Commit 278f7f22 authored by Ben Gamari's avatar Ben Gamari 🐢

Unconditionally flush update remembered set during minor GC

Flush the update remembered set. The goal here is to flush periodically to
ensure that we don't end up with a thread who marks their stack on their
local update remembered set and doesn't flush until the nonmoving sync
period as this would result in a large fraction of the heap being marked
during the sync pause.
parent 0ab6dd70
......@@ -730,6 +730,14 @@ GarbageCollect (uint32_t collect_gen,
} // for all generations
// Flush the update remembered set. See Note [Eager update remembered set
// flushing] in NonMovingMark.c
if (RtsFlags.GcFlags.useNonmoving) {
// Mark and sweep the oldest generation.
// N.B. This can only happen after we've moved
// oldest_gen->scavenged_large_objects back to oldest_gen->large_objects.
......@@ -131,6 +131,49 @@ StgIndStatic *debug_caf_list_snapshot = (StgIndStatic*)END_OF_CAF_LIST;
* The mark phase is responsible for freeing update remembered set block
* allocations.
* Note that we eagerly flush update remembered sets during minor GCs as
* described in Note [Eager update remembered set flushing].
* Note [Eager update remembered set flushing]
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* We eagerly flush update remembered sets during minor GCs to avoid scenarios
* like the following which could result in long sync pauses:
* 1. We start a major GC, all thread stacks are added to the mark queue.
* 2. The concurrent mark thread starts.
* 3. The mutator is allowed to resume. One mutator thread T is scheduled and marks its
* stack to its local update remembered set.
* 4. The mark thread eventually encounters the mutator thread's stack but
* sees that it has already been marked; skips it.
* 5. Thread T continues running but does not push enough to its update
* remembered set to require a flush.
* 6. Eventually the mark thread finished marking and requests a final sync.
* 7. The thread T flushes its update remembered set.
* 8. We find that a large fraction of the heap (namely the subset that is
* only reachable from the thread T's stack) needs to be marked, incurring
* a large sync pause
* We avoid this by periodically (during minor GC) forcing a flush of the
* update remembered set.
* A better (but more complex) approach that would be worthwhile trying in the
* future would be to rather do the following:
* 1. Concurrent mark phase is on-going
* 2. Mark thread runs out of things to mark
* 3. Mark thread sends a signal to capabilities requesting that they send
* their update remembered sets without suspending their execution
* 4. The mark thread marks everything it was sent; runs out of things to mark
* 5. Mark thread initiates a sync
* 6. Capabilities send their final update remembered sets and suspend execution
* 7. Mark thread marks everything is was sent
* 8. Mark thead allows capabilities to resume.
* However, this is obviously a fair amount of complexity and so far the
* periodic eager flushing approach has been sufficient.
* Note [Concurrent read barrier on deRefWeak#]
* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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