Class UnifiedMemoryManager

  • public class UnifiedMemoryManager
    extends Object
    Unified Memory Manager - Initial Design Motivation: The Unified Memory Manager, henceforth UMM, will act as a central manager of in-memory matrix (uncompressed and compressed), frame, and tensor blocks within SystemDS control program. So far, operation memory (70%) and buffer pool memory (15%, LazyWriteBuffer) are managed independently, which causes unnecessary evictions. New components like the LineageCache also use and manage statically provisioned memory areas. Ultimately, the UMM aims to eliminate these shortcomings by providing a central, potentially thread-local, memory management. Memory Areas: Initially, the UMM only handles CacheBlock objects (e.g., MatrixBlock, FrameBlock, and TensorBlock), and manages two memory areas: (1) operation memory (pinned cache blocks and reserved memory) and (2) dirty objects (dirty cache blocks that need to be written to local FS before eviction) The UMM is configured with a capacity (absolute size in byte). Relative to this capacity, the operations and buffer pool memory areas each will have a min and max amount of memory they can occupy, meaning that the boundary for the areas can shift dynamically depending on the current load. Most importantly, though, dirty objects must not be counted twice when pinning such an object for an operation. The min/max constraints are not exposed but configured internally. A good starting point are the following constraints (relative to JVM max heap size): ___________________________ | operations | 0% | 70% | (pin requests always accepted) | buffer pool | 15% | 85% | (eviction on demand) Object Lifecycle: The UMM will also need to keep track of the current state of individual cache blocks, for which it will have a few member variables. A queue similar to the current EvictionQueue is used to add/remove entries with LRU as its eviction policy. In general, there are three properties of object status to consider: (1) Non-dirty/dirty: non-dirty objects have a representation on HDFS or can be recomputed from lineage trace (e.g., rand/seq outputs), while dirty objects need to be preserved. (2) FS Persisted: on eviction, dirty objects need to be written to local file system. As long the local file representation exist, dirty objects can simply be dropped. (3) Pinned/unpinned: For operations, objects are pinned into memory to guard against eviction. All pin requests have to be accepted, and once a non-dirty object is released (unpinned) it can be dropped without persisting it to local FS. Example Scenarios for an Operation: (1) Inputs are available in the UMM, enough space left for the output. (2) Some inputs are pre-evicted. Read and pin those in the operational memory. (3) Inputs are available in the UMM, not enough space left for the output. Evict cached objects to reserve worst-case output memory. (4) Some inputs are pre-evicted and not enough space left for the inputs and output. Evict cached objects to make space for the inputs. Evict cached objects to reserve worst-case output memory. Thread-safeness: Initially, the UMM will be used in an instance-based manner. For global visibility and use in parallel for loops, the UMM would need to provide a static, synchronized API, but this constitutes a source of severe contention. In the future, we will consider a design with thread-local UMMs for the individual parfor workers.
    • Constructor Detail

      • UnifiedMemoryManager

        public UnifiedMemoryManager()
    • Method Detail

      • reserveOutputMem

        public static void reserveOutputMem()
      • init

        public static void init()
      • cleanup

        public static void cleanup()
      • printStatus

        public static void printStatus​(String operation)
        Print current status of UMM, including all entries. NOTE: use only for debugging or testing.
        operation - e.g. BEFORE PIN, AFTER UNPIN, AT MAKESPACE
      • setUMMLimit

        public static void setUMMLimit​(long val)
      • getUMMSize

        public static long getUMMSize()
      • getUMMFree

        public static long getUMMFree()
      • makeSpace

        public static int makeSpace​(long reqSpace)
      • getCacheBlockSize

        public static long getCacheBlockSize​(CacheBlock<?> cb)
      • deleteBlock

        public static void deleteBlock​(String fname)
      • deleteAll

        public void deleteAll()
        Removes all cache blocks from all memory areas and deletes all evicted representations (files in local FS). All internally thread pools must be shut down in a graceful manner (e.g., wait for pending deletes).
      • forceEviction

        public static void forceEviction()
                                  throws IOException
        Evicts all buffer pool entries. NOTE: use only for debugging or testing.
        IOException - if IOException occurs