com.yahoo.memory

Class MemoryMappedFile

java.lang.Object

com.yahoo.memory.NativeMemory

com.yahoo.memory.MemoryMappedFile

All Implemented Interfaces:

Memory

public final class MemoryMappedFile
extends NativeMemory

MemoryMappedFile class extends NativeMemory and is used to memory map files (including those > 2GB) off heap.

This class leverages the JVM Cleaner class that replaces Object.finalize() and serves as a back-up if the calling class does not call freeMemory().

Author:

Praveenkumar Venkatesan

Method Summary

Modifier and Type	Method and Description
void	force() Forces any changes made to this content to be written to the storage device containing the mapped file.
void	freeMemory() Because the Memory classes now use the JVM Cleaner, calling freeMemory(), which also calls Cleaner, is optional.
static MemoryMappedFile	getInstance(File file, long position, long len) Factory method for creating a memory mapping a file.
boolean	isLoaded() Tells whether or not the content is resident in physical memory.
void	load() Loads content into physical memory.

Methods inherited from class com.yahoo.memory.NativeMemory

addAndGetInt, addAndGetLong, array, asReadOnlyMemory, byteBuffer, clear, clear, clearBits, compareAndSwapInt, compareAndSwapLong, copy, copy, copy, fill, fill, getAddress, getAndSetInt, getAndSetLong, getBoolean, getBooleanArray, getByte, getByteArray, getCapacity, getChar, getCharArray, getCumulativeOffset, getDouble, getDoubleArray, getFloat, getFloatArray, getInt, getIntArray, getLong, getLongArray, getMemoryRequest, getParent, getShort, getShortArray, hasArray, hasByteBuffer, isAllBitsClear, isAllBitsSet, isAllocated, isAnyBitsClear, isAnyBitsSet, isDirect, isReadOnly, putBoolean, putBooleanArray, putByte, putByteArray, putChar, putCharArray, putDouble, putDoubleArray, putFloat, putFloatArray, putInt, putIntArray, putLong, putLongArray, putShort, putShortArray, setBits, setMemoryRequest, toHexString, wrap

Methods inherited from class java.lang.Object

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

Method Detail

getInstance

public static MemoryMappedFile getInstance(File file,
                                           long position,
                                           long len)
                                    throws Exception

Factory method for creating a memory mapping a file.

Memory maps a file directly in off heap leveraging native map0 method used in FileChannelImpl.c. The owner will have read write access to that address space.

Parameters:

file - File to be mapped

position - Memory map starting from this position in the file

len - Memory map at most len bytes > 0 starting from position

Returns:

A new MemoryMappedFile

Throws:

Exception - file not found or RuntimeException, etc.

load

public void load()

Loads content into physical memory. This method makes a best effort to ensure that, when it returns, this buffer's content is resident in physical memory. Invoking this method may cause some number of page faults and I/O operations to occur.

See Also:

java/nio/MappedByteBuffer.load

isLoaded

public boolean isLoaded()

Tells whether or not the content is resident in physical memory. A return value of true implies that it is highly likely that all of the data in this buffer is resident in physical memory and may therefore be accessed without incurring any virtual-memory page faults or I/O operations. A return value of false does not necessarily imply that the content is not resident in physical memory. The returned value is a hint, rather than a guarantee, because the underlying operating system may have paged out some of the buffer's data by the time that an invocation of this method returns.

Returns:

true if loaded

See Also:

java /nio/MappedByteBuffer.isLoaded

force

public void force()

Forces any changes made to this content to be written to the storage device containing the mapped file.

If the file mapped into this buffer resides on a local storage device then when this method returns it is guaranteed that all changes made to the buffer since it was created, or since this method was last invoked, will have been written to that device.

If the file does not reside on a local device then no such guarantee is made.

If this buffer was not mapped in read/write mode (java.nio.channels.FileChannel.MapMode.READ_WRITE) then invoking this method has no effect.

See Also:

java/ nio/MappedByteBuffer.force

freeMemory

public void freeMemory()

Description copied from interface: Memory

Because the Memory classes now use the JVM Cleaner, calling freeMemory(), which also calls Cleaner, is optional. In any case, calling freeMemory() is only relevant for Memory objects that have actually allocated native memory, which are those that have been allocated using AllocMemory or MemoryMappedFile.

Preemptively calling freeMemory() may reduce the load on the JVM GarbageCollector, but the significance of this will have to be verified in the target system environment. Calling freeMemory() always disables the current instance by setting the capacity to zero.

Specified by:

freeMemory in interface Memory

Overrides:

freeMemory in class NativeMemory