BlockInfoManager

BlockInfoManager manages memory blocks (aka memory pages). It controls concurrent access to memory blocks by read and write locks (for existing and new ones).

Note	Locks are the mechanism to control concurrent access to data and prevent destructive interaction between operations that use the same resource.

Table 1. `BlockInfoManager` Internal Registries and Counters
Name	Description
`infos`	Tracks BlockInfo per block (as BlockId).
`readLocksByTask`	Tracks tasks (by `TaskAttemptId`) and the blocks they locked for reading (as BlockId).
`writeLocksByTask`	Tracks tasks (by `TaskAttemptId`) and the blocks they locked for writing (as BlockId).

Note	`BlockInfoManager` is a `private[storage]` class that belongs to `org.apache.spark.storage` package.

Tip	Enable `TRACE` logging level for `org.apache.spark.storage.BlockInfoManager` logger to see what happens inside. Add the following line to `conf/log4j.properties`: `log4j.logger.org.apache.spark.storage.BlockInfoManager=TRACE` Refer to Logging.

`registerTask` Method

Caution

FIXME

Downgrading Exclusive Write Lock For Block to Shared Read Lock — `downgradeLock` Method

downgradeLock(blockId: BlockId): Unit

downgradeLock…FIXME

Obtaining Read Lock For Block — `lockForReading` Method

lockForReading(
  blockId: BlockId,
  blocking: Boolean = true): Option[BlockInfo]

lockForReading locks blockId memory block for reading when the block was registered earlier and no writer tasks use it.

When executed, lockForReading prints out the following TRACE message to the logs:

TRACE BlockInfoManager: Task [currentTaskAttemptId] trying to acquire read lock for [blockId]

lockForReading looks up the metadata of the blockId block (in infos registry).

If no metadata could be found, it returns None which means that the block does not exist or was removed (and anybody could acquire a write lock).

Otherwise, when the metadata was found, i.e. registered, it checks so-called writerTask. Only when the block has no writer tasks, a read lock can be acquired. If so, the readerCount of the block metadata is incremented and the block is recorded (in the internal readLocksByTask registry). You should see the following TRACE message in the logs:

TRACE BlockInfoManager: Task [taskAttemptId] acquired read lock for [blockId]

The BlockInfo for the blockId block is returned.

Note	`-1024` is a special `taskAttemptId`, aka NON_TASK_WRITER, used to mark a non-task thread, e.g. by a driver thread or by unit test code.

For blocks with writerTask other than NO_WRITER, when blocking is enabled, lockForReading waits (until another thread invokes the Object.notify method or the Object.notifyAll methods for this object).

With blocking enabled, it will repeat the waiting-for-read-lock sequence until either None or the lock is obtained.

When blocking is disabled and the lock could not be obtained, None is returned immediately.

Note	`lockForReading` is a `synchronized` method, i.e. no two objects can use this and other instance methods.

Obtaining Write Lock for Block — `lockForWriting` Method

lockForWriting(
  blockId: BlockId,
  blocking: Boolean = true): Option[BlockInfo]

When executed, lockForWriting prints out the following TRACE message to the logs:

TRACE Task [currentTaskAttemptId] trying to acquire write lock for [blockId]

It looks up blockId in the internal infos registry. When no BlockInfo could be found, None is returned. Otherwise, blockId block is checked for writerTask to be BlockInfo.NO_WRITER with no readers (i.e. readerCount is 0) and only then the lock is returned.

When the write lock can be returned, BlockInfo.writerTask is set to currentTaskAttemptId and a new binding is added to the internal writeLocksByTask registry. You should see the following TRACE message in the logs:

TRACE Task [currentTaskAttemptId] acquired write lock for [blockId]

If, for some reason, blockId has a writer or the number of readers is positive (i.e. BlockInfo.readerCount is greater than 0), the method will wait (based on the input blocking flag) and attempt the write lock acquisition process until it finishes with a write lock.

Note	(deadlock possible) The method is `synchronized` and can block, i.e. `wait` that causes the current thread to wait until another thread invokes `Object.notify` or `Object.notifyAll` methods for this object.

lockForWriting return None for no blockId in the internal infos registry or when blocking flag is disabled and the write lock could not be acquired.

Obtaining Write Lock for New Block — `lockNewBlockForWriting` Method

lockNewBlockForWriting(
  blockId: BlockId,
  newBlockInfo: BlockInfo): Boolean

lockNewBlockForWriting obtains a write lock for blockId but only when the method could register the block.

Note	`lockNewBlockForWriting` is similar to lockForWriting method but for brand new blocks.

When executed, lockNewBlockForWriting prints out the following TRACE message to the logs:

TRACE Task [currentTaskAttemptId] trying to put [blockId]

If some other thread has already created the block, it finishes returning false. Otherwise, when the block does not exist, newBlockInfo is recorded in the internal infos registry and the block is locked for this client for writing. It then returns true.

Note	`lockNewBlockForWriting` executes itself in `synchronized` block so once the BlockInfoManager is locked the other internal registries should be available only for the currently-executing thread.

`currentTaskAttemptId` Method

Caution

FIXME

Releasing Lock on Block — `unlock` Method

unlock(blockId: BlockId): Unit

unlock releases…FIXME

When executed, unlock starts by printing out the following TRACE message to the logs:

TRACE BlockInfoManager: Task [currentTaskAttemptId] releasing lock for [blockId]

unlock gets the metadata for blockId. It may throw a IllegalStateException if the block was not found.

If the writer task for the block is not NO_WRITER, it becomes so and the blockId block is removed from the internal writeLocksByTask registry for the current task attempt.

Otherwise, if the writer task is indeed NO_WRITER, it is assumed that the blockId block is locked for reading. The readerCount counter is decremented for the blockId block and the read lock removed from the internal readLocksByTask registry for the current task attempt.

In the end, unlock wakes up all the threads waiting for the BlockInfoManager (using Java’s Object.notifyAll).

Caution

FIXME What threads could wait?

Releasing All Locks Obtained by Task — `releaseAllLocksForTask` Method

Caution

FIXME

Removing Memory Block — `removeBlock` Method

Caution

FIXME

`assertBlockIsLockedForWriting` Method

Caution

FIXME

BlockInfoManager