Motivation

Each Paimon task in Flink/Spark job will create TableWriteImpl for Paimon Table, which uses FileStoreWrite to write data to file store. Paimon will create KeyValueFileStoreWrite for key-value file store and initialize memory pool for the writer.

KeyValueFileStoreWrite create MergeTreeWriter for each bucket according to data partition and bucket number in each partition, there are multiple writers in one KeyValueFileStoreWrite. MergeTreeWriter has a writer buffer pool, it writes data to the buffer pool first, and flush the data to file system when the buffer is full. The writers in one task will share the memory pool, and the memory architecture of writers are as follows.

There are some problems in above memory architecture for writer buffer

1. Currently Paimon writer only supports heap which may cause fullgc and unstable writing data when it uses too much memory.

2. Each task allocates and manages its own memory pool, if there are too many tasks in one Executor, it may cause performance issues and even OOM.

To address the above issues, this PIP aims to support writer buffer based on Executor Memory Manager  (such as Flink Managed Memory) for Paimon. Executor in engine manages the total memory usage for tasks to avoid FullGC or task killed due to excessive memory usage.

Public Interfaces

MemorySegmentAllocator

MemorySegmentAllocator is added in Paimon to allocate and release segments for MemorySegmentPool . Paimon sink creates allocator according to job configuration to allocate segments from Executor Memory Manager or physical memory, such as heap.

/**
 * Memory segment allocator will allocate segments from physical memory or memory manager in executor.
 **/
@Public
public interface MemorySegmentAllocator {
    /**
     * Allocates a set of memory segments for memory pool.
     *
     * @param target The list into which to put the allocated memory pages.
     * @param numberOfPages The number of pages to allocate.
     * @throws MemoryAllocationException Thrown, if this memory manager does not
     *     have the requested amount of memory pages any more.
     */
    void allocate(
        Collection<MemorySegment> target,
        int numberOfPages) throws MemoryAllocationException;
    
    /* Release the segments allocated by the allocator if the task is closed. */
    void release();
}

Paimon sink needs to create memory pool based on allocator above, the memory pool size is from existing option write-buffer-size or calculated by sink operator. So MemorySegmentPool will be created according to allocator and memory pool size when the sink operator is initialized, and then the memory pool is added in WriterBuilder to create Paimon writer.

/* Add memory segment pool to writer builder. */
@Public
public interface WriteBuilder {
    WriterBuilder withMemoryPool(MemorySegmentPool pool);
}

MemorySegment

MemorySegment is a final class in Paimon at the moment and task may allocates existing segment instances from memory manager in different engine (Flink/Spark). Paimon needs to define MemorySegment as an interface to facilitate adapting memory segments from different computing engines to Paimon's segments.

@Public
public interface MemorySegment {
    ....
}

Proposed Changes

Architecture

As shown in above architecture, Paimon task creates different memory segment allocators and memory pool for writer according to options when it is initialized. Each writer allocates segments from the allocator and release the memory when the task is finished.

Memory Allocator For Flink

Options And Allocator Definitions

FlinkMemorySegmentAllocator allocates segments from Managed Memory for Paimon writer. This mechanism has the following constraints

1. Option write-buffer-size is ignored and the memory pool will use the memory size calculated from MemoryManager in Flink.

2. Option page-size is ignored and the memory pool will use Flink segment size.

When the task in Flink is closed, the allocator will release all segments it allocated.

/* Add memory allocator related options in flink connector. */
public class FlinkConnectorOptions {
    /* Sink writer allocate segments from managed memory. */
    public static final ConfigOption<Boolean> SINK_USE_MANAGED_MEMORY =
        ConfigOptions.key("sink.use-managed-memory-allocator")
                .booleanType()
                .defaultValue(false)
                .withDescription(
                        "If true, flink sink will use managed memory for merge tree; otherwise, it will create an independent memory allocator.");

    /**
     * Weight of writer buffer in managed memory, Flink will compute the memory size for writer according 
     * to the weight, the actual memory used depends on the running environment.
     */
    public static final ConfigOption<Integer> SINK_MANAGED_WRITER_BUFFER_WEIGHT =
        ConfigOptions.key("sink.managed.writer-buffer-weight")
                .intType()
                .defaultValue(256)
                .withDescription("Weight of writer buffer in managed memory, Flink will compute the memory size " +
                     "for writer according to the weight, the actual memory used depends on the running environment.");
}

/* Memory allocator for flink task which allocates segments from MemoryManager. */
public class FlinkMemorySegmentAllocator implements MemoryAllocator {
    /* The flink task which will be used to allocate and release segments. */
    private final Object owner;
    /* The memory manager from flink task. */
    private final MemoryManager memoryManager;
    
    /* Allocate segments from memory manager. */
    void allocate(
            Collection<MemorySegment> target,
            int numberOfPages) throws MemoryAllocationException {
        ...;
        memoryManager.allocatePages(owner, target, numOfPages);
        ...;
    }
    
    /* Release the segments allocated by the allocator if the task is closed. */
    void release();
}

/* Memory segment wrapper for flink. */
public class FlinkMemorySegment implements MemorySegment {
    org.apache.flink.core.memory.MemorySegment segment;
    
    public FlinkMemorySegment(MemorySegment segment);
}

Use Allocator In SQL

Users can set memory weight in SQL for Flink Managed Memory, then Flink sink operator will get the memory pool size  and create allocator for Paimon writer.

INSERT INTO paimon_table /*+ OPTIONS('sink.use-managed-memory-allocator'='true', 'sink.managed.writer-buffer-weight'='256') */
	SELECT * FROM ....;

Use Allocator In Flink

The process of using Managed Memory Allocator in Flink is divided into two parts: compilation and runtime.

In compilation, Paimon creates sink operator in FlinkSink. Paimon can get Transformation from the sink operator, declare managed memory by Transformation.declareManagedMemoryUseCaseAtOperatorScope(OPERATOR, weight of buffer)  according to options sink.use-menaged-memory-allocator and sink.managed.writer-buffer-weight .

In runtime, Paimon sink operator calculates writer buffer size based on StreamConfig and MemoryManager in AbstractStoreWriteOperator.setup , then creates FlinkMemorySegmentAllocator and MemorySegmentPool . Finally Paimon writer writes data to the memory pool and flush data to file system.

Default Memory Segment And Allocator

If users do not config allocator, Paimon writer will create default allocator to allocate DefaultMemorySegment  from JVM heap as it is doing now.

/* Default allocator for paimon which allocates segments from heap. */
public class HeapMemorySegmentAllocator implements MemorySegmentAllocator {
}

/* Default memory segment for paimon. */
public class DefaultMemorySegment implements MemorySegment {
}