Package org.neo4j.gds.core.utils.paged
Class HugeAtomicByteArray
- java.lang.Object
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- org.neo4j.gds.core.utils.paged.HugeAtomicByteArray
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- All Implemented Interfaces:
HugeCursorSupport<byte[]>
public abstract class HugeAtomicByteArray extends java.lang.Object implements HugeCursorSupport<byte[]>
A long-indexable array of atomic bytes that can contain more than 2 bn. elements.It is implemented by paging of smaller byte-arrays (
byte[][]
) to support approx. 32k bn. elements. If the the provided size is small enough, an optimized view of a singlebyte[]
might be used.- The array is of a fixed size and cannot grow or shrink dynamically.
- The array is not optimized for sparseness and has a large memory overhead if the values written to it are very sparse.
- The array does not support default values and returns the same default for unset values that a regular
byte[]
does (0
). - It only supports a minimal subset of the atomic operations that
AtomicLongArray
provides.
Basic Usage
AllocationTracker allocationTracker = ...; long arraySize = 42L; HugeAtomicByteArray array = HugeAtomicByteArray.newArray(arraySize, allocationTracker); array.set(13L, (byte)37); byte value = array.get(13L); // value = (byte)37;
Implementation is similar to the AtomicLongArray (but for bytes), based on sun.misc.Unsafe https://hg.openjdk.java.net/jdk/jdk13/file/9e0c80381e32/jdk/src/java.base/share/classes/java/util/concurrent/atomic/AtomicLongArray.java
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Constructor Summary
Constructors Constructor Description HugeAtomicByteArray()
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Method Summary
All Methods Static Methods Instance Methods Abstract Methods Concrete Methods Modifier and Type Method Description abstract byte
compareAndExchange(long index, byte expect, byte update)
Atomically sets the element at positionindex
to the given updated value if the current value, referred to as the witness value,==
the expected value.abstract boolean
compareAndSet(long index, byte expect, byte update)
Atomically sets the element at positionindex
to the given updated value if the current value==
the expected value.abstract byte
get(long index)
abstract byte
getAndAdd(long index, byte delta)
Atomically adds the given delta to the value at the given index.static long
memoryEstimation(long size)
static HugeAtomicByteArray
newArray(long size)
Creates a new array of the given size.abstract HugeCursor<byte[]>
newCursor()
Returns a newHugeCursor
for this array.abstract long
release()
Destroys the data, allowing the underlying storage arrays to be collected as garbage.abstract void
set(long index, byte value)
Sets the byte value at the given index to the given value.abstract void
setAll(byte value)
Set all entries in the array to the given value.abstract long
size()
Returns the length of this array.abstract long
sizeOf()
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Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
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Methods inherited from interface org.neo4j.gds.core.utils.paged.HugeCursorSupport
initCursor, initCursor
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Method Detail
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newCursor
public abstract HugeCursor<byte[]> newCursor()
Returns a newHugeCursor
for this array. The cursor is not positioned and in an invalid state. To position the cursor you must callHugeCursorSupport.initCursor(HugeCursor)
orHugeCursorSupport.initCursor(HugeCursor, long, long)
. Then the cursor needs to be put in a valid state by callingHugeCursor.next()
. Obtaining aHugeCursor
for an empty array (whereHugeCursorSupport.size()
returns0
) is undefined and might result in aNullPointerException
or anotherRuntimeException
.- Specified by:
newCursor
in interfaceHugeCursorSupport<byte[]>
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get
public abstract byte get(long index)
- Returns:
- the byte value at the given index
- Throws:
java.lang.ArrayIndexOutOfBoundsException
- if the index is not withinsize()
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getAndAdd
public abstract byte getAndAdd(long index, byte delta)
Atomically adds the given delta to the value at the given index.- Parameters:
index
- the indexdelta
- the value to add- Returns:
- the previous value at index
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set
public abstract void set(long index, byte value)
Sets the byte value at the given index to the given value.- Throws:
java.lang.ArrayIndexOutOfBoundsException
- if the index is not withinsize()
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compareAndSet
public abstract boolean compareAndSet(long index, byte expect, byte update)
Atomically sets the element at positionindex
to the given updated value if the current value==
the expected value.- Parameters:
index
- the indexexpect
- the expected valueupdate
- the new value- Returns:
true
if successful. False return indicates that the actual value was not equal to the expected value.
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compareAndExchange
public abstract byte compareAndExchange(long index, byte expect, byte update)
Atomically sets the element at positionindex
to the given updated value if the current value, referred to as the witness value,==
the expected value. This operation works as if implemented asif (this.compareAndSet(index, expect, update)) { return expect; } else { return this.get(index); }
The actual implementation is done with a single atomic operation so that the returned witness value is the value that was failing the update, not one that needs be read again after the failed update. This allows one to write CAS-loops in a different way, which removes one volatile read per loop iterationvar oldValue = this.get(index); while (true) { var newValue = updateFunction(oldValue); var witnessValue = this.compareAndExchange(index, oldValue, newValue); if (witnessValue == oldValue) { // update successful break; } // update unsuccessful set, loop and try again. // Here we already have the updated witness value and don't need to issue // a new read oldValue = witnessValue; }
- Parameters:
index
- the indexexpect
- the expected valueupdate
- the new value- Returns:
- the result that is the witness value, which will be the same as the expected value if successful or the new current value if unsuccessful.
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size
public abstract long size()
Returns the length of this array.If the size is greater than zero, the highest supported index is
size() - 1
The behavior is identical to calling
array.length
on primitive arrays.- Specified by:
size
in interfaceHugeCursorSupport<byte[]>
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sizeOf
public abstract long sizeOf()
- Returns:
- the amount of memory used by the instance of this array, in bytes.
This should be the same as returned from
release()
without actually releasing the array.
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setAll
public abstract void setAll(byte value)
Set all entries in the array to the given value. This method is not atomic!
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release
public abstract long release()
Destroys the data, allowing the underlying storage arrays to be collected as garbage. The array is unusable after calling this method and will throwNullPointerException
s on virtually every method invocation.Note that the data might not immediately collectible if there are still cursors alive that reference this array. You have to
HugeCursor.close()
every cursor instance as well.- Returns:
- the amount of memory freed, in bytes.
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newArray
public static HugeAtomicByteArray newArray(long size)
Creates a new array of the given size.
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memoryEstimation
public static long memoryEstimation(long size)
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