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= [[UnifiedMemoryManager]] UnifiedMemoryManager

UnifiedMemoryManager is the default[MemoryManager] (based on[spark.memory.useLegacyMode] configuration property).

== [[creating-instance]] Creating Instance

UnifiedMemoryManager takes the following to be created:

  • [[conf]][SparkConf]
  • [[maxHeapMemory]] Maximum heap memory
  • [[onHeapStorageRegionSize]] Size of the on-heap storage region
  • [[numCores]] Number of CPU cores

UnifiedMemoryManager requires that:

  • Sum of the pool size of the[on-heap ExecutionMemoryPool] and[on-heap StorageMemoryPool] is exactly the <>

  • Sum of the pool size of the[off-heap ExecutionMemoryPool] and[off-heap StorageMemoryPool] is exactly the maximum off-heap memory (based on[spark.memory.offHeap.size] configuration property)

== [[apply]] Creating UnifiedMemoryManager

[source, scala]

apply( conf: SparkConf, numCores: Int): UnifiedMemoryManager

apply computes the <> (using the input[SparkConf]).

apply computes the size of the on-heap storage region which is a fraction of the maximum heap memory based on[spark.memory.storageFraction] configuration property (default: 0.5).

In the end, apply creates a <> (with the given and computed values).

apply is used when SparkEnv utility is used to[create a SparkEnv] (for the driver and executors).

== [[getMaxMemory]] Calculating Maximum Heap Memory

[source, scala]

getMaxMemory( conf: SparkConf): Long

getMaxMemory calculates the maximum memory to use for execution and storage.

[source, scala]

// local mode with --conf spark.driver.memory=2g scala> sc.getConf.getSizeAsBytes("spark.driver.memory") res0: Long = 2147483648

scala> val systemMemory = Runtime.getRuntime.maxMemory

// fixed amount of memory for non-storage, non-execution purposes val reservedMemory = 300 * 1024 * 1024

// minimum system memory required val minSystemMemory = (reservedMemory * 1.5).ceil.toLong

val usableMemory = systemMemory - reservedMemory

val memoryFraction = sc.getConf.getDouble("spark.memory.fraction", 0.6) scala> val maxMemory = (usableMemory * memoryFraction).toLong maxMemory: Long = 956615884

import scala> JavaUtils.byteStringAsMb(maxMemory + "b") res1: Long = 912

getMaxMemory reads <> and decrements it by <> (for non-storage and non-execution purposes).

getMaxMemory makes sure that the following requirements are met:

  1. System memory is not smaller than about 1,5 of the reserved system memory.
  2.[spark.executor.memory] is not smaller than about 1,5 of the reserved system memory.

Ultimately, getMaxMemory returns <> of the maximum amount of memory for the JVM (minus the reserved system memory).

CAUTION: FIXME omnigraffle it.

== [[acquireExecutionMemory]] acquireExecutionMemory Method

[source, scala]

acquireExecutionMemory( numBytes: Long, taskAttemptId: Long, memoryMode: MemoryMode): Long

NOTE: acquireExecutionMemory is part of the[MemoryManager] contract

acquireExecutionMemory does...FIXME

Internally, acquireExecutionMemory varies per MemoryMode, i.e. ON_HEAP and OFF_HEAP.

.acquireExecutionMemory and MemoryMode [cols="1m,1m,1m",options="header",width="100%"] |=== | | ON_HEAP | OFF_HEAP

| executionPool | onHeapExecutionMemoryPool | offHeapExecutionMemoryPool

| storagePool | onHeapStorageMemoryPool | offHeapStorageMemoryPool

| storageRegionSize | onHeapStorageRegionSize | offHeapStorageMemory

| maxMemory | maxHeapMemory | maxOffHeapMemory |===


== [[acquireStorageMemory]] acquireStorageMemory Method

[source, scala]

acquireStorageMemory( blockId: BlockId, numBytes: Long, memoryMode: MemoryMode): Boolean

NOTE: acquireStorageMemory is part of the[MemoryManager] contract.

acquireStorageMemory has two modes of operation per memoryMode, i.e. MemoryMode.ON_HEAP or MemoryMode.OFF_HEAP, for execution and storage pools, and the maximum amount of memory to use.

CAUTION: FIXME Where are they used?

In MemoryMode.ON_HEAP, onHeapExecutionMemoryPool, onHeapStorageMemoryPool, and <> are used.

In MemoryMode.OFF_HEAP, offHeapExecutionMemoryPool, offHeapStorageMemoryPool, and maxOffHeapMemory are used.

CAUTION: FIXME What is the difference between them?

It makes sure that the requested number of bytes numBytes (for a block to store) fits the available memory. If it is not the case, you should see the following INFO message in the logs and the method returns false.

INFO Will not store [blockId] as the required space ([numBytes] bytes) exceeds our memory limit ([maxMemory] bytes)

If the requested number of bytes numBytes is greater than memoryFree in the storage pool, acquireStorageMemory will attempt to use the free memory from the execution pool.

NOTE: The storage pool can use the free memory from the execution pool.

It will take as much memory as required to fit numBytes from memoryFree in the execution pool (up to the whole free memory in the pool).

Ultimately, acquireStorageMemory requests the storage pool for numBytes for blockId.


acquireStorageMemory is used when MemoryStore[acquires storage memory to putBytes] or[putIteratorAsValues] and[putIteratorAsBytes].

It is also used internally when UnifiedMemoryManager <>.

== [[acquireUnrollMemory]] acquireUnrollMemory Method

NOTE: acquireUnrollMemory is part of the[MemoryManager] contract.

acquireUnrollMemory simply forwards all the calls to <>.

== [[maxOnHeapStorageMemory]] maxOnHeapStorageMemory Method

[source, scala]

maxOnHeapStorageMemory: Long

NOTE: maxOnHeapStorageMemory is part of the[MemoryManager] contract

maxOnHeapStorageMemory is the difference between maxHeapMemory of the UnifiedMemoryManager and the memory currently in use in onHeapExecutionMemoryPool execution memory pool.

Last update: 2020-11-27
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