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Ömer Sinan Ağacan authored Feb 22, 2019 and Ben Gamari committed Feb 22, 2019

Write barriers push old values of updated field, e.g. when we have

    *q = p;

we do

    updateRemembSetPushClosure(*q, q);
    *q = p;

Here the second argument ("origin") is not useful because by the time we
do the update we invalidate "origin" (`q` is no longer origin of old
`*q`).

In general it doesn't make sense to record origins in write barriers so
we remove all origin arguments from write barriers.

Fixes #170.

Ömer Sinan Ağacan authored Feb 19, 2019 and Ben Gamari committed Feb 22, 2019

Fixes #162

Ömer Sinan Ağacan authored Feb 16, 2019 and Ben Gamari committed Feb 22, 2019

These flush calls cause capabilities to flush their UpdRemSets too
early, then set `upd_rem_set_syncd = true`. This causes NOT syncing
UpdRemSets in nonmovingBeginFlush and losing track of some objects.

Fixes #159 (return_mem_to_os)

Ömer Sinan Ağacan authored Feb 15, 2019 and Ben Gamari committed Feb 22, 2019

This fixes a bug that happens when we start exit sequence while
concurrent mark is running. Mark copies oldest gen threads to
nonmoving_weak_ptr_list and nonmoving_old_weak_ptr_list (which also act
as the snapshot for weaks) before releasing capabilities to allow
concurrent minor collections (which may add weaks to oldest_gen weak
list). We need to move these weaks back to oldest gen weak list to be
able to run C finalizers of them in runAllCFinalizers in hs_exit_.

Ömer Sinan Ağacan authored Feb 14, 2019 and Ben Gamari committed Feb 22, 2019

`resurrectThreads` runs a code that runs a write barrier, causing
pushing objects to UpdRemSet. This causes marking some objects in the
next GC cycle. We fix this by resetting UpdRemSets before releasing the
capabilities. See also comments in the code.

Fixes #142

Otherwise we may potentially race with the nonmoving collector. I
believe this was the cause of #155.

Fixes #154.

These two collection strategies are mutually exclusive.
Fixes #132.

Previously we relied on blatantly undefined behavior. Now we at very
least use volatile loads.

This uses the nonmoving collector when compiling the testcases.

Previously we would fail to account for changes by putMVar#'s wakeup
loop.

This may happen if two threads enter the thunk at the same time.

Ömer Sinan Ağacan authored Feb 11, 2019 and Ben Gamari committed Feb 22, 2019

We need to push the SRT to UpdRemSet when updating THUNK_STATICs as when
they become IND_STATIC we lose the SRT which is in the snapshot.

Fixes #145, #124

(cherry picked from commit 6a88e1786f0d5cb287d5247627c6ef1b69acb4aa)

The nonmoving GC doesn't support `+RTS -G1`, which this test insists on.

Ben Gamari authored Feb 05, 2019 and Ben Gamari committed Feb 22, 2019

This introduces a simple census of the non-moving heap (not to be
confused with the heap census used by the heap profiler). This
collects basic heap usage information (number of allocated and free
blocks) which is useful when characterising fragmentation of the
nonmoving heap.

Ben Gamari authored Feb 05, 2019 and Ben Gamari committed Feb 22, 2019

This introduces a few events to mark key points in the nonmoving
garbage collection cycle. These include:

 * `EVENT_CONC_MARK_BEGIN`, denoting the beginning of a round of
   marking. This may happen more than once in a single major collection
   since we the major collector iterates until it hits a fixed point.

 * `EVENT_CONC_MARK_END`, denoting the end of a round of marking.

 * `EVENT_CONC_SYNC_BEGIN`, denoting the beginning of the post-mark
   synchronization phase

 * `EVENT_CONC_UPD_REM_SET_FLUSH`, indicating that a capability has
   flushed its update remembered set.

 * `EVENT_CONC_SYNC_END`, denoting that all mutators have flushed their
   update remembered sets.

 * `EVENT_CONC_SWEEP_BEGIN`, denoting the beginning of the sweep portion
   of the major collection.

 * `EVENT_CONC_SWEEP_END`, denoting the end of the sweep portion of the
   major collection.

Ben Gamari authored Feb 05, 2019 and Ben Gamari committed Feb 22, 2019

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>

Ben Gamari authored Feb 05, 2019 and Ben Gamari committed Feb 22, 2019

This simply runs the compile_and_run tests with `-xn`, enabling the
nonmoving oldest generation.

Ömer Sinan Ağacan authored Feb 05, 2019 and Ben Gamari committed Feb 22, 2019

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 <ben@well-typed.com>

Ben Gamari authored Feb 05, 2019 and Ben Gamari committed Feb 22, 2019

This flag will enable the use of a non-moving oldest generation.