[SPARK-12817] Simplify CacheManager code and remove unused BlockManager methods #10748
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/cc @andrewor14 for review. |
| values: Array[Any], | ||
| level: StorageLevel, | ||
| returnValues: Boolean): PutResult = { | ||
| putIterator(blockId, values.toIterator, level, returnValues) |
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The fact that these putArray() methods just turned around and called putIterator() suggests to me that this isn't a terribly useful method to expose.
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Jenkins, retest this please. |
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Test build #49370 has finished for PR 10748 at commit
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Hmm, it looks like two tests are failing: and My hunch is that the spill-to-disk fallback path works slightly differently when called in I'll take a look to see if I can spot what's going on. I think that the control flow of the disk fallback path could be a bit better documented, so I'll see about adding some comments to the existing code. |
| /* | ||
| * This RDD is to be cached in memory. In this case we cannot pass the computed values | ||
| * to the BlockManager as an iterator and expect to read it back later. This is because | ||
| * we may end up dropping a partition from memory store before getting it back. |
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This problem can be addressed via my other patch for locking in the block manager: we can have a put() implicitly retain a lock to the block which was just stored.
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Going to close this for now to declutter the queue but will re-open as soon as my other patch is merged. |
asfgit
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Feb 26, 2016
## Motivation As a pre-requisite to off-heap caching of blocks, we need a mechanism to prevent pages / blocks from being evicted while they are being read. With on-heap objects, evicting a block while it is being read merely leads to memory-accounting problems (because we assume that an evicted block is a candidate for garbage-collection, which will not be true during a read), but with off-heap memory this will lead to either data corruption or segmentation faults. ## Changes ### BlockInfoManager and reader/writer locks This patch adds block-level read/write locks to the BlockManager. It introduces a new `BlockInfoManager` component, which is contained within the `BlockManager`, holds the `BlockInfo` objects that the `BlockManager` uses for tracking block metadata, and exposes APIs for locking blocks in either shared read or exclusive write modes. `BlockManager`'s `get*()` and `put*()` methods now implicitly acquire the necessary locks. After a `get()` call successfully retrieves a block, that block is locked in a shared read mode. A `put()` call will block until it acquires an exclusive write lock. If the write succeeds, the write lock will be downgraded to a shared read lock before returning to the caller. This `put()` locking behavior allows us store a block and then immediately turn around and read it without having to worry about it having been evicted between the write and the read, which will allow us to significantly simplify `CacheManager` in the future (see #10748). See `BlockInfoManagerSuite`'s test cases for a more detailed specification of the locking semantics. ### Auto-release of locks at the end of tasks Our locking APIs support explicit release of locks (by calling `unlock()`), but it's not always possible to guarantee that locks will be released prior to the end of the task. One reason for this is our iterator interface: since our iterators don't support an explicit `close()` operator to signal that no more records will be consumed, operations like `take()` or `limit()` don't have a good means to release locks on their input iterators' blocks. Another example is broadcast variables, whose block locks can only be released at the end of the task. To address this, `BlockInfoManager` uses a pair of maps to track the set of locks acquired by each task. Lock acquisitions automatically record the current task attempt id by obtaining it from `TaskContext`. When a task finishes, code in `Executor` calls `BlockInfoManager.unlockAllLocksForTask(taskAttemptId)` to free locks. ### Locking and the MemoryStore In order to prevent in-memory blocks from being evicted while they are being read, the `MemoryStore`'s `evictBlocksToFreeSpace()` method acquires write locks on blocks which it is considering as candidates for eviction. These lock acquisitions are non-blocking, so a block which is being read will not be evicted. By holding write locks until the eviction is performed or skipped (in case evicting the blocks would not free enough memory), we avoid a race where a new reader starts to read a block after the block has been marked as an eviction candidate but before it has been removed. ### Locking and remote block transfer This patch makes small changes to to block transfer and network layer code so that locks acquired by the BlockTransferService are released as soon as block transfer messages are consumed and released by Netty. This builds on top of #11193, a bug fix related to freeing of network layer ManagedBuffers. ## FAQ - **Why not use Java's built-in [`ReadWriteLock`](https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/locks/ReadWriteLock.html)?** Our locks operate on a per-task rather than per-thread level. Under certain circumstances a task may consist of multiple threads, so using `ReadWriteLock` would mean that we might call `unlock()` from a thread which didn't hold the lock in question, an operation which has undefined semantics. If we could rely on Java 8 classes, we might be able to use [`StampedLock`](https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/locks/StampedLock.html) to work around this issue. - **Why not detect "leaked" locks in tests?**: See above notes about `take()` and `limit`. Author: Josh Rosen <joshrosen@databricks.com> Closes #10705 from JoshRosen/pin-pages.
asfgit
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CacheManager directly calls MemoryStore.unrollSafely() and has its own logic for handling graceful fallback to disk when cached data does not fit in memory. However, this logic also exists inside of the MemoryStore itself, so this appears to be unnecessary duplication. Thanks to the addition of block-level read/write locks in #10705, we can refactor the code to remove the CacheManager and replace it with an atomic `BlockManager.getOrElseUpdate()` method. This pull request replaces / subsumes #10748. /cc andrewor14 and nongli for review. Note that this changes the locking semantics of a couple of internal BlockManager methods (`doPut()` and `lockNewBlockForWriting`), so please pay attention to the Scaladoc changes and new test cases for those methods. Author: Josh Rosen <joshrosen@databricks.com> Closes #11436 from JoshRosen/remove-cachemanager.
roygao94
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Mar 22, 2016
CacheManager directly calls MemoryStore.unrollSafely() and has its own logic for handling graceful fallback to disk when cached data does not fit in memory. However, this logic also exists inside of the MemoryStore itself, so this appears to be unnecessary duplication. Thanks to the addition of block-level read/write locks in apache#10705, we can refactor the code to remove the CacheManager and replace it with an atomic `BlockManager.getOrElseUpdate()` method. This pull request replaces / subsumes apache#10748. /cc andrewor14 and nongli for review. Note that this changes the locking semantics of a couple of internal BlockManager methods (`doPut()` and `lockNewBlockForWriting`), so please pay attention to the Scaladoc changes and new test cases for those methods. Author: Josh Rosen <joshrosen@databricks.com> Closes apache#11436 from JoshRosen/remove-cachemanager.
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CacheManager directly calls MemoryStore.unrollSafely() and has its own logic for handling graceful fallback to disk when cached data does not fit in memory. However, this logic also exists inside of the MemoryStore itself, so this appears to be unnecessary duplication.
We can remove this duplication and delete a significant amount of BlockManager code which existed only to support this CacheManager code.