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:

• shallowSizeOf(Object)
• shallowSizeOfInstance(Class)

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(Accountable a)
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:

• Constant Field Values

ONE_MB

public static final long ONE_MB

One megabyte bytes.

See Also:

• Constant Field Values

ONE_GB

public static final long ONE_GB

One gigabyte bytes.

See Also:

• Constant Field Values

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:

• Constant Field Values

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:

• Constant Field Values

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(Accountable a)

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:

• shallowSizeOf(Object)

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).