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  1. 19 Oct, 2019 6 commits
    • Ben Gamari's avatar
      226ea0f9
    • Ben Gamari's avatar
      NonMoving: Introduce nonmovingSegmentLogBlockSize acccessor · e5258987
      Ben Gamari authored
      This will allow us to easily move the block size elsewhere.
      e5258987
    • Ben Gamari's avatar
      NonMoving: Don't do major GC if one is already running · 1d7b6fac
      Ben Gamari authored
      Previously we would perform a preparatory moving collection, resulting
      in many things being added to the mark queue. When we finished with this
      we would realize in nonmovingCollect that there was already a collection
      running, in which case we would simply not run the nonmoving collector.
      
      However, it was very easy to end up in a "treadmilling" situation: all
      subsequent GC following the first failed major GC would be scheduled as
      major GCs. Consequently we would continuously feed the concurrent
      collector with more mark queue entries and it would never finish.
      
      This patch aborts the major collection far earlier, meaning that we
      avoid adding nonmoving objects to the mark queue and allowing the
      concurrent collector to finish.
      1d7b6fac
    • Ben Gamari's avatar
      NonMoving: Optimise allocator cache behavior · f7584d48
      Ben Gamari authored
      Previously we would look at the segment header to determine the block
      size despite the fact that we already had the block size at hand.
      f7584d48
    • Ben Gamari's avatar
      NonMoving: Prefetch segment header · 662bffb0
      Ben Gamari authored
      662bffb0
    • Ben Gamari's avatar
      NonMoving: Eliminate integer division in nonmovingBlockCount · 9ef76b05
      Ben Gamari authored
      Perf showed that the this single div was capturing up to 10% of samples
      in nonmovingMark. However, the overwhelming majority of cases is looking
      at small block sizes. These cases we can easily compute explicitly,
      allowing the compiler to turn the division into a significantly more
      efficient division-by-constant.
      
      While the increase in source code looks scary, this all optimises down
      to very nice looking assembler. At this point the only remaining
      hotspots in nonmovingBlockCount are due to memory access.
      9ef76b05
  2. 18 Oct, 2019 3 commits
    • Ben Gamari's avatar
      Don't cleanup until we've stopped the collector · 47d6b1a2
      Ben Gamari authored
      This requires that we break nonmovingExit into two pieces since we need
      to first stop the collector to relinquish any capabilities, then we need
      to shutdown the scheduler, then we need to free the nonmoving
      allocators.
      47d6b1a2
    • Ben Gamari's avatar
      rts: Implement concurrent collection in the nonmoving collector · d7017446
      Ben Gamari authored
      This extends the non-moving collector to allow concurrent collection.
      
      The full design of the collector implemented here is described in detail
      in a technical note
      
          B. Gamari. "A Concurrent Garbage Collector For the Glasgow Haskell
          Compiler" (2018)
      
      This extension involves the introduction of a capability-local
      remembered set, known as the /update remembered set/, which tracks
      objects which may no longer be visible to the collector due to mutation.
      To maintain this remembered set we introduce a write barrier on
      mutations which is enabled while a concurrent mark is underway.
      
      The update remembered set representation is similar to that of the
      nonmoving mark queue, being a chunked array of `MarkEntry`s. Each
      `Capability` maintains a single accumulator chunk, which it flushed
      when it (a) is filled, or (b) when the nonmoving collector enters its
      post-mark synchronization phase.
      
      While the write barrier touches a significant amount of code it is
      conceptually straightforward: the mutator must ensure that the referee
      of any pointer it overwrites is added to the update remembered set.
      However, there are a few details:
      
       * In the case of objects with a dirty flag (e.g. `MVar`s) we can
         exploit the fact that only the *first* mutation requires a write
         barrier.
      
       * Weak references, as usual, complicate things. In particular, we must
         ensure that the referee of a weak object is marked if dereferenced by
         the mutator. For this we (unfortunately) must introduce a read
         barrier, as described in Note [Concurrent read barrier on deRefWeak#]
         (in `NonMovingMark.c`).
      
       * Stable names are also a bit tricky as described in Note [Sweeping
         stable names in the concurrent collector] (`NonMovingSweep.c`).
      
      We take quite some pains to ensure that the high thread count often seen
      in parallel Haskell applications doesn't affect pause times. To this end
      we allow thread stacks to be marked either by the thread itself (when it
      is executed or stack-underflows) or the concurrent mark thread (if the
      thread owning the stack is never scheduled). There is a non-trivial
      handshake to ensure that this happens without racing which is described
      in Note [StgStack dirtiness flags and concurrent marking].
      Co-Authored-by: Ömer Sinan Ağacan's avatarÖmer Sinan Ağacan <omer@well-typed.com>
      d7017446
    • Ömer Sinan Ağacan's avatar
      rts: Non-concurrent mark and sweep · 2309789a
      Ömer Sinan Ağacan authored
      This implements the core heap structure and a serial mark/sweep
      collector which can be used to manage the oldest-generation heap.
      This is the first step towards a concurrent mark-and-sweep collector
      aimed at low-latency applications.
      
      The full design of the collector implemented here is described in detail
      in a technical note
      
          B. Gamari. "A Concurrent Garbage Collector For the Glasgow Haskell
          Compiler" (2018)
      
      The basic heap structure used in this design is heavily inspired by
      
          K. Ueno & A. Ohori. "A fully concurrent garbage collector for
          functional programs on multicore processors." /ACM SIGPLAN Notices/
          Vol. 51. No. 9 (presented by ICFP 2016)
      
      This design is intended to allow both marking and sweeping
      concurrent to execution of a multi-core mutator. Unlike the Ueno design,
      which requires no global synchronization pauses, the collector
      introduced here requires a stop-the-world pause at the beginning and end
      of the mark phase.
      
      To avoid heap fragmentation, the allocator consists of a number of
      fixed-size /sub-allocators/. Each of these sub-allocators allocators into
      its own set of /segments/, themselves allocated from the block
      allocator. Each segment is broken into a set of fixed-size allocation
      blocks (which back allocations) in addition to a bitmap (used to track
      the liveness of blocks) and some additional metadata (used also used
      to track liveness).
      
      This heap structure enables collection via mark-and-sweep, which can be
      performed concurrently via a snapshot-at-the-beginning scheme (although
      concurrent collection is not implemented in this patch).
      
      The mark queue is a fairly straightforward chunked-array structure.
      The representation is a bit more verbose than a typical mark queue to
      accomodate a combination of two features:
      
       * a mark FIFO, which improves the locality of marking, reducing one of
         the major overheads seen in mark/sweep allocators (see [1] for
         details)
      
       * the selector optimization and indirection shortcutting, which
         requires that we track where we found each reference to an object
         in case we need to update the reference at a later point (e.g. when
         we find that it is an indirection). See Note [Origin references in
         the nonmoving collector] (in `NonMovingMark.h`) for details.
      
      Beyond this the mark/sweep is fairly run-of-the-mill.
      
      [1] R. Garner, S.M. Blackburn, D. Frampton. "Effective Prefetch for
          Mark-Sweep Garbage Collection." ISMM 2007.
      Co-Authored-By: Ben Gamari's avatarBen Gamari <ben@well-typed.com>
      2309789a