• 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
Rts.h 8.93 KB