com.persistit

Class BufferPool

java.lang.Object

com.persistit.BufferPool

public class BufferPool
extends Object

A pool of Buffer objects, maintained on various lists that permit rapid lookup and replacement of pages images within Buffers.

Version:

2.1

Field Summary

Fields

Modifier and Type	Field and Description
static int	MAXIMUM_POOL_COUNT Maximum number of buffers this pool may have
static int	MINIMUM_POOL_COUNT Minimum number of buffers this pool may have

Method Summary

Modifier and Type	Method and Description
Buffer	getBufferCopy(int index)
Buffer	getBufferCopy(Volume vol, long page) Returns a copy of Buffer.
int	getBufferCount()
int	getBufferSize()
long	getEarliestDirtyTimestamp()
long	getForcedCheckpointWriteCounter() This counter is incremented each time a application modifies a buffer that is (a) dirty, and (b) required to be written as part of a checkpoint.
long	getForcedWriteCounter() This counter is incremented ach time the eviction algorithm selects a dirty buffer to evict.
long	getHitCounter()
double	getHitRatio() Get the "hit ratio" - the number of hits divided by the number of overall gets.
long	getMissCounter()
long	getNewCounter()
void	resetCounters() Resets the get and hit counters to zero.
String	toString()

Methods inherited from class java.lang.Object

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

Field Detail

MINIMUM_POOL_COUNT

public static final int MINIMUM_POOL_COUNT

Minimum number of buffers this pool may have

See Also:

Constant Field Values

MAXIMUM_POOL_COUNT

public static final int MAXIMUM_POOL_COUNT

Maximum number of buffers this pool may have

See Also:

Constant Field Values

Method Detail

getBufferSize

public int getBufferSize()

Returns:

Size (in bytes) of each Buffer managed by this pool.

getBufferCount

public int getBufferCount()

Returns:

The count of Buffers managed by this pool.

getMissCounter

public long getMissCounter()

Returns:

The count of lookup operations for pages images in this pool that required a physical read operation. This number, in comparison with the hit counter, indicates how effective the cache is in reducing disk I/O.

getHitCounter

public long getHitCounter()

Returns:

The count of lookup operations for pages images in this pool for which the page image was already found in this BufferPool. This number, in comparison with the get counter, indicates how effective the cache is in reducing disk I/O.

getNewCounter

public long getNewCounter()

Returns:

The count of buffers newly created in this pool. Each time a new page is added to a Volume, this counter is incremented.

getForcedWriteCounter

public long getForcedWriteCounter()

This counter is incremented ach time the eviction algorithm selects a dirty buffer to evict. Normally dirty pages are written by the background PAGE_WRITER thread, and therefore an abnormally large forcedWrite count indicates the PAGE_WRITER thread is falling behind.

Returns:

The count of buffers written to disk when evicted.

getForcedCheckpointWriteCounter

public long getForcedCheckpointWriteCounter()

This counter is incremented each time a application modifies a buffer that is (a) dirty, and (b) required to be written as part of a checkpoint. An abnormally large count indicates that the PAGE_WRITER thread is falling behind.

Returns:

The count of buffers written to disk due to a checkpoint.

resetCounters

public void resetCounters()

Resets the get and hit counters to zero.

getHitRatio

public double getHitRatio()

Get the "hit ratio" - the number of hits divided by the number of overall gets. A value close to 1.0 indicates that most attempts to find data in the BufferPool are successful - i.e., that the cache is effectively reducing the need for disk read operations.

Returns:

The ratio

getBufferCopy

public Buffer getBufferCopy(Volume vol,
                            long page)
                     throws InvalidPageAddressException,
                            InvalidPageStructureException,
                            VolumeClosedException,
                            InUseException,
                            PersistitIOException,
                            PersistitInterruptedException

Returns a copy of Buffer. The returned buffer is newly created, is not a member of the buffer pool, and is not claimed. There is no guarantee that the content of this copy is internally consistent because another thread may be modifying the buffer while the copy is being made. The returned Buffer should be used only for display and diagnostic purposes.

Parameters:

vol -

page -

Returns:

Copy of the Buffer

Throws:

InvalidPageAddressException

InvalidPageStructureException

VolumeClosedException

PersistitInterruptedException

RetryException

IOException

InUseException

PersistitIOException

getBufferCopy

public Buffer getBufferCopy(int index)
                     throws IllegalArgumentException

Throws:

IllegalArgumentException

getEarliestDirtyTimestamp

public long getEarliestDirtyTimestamp()

Returns:

Earliest timestamp of any dirty page in this BufferPool.

toString

public String toString()

Overrides:

toString in class Object