Package io.github.jbellis.jvector.util

Class RamUsageEstimator

java.lang.Object
io.github.jbellis.jvector.util.RamUsageEstimator
public final class RamUsageEstimator extends Object
Estimates the size (memory representation) of Java objects.

This class uses assumptions that were discovered for the Hotspot virtual machine. If you use a non-OpenJDK/Oracle-based JVM, the measurements may be slightly wrong.

See Also:

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    static final boolean
    COMPRESSED_REFS_ENABLED
    True, iff compressed references (oops) are enabled by this JVM
    static final long
    HASHTABLE_RAM_BYTES_PER_ENTRY
    Approximate memory usage that we assign to a Hashtable / HashMap entry.
    static final int
    MAX_DEPTH
    Recurse only into immediate descendants.
    static final int
    NUM_BYTES_ARRAY_HEADER
    Number of bytes to represent an array header (no content, but with alignments).
    static final int
    NUM_BYTES_OBJECT_ALIGNMENT
    A constant specifying the object alignment boundary inside the JVM.
    static final int
    NUM_BYTES_OBJECT_HEADER
    Number of bytes to represent an object header (no fields, no alignments).
    static final int
    NUM_BYTES_OBJECT_REF
    Number of bytes this JVM uses to represent an object reference.
    static final long
    ONE_GB
    One gigabyte bytes.
    static final long
    ONE_KB
    One kilobyte bytes.
    static final long
    ONE_MB
    One megabyte bytes.
    static final Map<Class<?>,Integer>
    primitiveSizes
    Sizes of primitive classes.
    static final int
    UNKNOWN_DEFAULT_RAM_BYTES_USED
    Approximate memory usage that we assign to all unknown objects - this maps roughly to a few primitive fields and a couple short String-s.

  • Method Summary

    Modifier and Type
    Method
    Description
    static long
    adjustForField(long sizeSoFar, Field f)
    This method returns the maximum representation size of an object.
    static long
    alignObjectSize(long size)
    Aligns an object size to be the next multiple of NUM_BYTES_OBJECT_ALIGNMENT.
    static String
    humanReadableUnits(long bytes, DecimalFormat df)
    Returns size in human-readable units (GB, MB, KB or bytes).
    static long
    shallowSizeOf(Object obj)
    Estimates a "shallow" memory usage of the given object.
    static long
    shallowSizeOf(Object[] arr)
    Returns the shallow size in bytes of the Object[] object.
    static long
    shallowSizeOfInstance(Class<?> clazz)
    Returns the shallow instance size in bytes an instance of the given class would occupy.
    static long
    sizeOf(boolean[] arr)
    Returns the size in bytes of the boolean[] object.
    static long
    sizeOf(byte[] arr)
    Returns the size in bytes of the byte[] object.
    static long
    sizeOf(char[] arr)
    Returns the size in bytes of the char[] object.
    static long
    sizeOf(double[] arr)
    Returns the size in bytes of the double[] object.
    static long
    sizeOf(float[] arr)
    Returns the size in bytes of the float[] object.
    static long
    sizeOf(int[] arr)
    Returns the size in bytes of the int[] object.
    static long
    sizeOf(long[] arr)
    Returns the size in bytes of the long[] object.
    static long
    sizeOf(short[] arr)
    Returns the size in bytes of the short[] object.
    static long
    sizeOf(Integer ignored)
    Return the shallow size of the provided Integer object.
    static long
    sizeOf(Long ignored)
    Return the shallow size of the provided Long object.
    static long
    sizeOf(String s)
    Returns the size in bytes of the String object.
    static long
    sizeOf(String[] arr)
    Returns the size in bytes of the String[] object.

    Methods inherited from class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Field Details

    • ONE_KB

      public static final long ONE_KB
      One kilobyte bytes.
      See Also:

    • ONE_MB

      public static final long ONE_MB
      One megabyte bytes.
      See Also:

    • ONE_GB

      public static final long ONE_GB
      One gigabyte bytes.
      See Also:

    • COMPRESSED_REFS_ENABLED

      public static final boolean COMPRESSED_REFS_ENABLED
      True, iff compressed references (oops) are enabled by this JVM

    • NUM_BYTES_OBJECT_REF

      public static final int NUM_BYTES_OBJECT_REF
      Number of bytes this JVM uses to represent an object reference.

    • NUM_BYTES_OBJECT_HEADER

      public static final int NUM_BYTES_OBJECT_HEADER
      Number of bytes to represent an object header (no fields, no alignments).

    • NUM_BYTES_ARRAY_HEADER

      public static final int NUM_BYTES_ARRAY_HEADER
      Number of bytes to represent an array header (no content, but with alignments).

    • NUM_BYTES_OBJECT_ALIGNMENT

      public static final int NUM_BYTES_OBJECT_ALIGNMENT
      A constant specifying the object alignment boundary inside the JVM. Objects will always take a full multiple of this constant, possibly wasting some space.

    • UNKNOWN_DEFAULT_RAM_BYTES_USED

      public static final int UNKNOWN_DEFAULT_RAM_BYTES_USED
      Approximate memory usage that we assign to all unknown objects - this maps roughly to a few primitive fields and a couple short String-s.
      See Also:

    • primitiveSizes

      public static final Map<Class<?>,Integer> primitiveSizes
      Sizes of primitive classes.

    • HASHTABLE_RAM_BYTES_PER_ENTRY

      public static final long HASHTABLE_RAM_BYTES_PER_ENTRY
      Approximate memory usage that we assign to a Hashtable / HashMap entry.

    • MAX_DEPTH

      public static final int MAX_DEPTH
      Recurse only into immediate descendants.
      See Also:

  • Method Details

    • alignObjectSize

      public static long alignObjectSize(long size)
      Aligns an object size to be the next multiple of NUM_BYTES_OBJECT_ALIGNMENT.

    • sizeOf

      public static long sizeOf(Integer ignored)
      Return the shallow size of the provided Integer object. Ignores the possibility that this object is part of the VM IntegerCache

    • sizeOf

      public static long sizeOf(Long ignored)
      Return the shallow size of the provided Long object. Ignores the possibility that this object is part of the VM LongCache

    • sizeOf

      public static long sizeOf(byte[] arr)
      Returns the size in bytes of the byte[] object.

    • sizeOf

      public static long sizeOf(boolean[] arr)
      Returns the size in bytes of the boolean[] object.

    • sizeOf

      public static long sizeOf(char[] arr)
      Returns the size in bytes of the char[] object.

    • sizeOf

      public static long sizeOf(short[] arr)
      Returns the size in bytes of the short[] object.

    • sizeOf

      public static long sizeOf(int[] arr)
      Returns the size in bytes of the int[] object.

    • sizeOf

      public static long sizeOf(float[] arr)
      Returns the size in bytes of the float[] object.

    • sizeOf

      public static long sizeOf(long[] arr)
      Returns the size in bytes of the long[] object.

    • sizeOf

      public static long sizeOf(double[] arr)
      Returns the size in bytes of the double[] object.

    • sizeOf

      public static long sizeOf(String[] arr)
      Returns the size in bytes of the String[] object.

    • sizeOf

      public static long sizeOf(String s)
      Returns the size in bytes of the String object.

    • shallowSizeOf

      public static long shallowSizeOf(Object[] arr)
      Returns the shallow size in bytes of the Object[] object.

    • shallowSizeOf

      public static long shallowSizeOf(Object obj)
      Estimates a "shallow" memory usage of the given object. For arrays, this will be the memory taken by array storage (no subreferences will be followed). For objects, this will be the memory taken by the fields.

      JVM object alignments are also applied.

    • shallowSizeOfInstance

      public static long shallowSizeOfInstance(Class<?> clazz)
      Returns the shallow instance size in bytes an instance of the given class would occupy. This works with all conventional classes and primitive types, but not with arrays (the size then depends on the number of elements and varies from object to object).
      Throws:
      IllegalArgumentException - if clazz is an array class.
      See Also:

    • adjustForField

      public static long adjustForField(long sizeSoFar, Field f)
      This method returns the maximum representation size of an object. sizeSoFar is the object's size measured so far. f is the field being probed.

      The returned offset will be the maximum of whatever was measured so far and f field's offset and representation size (unaligned).

    • humanReadableUnits

      public static String humanReadableUnits(long bytes, DecimalFormat df)
      Returns size in human-readable units (GB, MB, KB or bytes).