A Blocstructured Heap Simplifies Parallel GC一个块结构简化了并行GC堆.pptVIP

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A Blocstructured Heap Simplifies Parallel GC一个块结构简化了并行GC堆.ppt

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A Blocstructured Heap Simplifies Parallel GC一个块结构简化了并行GC堆

A Block-structured Heap Simplifies Parallel GC Simon Marlow (Microsoft Research) Roshan James (U. Indiana) Tim Harris (Microsoft Research) Simon Peyton Jones (Microsoft Research) Problem Domain Stop the world and collect using multiple threads. we are not tackling the problem of GC running concurrently with program execution, for now. we are not tackling the problem of independent GC in a program running on multiple CPUs (but plan to later). Our existing GC is quite complex: Multi-generational Arbitrary aging per generation Eager promotion: promote an object early if it is referenced by an old generation. Copying or compaction for the old generation (parallelise copying only for now) Typical allocation rate: 100Mb-1Gb/s Background: copying collection How can we parallelise this? The main problem is finding an effective way to partition the problem, so we can keep N CPUs busy all the time. Static partitioning (eg. partition the heap by address) isn’t good: live data might not be evenly distributed Need synchronisation when pointers cross partition boundaries Work queues So typically, we need dynamic partitioning for GC. The available work (pointers to object to be scanned) is kept on a queue CPUs remove items from the queue, scan the object, and add more roots to the queue. eg. Flood, Detlefs, Shavit, Zhang (2001) Good work partitioning, but need separate work queues: in single-threaded GC, the to-space is the work queue. clever lock-free data structures extra administrative overhead some strategy for overflow (GC can’t use arbitrary extra memory!) A block-structured heap Heap is divided into blocks, e.g. 4k Blocks can be linked together in lists GC sits on top of a block allocator, which manages a free list of blocks. Each block has a “block descriptor”: a small data structure including the link field, which generation it belongs to, … Getting to the block descriptor from an arbitrary address is a pure function (~6 instructions) Block-structured heap Advantages