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+# Proposal: Concurrent stack re-scanning
+
+Author(s): Austin Clements, Rick Hudson
+
+Last updated: 2016-10-18
+
+Discussion at https://golang.org/issue/17505.
+
+**Note:** We are not actually proposing this.
+This design was developed before proposal #17503, which is a
+dramatically simpler solution to the problem of stack re-scanning.
+We're posting this design doc for its historical value.
+
+
+## Abstract
+
+Since the release of the concurrent garbage collector in Go 1.5, each
+subsequent release has further reduced stop-the-world (STW) time by
+moving more tasks to the concurrent phase.
+As of Go 1.7, the only non-trivial STW task is stack re-scanning.
+We propose to make stack re-scanning concurrent for Go 1.8, likely
+resulting in sub-millisecond worst-case STW times.
+
+
+## Background
+
+Go's concurrent garbage collector consists of four phases: mark, mark
+termination, sweep, and sweep termination.
+The mark and sweep phases are *concurrent*, meaning that the
+application (the *mutator*) continues to run during these phases,
+while the mark termination and sweep termination phases are
+*stop-the-world* (STW), meaning that the garbage collector pauses the
+mutator for the duration of the phase.
+
+Since Go 1.5, we've been steadily moving tasks from the STW phases to
+the concurrent phases, with a particular focus on tasks that take time
+proportional to something under application control, such as heap size
+or number of goroutines.
+As a result, in Go 1.7, most applications have sub-millisecond STW
+times.
+
+As of Go 1.7, the only remaining application-controllable STW task is
+*stack re-scanning*.
+Because of this one task, applications with large numbers of active
+goroutines can still experience STW times in excess of 10ms.
+
+Stack re-scanning is necessary because stacks are *permagray* in the
+Go garbage collector.
+Specifically, for performance reasons, there are no write barriers for
+writes to pointers in the current stack frame.
+As a result, even though the garbage collector scans all stacks at the
+beginning of the mark phase, it must re-scan all modified stacks with
+the world is stopped to catch any pointers the mutator "hid" on the
+stack.
+
+Unfortunately, this makes STW time proportional to the total amount of
+stack that needs to be rescanned.
+Worse, stack scanning is relatively expensive (~5ms/MB).
+Hence, applications with a large number of active goroutines can
+quickly drive up STW time.
+
+
+## Proposal
+
+We propose to make stack re-scanning concurrent using a *transitive
+mark* write barrier.
+
+In this design, we add a new concurrent phase between mark and mark
+termination called *stack re-scan*.
+This phase starts as soon as the mark phase has marked all objects
+reachable from roots *other than stacks*.
+The phase re-scans stacks that have been modified since their initial
+scan, and enables a special *transitive mark* write barrier.
+
+Re-scanning and the write barrier ensure the following invariant
+during this phase:
+
+> *After a goroutine stack G has been re-scanned, all objects locally
+> reachable to G are black.*
+
+This depends on a goroutine-local notion of reachability, which is the
+set of objects reachable from globals or a given goroutine's stack or
+registers.
+Unlike regular global reachability, this is not stable: as goroutines
+modify heap pointers or communicate, an object that was locally
+unreachable to a given goroutine may become locally reachable.
+However, the concepts are closely related: a globally reachable object
+must be locally reachable by at least one goroutine, and, conversely,
+an object that is not locally reachable by any goroutine is not
+globally reachable.
+
+This invariant ensures that re-scanning a stack *blackens* that stack,
+and that the stack remains black since the goroutine has no way to
+find a white object once its stack has been re-scanned.
+
+Furthermore, once every goroutine stack has been re-scanned, marking
+is complete.
+Every globally reachable object must be locally reachable by some
+goroutine and, once every stack has been re-scanned, every object
+locally reachable by some goroutine is black, so it follows that every
+globally reachable object is black once every stack has been
+re-scanned.
+
+### Transitive mark write barrier
+
+The transitive mark write barrier for an assignment `*dst = src`
+(where `src` is a pointer) ensures that all objects reachable from
+`src` are black *before* writing `src` to `*dst`.
+Writing `src` to `*dst` may make any object reachable from `src`
+(including `src` itself) locally reachable to some goroutine that has
+been re-scanned.
+Hence, to maintain the invariant, we must ensure these objects are all
+black.
+
+To do this, the write barrier greys `src` and then drains the mark
+work queue until there are no grey objects (using the same work queue
+logic that drives the mark phase).
+At this point, it writes `src` to `*dst` and allows the goroutine to
+proceed.
+
+The write barrier must not perform the write until all simultaneous
+write barriers are also ready to perform the write.
+We refer to this *mark quiescence*.
+To see why this is necessary, consider two simultaneous write barriers
+for `*D1 = S1` and `*D2 = S2` on an object graph that looks like this:
+
+    G1 [b] → D1 [b]   S1 [w]
+                            ↘
+                             O1 [w] → O2 [w] → O3 [w]
+                            ↗
+             D2 [b]   S2 [w]
+
+Goroutine *G1* has been re-scanned (so *D1* must be black), while *Sn*
+and *On* are all white.
+
+Suppose the *S2* write barrier blackens *S2* and *O1* and greys *O2*,
+then the *S1* write barrier blackens *S1* and observes that *O1* is
+already black:
+
+    G1 [b] → D1 [b]   S1 [b]
+                            ↘
+                             O1 [b] → O2 [g] → O3 [w]
+                            ↗
+             D2 [b]   S2 [b]
+
+At this point, the *S1* barrier has run out of local work, but the
+*S2* barrier is still going.
+If *S1* were to complete and write `*D1 = S1` at this point, it would
+make white object *O3* reachable to goroutine *G1*, violating the
+invariant.
+Hence, the *S1* barrier cannot complete until the *S2* barrier is also
+ready to complete.
+
+This requirement sounds onerous, but it can be achieved in a simple
+and reasonably efficient manner by sharing a global mark work queue
+between the write barriers.
+This reuses the existing mark work queue and quiescence logic and
+allows write barriers to help each other to completion.
+
+### Stack re-scanning
+
+The stack re-scan phase re-scans the stacks of all goroutines that
+have run since the initial stack scan to find pointers to white
+objects.
+The process of re-scanning a stack is identical to that of the initial
+scan, except that it must participate in mark quiescence.
+Specifically, the re-scanned goroutine must not resume execution until
+the system has reached mark quiescence (even if no white pointers are
+found on the stack).
+Otherwise, the same sorts of races that were described above are
+possible.
+
+There are multiple ways to realize this.
+The whole stack scan could participate in mark quiescence, but this
+would block any contemporaneous stack scans or write barriers from
+completing during a stack scan if any white pointers were found.
+Alternatively, each white pointer found on the stack could participate
+individually in mark quiescence, blocking the stack scan at that
+pointer until mark quiescence, and the stack scan could again
+participate in mark quiescence once all frames had been scanned.
+
+We propose an intermediate: gather small batches of white pointers
+from a stack at a time and reach mark quiescence on each batch
+individually, as well as at the end of the stack scan (even if the
+final batch is empty).
+
+### Other considerations
+
+Goroutines that start during stack re-scanning cannot reach any white
+objects, so their stacks are immediately considered black.
+
+Goroutines can also share pointers through channels, which are often
+implemented as direct stack-to-stack copies.
+Hence, channel receives also require write barriers in order to
+maintain the invariant.
+Channel receives already have write barriers to maintain stack
+barriers, so there is no additional work here.
+
+
+## Rationale
+
+The primary drawback of this approach to concurrent stack re-scanning
+is that a write barrier during re-scanning could introduce significant
+mutator latency if the transitive mark finds a large unmarked region
+of the heap, or if overlapping write barriers significantly delay mark
+quiescence.
+However, we consider this situation unlikely in non-adversarial
+applications.
+Furthermore, the resulting delay should be no worse than the mark
+termination STW time applications currently experience, since mark
+termination has to do exactly the same amount of marking work, in
+addition to the cost of stack scanning.
+
+### Alternative approaches
+
+An alternative solution to concurrent stack re-scanning would be to
+adopt DMOS-style quiescence [Hudson '97].
+In this approach, greying any object during stack re-scanning (either
+by finding a pointer to a white object on a stack or by installing a
+pointer to a white object in the heap) forces the GC to drain this
+marking work and *restart* the stack re-scanning phase.
+
+This approach has a much simpler write barrier implementation that is
+constant time, so the write barrier would not induce significant
+mutator latency.
+However, unlike the proposed approach, the amount of work performed by
+DMOS-style stack re-scanning is potentially unbounded.
+This interacts poorly with Go's GC pacer.
+The pacer enforces the goal heap size making allocating and GC work
+proportional, but this requires an upper bound on possible GC work.
+As a result, if the pacer underestimates the amount of re-scanning
+work, it may need to block allocation entirely to avoid exceeding the
+goal heap size.
+This would be an effective STW.
+
+There is also a hybrid solution: we could use the proposed transitive
+marking write barrier, but bound the amount of work it can do (and
+hence the latency it can induce).
+If the write barrier exceeds this bound, it performs a DMOS-style
+restart.
+This is likely to get the best of both worlds, but also inherits the
+sum of their complexity.
+
+A final alternative would be to eliminate concurrent stack re-scanning
+entirely by adopting a *deletion-style* write barrier [Yuasa '90].
+This style of write barrier allows the initial stack scan to *blacken*
+the stack, rather than merely greying it (still without the need for
+stack write barriers).
+For full details, see proposal #17503.
+
+
+## Compatibility
+
+This proposal does not affect the language or any APIs and hence
+satisfies the Go 1 compatibility guidelines.
+
+
+## Implementation
+
+We do not plan to implement this proposal.
+Instead, we plan to implement proposal #17503.
+
+The implementation steps are as follows:
+
+1. While not strictly necessary, first make GC assists participate in
+   stack scanning.
+   Currently this is not possible, which increases mutator latency at
+   the beginning of the GC cycle.
+   This proposal would compound this effect by also blocking GC
+   assists at the end of the GC cycle, causing an effective STW.
+
+2. Modify the write barrier to be pre-publication instead of
+   post-publication.
+   Currently the write barrier occurs after the write of a pointer,
+   but this proposal requires that the write barrier complete
+   transitive marking *before* writing the pointer to its destination.
+   A pre-publication barrier is also necessary for
+   [ROC](https://golang.org/s/gctoc).
+
+3. Make the mark completion condition precise.
+   Currently it's possible (albeit unlikely) to enter mark termination
+   before all heap pointers have been marked.
+   This proposal requires that we not start stack re-scanning until
+   all objects reachable from globals are marked, which requires a
+   precise completion condition.
+
+4. Implement the transitive mark write barrier.
+   This can reuse the existing work buffer pool lists and logic,
+   including the global quiescence barrier in getfull.
+   It may be necessary to improve the performance characteristics of
+   the getfull barrier, since this proposal will lean far more heavily
+   on this barrier than we currently do.
+
+5. Check stack re-scanning code and make sure it is safe during
+   non-STW.
+   Since this only runs during STW right now, it may omit
+   synchronization that will be necessary when running during non-STW.
+   This is likely to be minimal, since most of the code is shared with
+   the initial stack scan, which does run concurrently.
+
+6. Make stack re-scanning participate in write barrier quiescence.
+
+7. Create a new stack re-scanning phase.
+   Make mark 2 completion transition to stack re-scanning instead of
+   mark termination and enqueue stack re-scanning root jobs.
+   Once all stack re-scanning jobs are complete, transition to mark
+   termination.
+
+
+## Acknowledgments
+
+We would like to thank Rhys Hiltner (@rhysh) for suggesting the idea
+of a transitive mark write barrier.
+
+
+## References
+
+[Hudson '97] R. L. Hudson, R. Morrison, J. E. B. Moss, and D. S.
+Munro. Garbage collecting the world: One car at a time. In *ACM
+SIGPLAN Notices* 32(10):162–175, October 1997.
+
+[Yuasa '90] T. Yuasa. Real-time garbage collection on general-purpose
+machines. *Journal of Systems and Software*, 11(3):181–198, 1990.