Package org.h2.util

Class MemoryEstimator

java.lang.Object

org.h2.util.MemoryEstimator

public final class MemoryEstimator
extends java.lang.Object

Class MemoryEstimator. Calculation of the amount of memory occupied by keys, values and pages of the MVTable may become expensive operation for complex data types like Row. On the other hand, result of the calculation is used by page cache to limit it's size and determine when eviction is needed. Another usage is to trigger auto commit, based on amount of unsaved changes. In both cases reasonable (lets say ~30%) approximation would be good enough and will do the job. This class replaces exact calculation with an estimate based on a sliding window average of last 256 values. If estimation gets close to the exact value, then next N calculations are skipped and replaced with the estimate, where N depends on the estimation error.

Method Summary

Modifier and Type	Method	Description
static <T> int	estimateMemory(java.util.concurrent.atomic.AtomicLong stats, DataType<T> dataType, T data)	Estimates memory size of the data based on previous values.
static <T> int	estimateMemory(java.util.concurrent.atomic.AtomicLong stats, DataType<T> dataType, T[] storage, int count)	Estimates memory size of the data set based on previous values.
static int	samplingPct(java.util.concurrent.atomic.AtomicLong stats)	Calculates percentage of how many times actual calculation happened (vs.

Methods inherited from class java.lang.Object

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

Method Detail

estimateMemory

public static <T> int estimateMemory(java.util.concurrent.atomic.AtomicLong stats,
                                     DataType<T> dataType,
                                     T data)

Estimates memory size of the data based on previous values.

Type Parameters:

T - type of the data

Parameters:

stats - AtomicLong holding statistical data about the estimated sequence

dataType - used for calculation of the next sequence value, if necessary

data - which size is to be calculated as the next sequence value, if necessary

Returns:

next estimated or calculated value of the sequence

estimateMemory

public static <T> int estimateMemory(java.util.concurrent.atomic.AtomicLong stats,
                                     DataType<T> dataType,
                                     T[] storage,
                                     int count)

Estimates memory size of the data set based on previous values.

Type Parameters:

T - type of the data in the storage

Parameters:

stats - AtomicLong holding statistical data about the estimated sequence

dataType - used for calculation of the next sequence value, if necessary

storage - of the data set, which size is to be calculated

count - number of data items in the storage

Returns:

next estimated or calculated size of the storage

samplingPct

public static int samplingPct(java.util.concurrent.atomic.AtomicLong stats)

Calculates percentage of how many times actual calculation happened (vs. estimation)

Parameters:

stats - AtomicLong holding statistical data about the estimated sequence

Returns:

sampling percentage in range 0 - 100