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authorBen Gamari <ben@smart-cactus.org>2019-05-17 11:42:11 -0400
committerBen Gamari <ben@smart-cactus.org>2019-10-22 12:20:46 -0400
commit116e4646f90178cd9f5c7d713074c1a049433134 (patch)
tree337e28a3312fdbde6150592cc5b5dfe5e7e9a5a7 /rts/sm
parent5b130b3d9d69f23d868765e97fb20d3afa6a6732 (diff)
downloadhaskell-116e4646f90178cd9f5c7d713074c1a049433134.tar.gz
NonMoving: Add summarizing Notewip/gc/docs
Diffstat (limited to 'rts/sm')
-rw-r--r--rts/sm/NonMoving.c177
-rw-r--r--rts/sm/NonMovingMark.c10
2 files changed, 186 insertions, 1 deletions
diff --git a/rts/sm/NonMoving.c b/rts/sm/NonMoving.c
index 6dc1010d43..b47dca1595 100644
--- a/rts/sm/NonMoving.c
+++ b/rts/sm/NonMoving.c
@@ -49,8 +49,183 @@ Mutex concurrent_coll_finished_lock;
/*
* Note [Non-moving garbage collector]
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * The sources rts/NonMoving*.c implement GHC's non-moving garbage collector
+ * for the oldest generation. In contrast to the throughput-oriented moving
+ * collector, the non-moving collector is designed to achieve low GC latencies
+ * on large heaps. It accomplishes low-latencies by way of a concurrent
+ * mark-and-sweep collection strategy on a specially-designed heap structure.
+ * While the design is described in detail in the design document found in
+ * docs/storage/nonmoving-gc, we briefly summarize the structure here.
+ *
+ *
+ * === Heap Structure ===
+ *
+ * The nonmoving heap (embodied by struct NonmovingHeap) consists of a family
+ * of allocators, each serving a range of allocation sizes. Each allocator
+ * consists of a set of *segments*, each of which contain fixed-size *blocks*
+ * (not to be confused with "blocks" provided by GHC's block allocator; this is
+ * admittedly an unfortunate overlap in terminology). These blocks are the
+ * backing store for the allocator. In addition to blocks, the segment also
+ * contains some header information (see struct NonmovingSegment in
+ * NonMoving.h). This header contains a *bitmap* encoding one byte per block
+ * (used by the collector to record liveness), as well as the index of the next
+ * unallocated block (and a *snapshot* of this field which will be described in
+ * the next section).
+ *
+ * Each allocator maintains three sets of segments:
+ *
+ * - A *current* segment for each capability; this is the segment which that
+ * capability will allocate into.
+ *
+ * - A pool of *active* segments, each of which containing at least one
+ * unallocated block. The allocate will take a segment from this pool when
+ * it fills its *current* segment.
+ *
+ * - A set of *filled* segments, which contain no unallocated blocks and will
+ * be collected during the next major GC cycle
+ *
+ * Storage for segments is allocated using the block allocator using an aligned
+ * group of NONMOVING_SEGMENT_BLOCKS blocks. This makes the task of locating
+ * the segment header for a clone a simple matter of bit-masking (as
+ * implemented by nonmovingGetSegment).
+ *
+ * In addition, to relieve pressure on the block allocator we keep a small pool
+ * of free blocks around (nonmovingHeap.free) which can be pushed/popped
+ * to/from in a lock-free manner.
+ *
+ *
+ * === Allocation ===
+ *
+ * The allocator (as implemented by nonmovingAllocate) starts by identifying
+ * which allocator the request should be made against. It then allocates into
+ * its local current segment and bumps the next_free pointer to point to the
+ * next unallocated block (as indicated by the bitmap). If it finds the current
+ * segment is now full it moves it to the filled list and looks for a new
+ * segment to make current from a few sources:
+ *
+ * 1. the allocator's active list (see pop_active_segment)
+ * 2. the nonmoving heap's free block pool (see nonmovingPopFreeSegment)
+ * 3. allocate a new segment from the block allocator (see
+ * nonmovingAllocSegment)
+ *
+ * Note that allocation does *not* involve modifying the bitmap. The bitmap is
+ * only modified by the collector.
+ *
+ *
+ * === Snapshot invariant ===
+ *
+ * To safely collect in a concurrent setting, the collector relies on the
+ * notion of a *snapshot*. The snapshot is a hypothetical frozen state of the
+ * heap topology taken at the beginning of the major collection cycle.
+ * With this definition we require the following property of the mark phase,
+ * which we call the *snapshot invariant*,
+ *
+ * All objects that were reachable at the time the snapshot was collected
+ * must have their mark bits set at the end of the mark phase.
+ *
+ * As the mutator might change the topology of the heap while we are marking
+ * this property requires some cooperation from the mutator to maintain.
+ * Specifically, we rely on a write barrier as described in Note [Update
+ * remembered set].
+ *
+ * To determine which objects were existent when the snapshot was taken we
+ * record a snapshot of each segments next_free pointer at the beginning of
+ * collection.
+ *
+ *
+ * === Collection ===
+ *
+ * Collection happens in a few phases some of which occur during a
+ * stop-the-world period (marked with [STW]) and others which can occur
+ * concurrently with mutation and minor collection (marked with [CONC]):
+ *
+ * 1. [STW] Preparatory GC: Here we do a standard minor collection of the
+ * younger generations (which may evacuate things to the nonmoving heap).
+ * References from younger generations into the nonmoving heap are recorded
+ * in the mark queue (see Note [Aging under the non-moving collector] in
+ * this file).
+ *
+ * 2. [STW] Snapshot update: Here we update the segment snapshot metadata
+ * (see nonmovingPrepareMark) and move the filled segments to
+ * nonmovingHeap.sweep_list, which is the set of segments which we will
+ * sweep this GC cycle.
+ *
+ * 3. [STW] Root collection: Here we walk over a variety of root sources
+ * and add them to the mark queue (see nonmovingCollect).
+ *
+ * 4. [CONC] Concurrent marking: Here we do the majority of marking concurrently
+ * with mutator execution (but with the write barrier enabled; see
+ * Note [Update remembered set]).
+ *
+ * 5. [STW] Final sync: Here we interrupt the mutators, ask them to
+ * flush their final update remembered sets, and mark any new references
+ * we find.
+ *
+ * 6. [CONC] Sweep: Here we walk over the nonmoving segments on sweep_list
+ * and place them back on either the active, current, or filled list,
+ * depending upon how much live data they contain.
+ *
+ *
+ * === Marking ===
+ *
+ * Ignoring large and static objects, marking a closure is fairly
+ * straightforward (implemented in NonMovingMark.c:mark_closure):
+ *
+ * 1. Check whether the closure is in the non-moving generation; if not then
+ * we ignore it.
+ * 2. Find the segment containing the closure's block.
+ * 3. Check whether the closure's block is above $seg->next_free_snap; if so
+ * then the block was not allocated when we took the snapshot and therefore
+ * we don't need to mark it.
+ * 4. Check whether the block's bitmap bits is equal to nonmovingMarkEpoch. If
+ * so then we can stop as we have already marked it.
+ * 5. Push the closure's pointers to the mark queue.
+ * 6. Set the blocks bitmap bits to nonmovingMarkEpoch.
+ *
+ * Note that the ordering of (5) and (6) is rather important, as described in
+ * Note [StgStack dirtiness flags and concurrent marking].
+ *
+ *
+ * === Other references ===
+ *
+ * Apart from the design document in docs/storage/nonmoving-gc and the Ueno
+ * 2016 paper (TODO citation) from which it drew inspiration, there are a
+ * variety of other relevant Notes scattered throughout the tree:
+ *
+ * - Note [Concurrent non-moving collection] (NonMoving.c) describes
+ * concurrency control of the nonmoving collector
+ *
+ * - Note [Live data accounting in nonmoving collector] (NonMoving.c)
+ * describes how we track the quantity of live data in the nonmoving
+ * generation.
+ *
+ * - Note [Aging under the non-moving collector] (NonMoving.c) describes how
+ * we accomodate aging
+ *
+ * - Note [Large objects in the non-moving collector] (NonMovingMark.c)
+ * describes how we track large objects.
+ *
+ * - Note [Update remembered set] (NonMovingMark.c) describes the function and
+ * implementation of the update remembered set used to realize the concurrent
+ * write barrier.
+ *
+ * - Note [Concurrent read barrier on deRefWeak#] (NonMovingMark.c) describes
+ * the read barrier on Weak# objects.
+ *
+ * - Note [Unintentional marking in resurrectThreads] (NonMovingMark.c) describes
+ * a tricky interaction between the update remembered set flush and weak
+ * finalization.
+ *
+ * - Note [Origin references in the nonmoving collector] (NonMovingMark.h)
+ * describes how we implement indirection short-cutting and the selector
+ * optimisation.
+ *
+ * - Note [StgStack dirtiness flags and concurrent marking] (TSO.h) describes
+ * the protocol for concurrent marking of stacks.
+ *
+ * - Note [Static objects under the nonmoving collector] (Storage.c) describes
+ * treatment of static objects.
*
- * TODO
*
* Note [Concurrent non-moving collection]
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
diff --git a/rts/sm/NonMovingMark.c b/rts/sm/NonMovingMark.c
index bb5d72bbf1..8dca11a417 100644
--- a/rts/sm/NonMovingMark.c
+++ b/rts/sm/NonMovingMark.c
@@ -119,6 +119,9 @@ StgIndStatic *debug_caf_list_snapshot = (StgIndStatic*)END_OF_CAF_LIST;
*
* - In the code generated by the STG code generator for pointer array writes
*
+ * - In thunk updates (e.g. updateWithIndirection) to ensure that the free
+ * variables of the original thunk remain reachable.
+ *
* There is also a read barrier to handle weak references, as described in
* Note [Concurrent read barrier on deRefWeak#].
*
@@ -559,6 +562,13 @@ inline void updateRemembSetPushThunk(Capability *cap, StgThunk *thunk)
updateRemembSetPushThunkEager(cap, (StgThunkInfoTable *) info, thunk);
}
+/* Push the free variables of a thunk to the update remembered set.
+ * This is called by the thunk update code (e.g. updateWithIndirection) before
+ * we update the indirectee to ensure that the thunk's free variables remain
+ * visible to the concurrent collector.
+ *
+ * See Note [Update rememembered set].
+ */
void updateRemembSetPushThunkEager(Capability *cap,
const StgThunkInfoTable *info,
StgThunk *thunk)