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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 <omer@well-typed.com>
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These will be needed when we implement sweeping in the nonmoving
collector.
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Summary:
Some comments in the `StableName` code still referred to
stable pointer details. Fix that.
Reviewers: bgamari, erikd, simonmar
Reviewed By: simonmar
Subscribers: rwbarton, carter
Differential Revision: https://phabricator.haskell.org/D5152
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Long ago, the stable name table and stable pointer tables were one.
Now, they are separate, and have significantly different
implementations. I believe the time has come to finish the split
that began in #7674.
* Divide `rts/Stable` into `rts/StableName` and `rts/StablePtr`.
* Give each table its own mutex.
* Add FFI functions `hs_lock_stable_ptr_table` and
`hs_unlock_stable_ptr_table` and document them.
These are intended to replace the previously undocumented
`hs_lock_stable_tables` and `hs_lock_stable_tables`,
which are now documented as deprecated synonyms.
* Make `eqStableName#` use pointer equality instead of unnecessarily
comparing stable name table indices.
Reviewers: simonmar, bgamari, erikd
Reviewed By: bgamari
Subscribers: rwbarton, carter
GHC Trac Issues: #15555
Differential Revision: https://phabricator.haskell.org/D5084
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