1. 05 Dec, 2019 1 commit
    • Ben Gamari's avatar
      nonmoving: Clear segment bitmaps during sweep · 69001f54
      Ben Gamari authored
      Previously we would clear the bitmaps of segments which we are going to
      sweep during the preparatory pause. However, this is unnecessary: the
      existence of the mark epoch ensures that the sweep will correctly
      identify non-reachable objects, even if we do not clear the bitmap.
      We now defer clearing the bitmap to sweep, which happens concurrently
      with mutation.
  2. 22 Oct, 2019 8 commits
  3. 21 Oct, 2019 3 commits
    • Ben Gamari's avatar
      Don't cleanup until we've stopped the collector · 10373416
      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
    • Ben Gamari's avatar
      rts: Implement concurrent collection in the nonmoving collector · bd8e3ff4
      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
       * 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>
    • Ömer Sinan Ağacan's avatar
      rts: Non-concurrent mark and sweep · 68e0647f
      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
       * 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>