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Posted to issues@flink.apache.org by "Stephan Ewen (JIRA)" <ji...@apache.org> on 2015/09/08 21:07:46 UTC
[jira] [Closed] (FLINK-1320) Add an off-heap variant of the managed
memory
[ https://issues.apache.org/jira/browse/FLINK-1320?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Stephan Ewen closed FLINK-1320.
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> Add an off-heap variant of the managed memory
> ---------------------------------------------
>
> Key: FLINK-1320
> URL: https://issues.apache.org/jira/browse/FLINK-1320
> Project: Flink
> Issue Type: Improvement
> Components: Local Runtime
> Reporter: Stephan Ewen
> Assignee: Stephan Ewen
> Priority: Minor
> Fix For: 0.10
>
>
> For (nearly) all memory that Flink accumulates (in the form of sort buffers, hash tables, caching), we use a special way of representing data serialized across a set of memory pages. The big work lies in the way the algorithms are implemented to operate on pages, rather than on objects.
> The core class for the memory is the {{MemorySegment}}, which has all methods to set and get primitives values efficiently. It is a somewhat simpler (and faster) variant of a HeapByteBuffer.
> As such, it should be straightforward to create a version where the memory segment is not backed by a heap byte[], but by memory allocated outside the JVM, in a similar way as the NIO DirectByteBuffers, or the Netty direct buffers do it.
> This may have multiple advantages:
> - We reduce the size of the JVM heap (garbage collected) and the number and size of long living alive objects. For large JVM sizes, this may improve performance quite a bit. Utilmately, we would in many cases reduce JVM size to 1/3 to 1/2 and keep the remaining memory outside the JVM.
> - We save copies when we move memory pages to disk (spilling) or through the network (shuffling / broadcasting / forward piping)
> The changes required to implement this are
> - Add a {{UnmanagedMemorySegment}} that only stores the memory adress as a long, and the segment size. It is initialized from a DirectByteBuffer.
> - Allow the MemoryManager to allocate these MemorySegments, instead of the current ones.
> - Make sure that the startup script pick up the mode and configure the heap size and the max direct memory properly.
> Since the MemorySegment is probably the most performance critical class in Flink, we must take care that we do this right. The following are critical considerations:
> - If we want both solutions (heap and off-heap) to exist side-by-side (configurable), we must make the base MemorySegment abstract and implement two versions (heap and off-heap).
> - To get the best performance, we need to make sure that only one class gets loaded (or at least ever used), to ensure optimal JIT de-virtualization and inlining.
> - We should carefully measure the performance of both variants. From previous micro benchmarks, I remember that individual byte accesses in DirectByteBuffers (off-heap) were slightly slower than on-heap, any larger accesses were equally good or slightly better.
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