8249196 and b) there was no test coverage at all. - The construction of the
MemoryMeterinstance has been refactored to a proper builder pattern to allow distinct classes for each implementation: Instrumentation, Unsafe, Specification). - A bunch of internal optimizations went into the existing guess-modes and performance for these modes has also been significantly improved (about 10x, see microbenchmark results above).
- Support for
@Unmeteredfor fields has been removed for the future implementation ofjava.lang.Runtime.deepSizeOf()with Java 16.