Proposal: Eliminate STW stack re-scanning
Author(s): Austin Clements, Rick Hudson
Last updated: 2016-10-18
Discussion at https://golang.org/issue/17503.
As of Go 1.7, the one remaining source of unbounded and potentially non-trivial stop-the-world (STW) time is stack re-scanning. We propose to eliminate the need for stack re-scanning by switching to a hybrid write barrier that combines a Yuasa-style deletion write barrier [Yuasa '90] and a Dijkstra-style insertion write barrier [Dijkstra '78]. Preliminary experiments show that this can reduce worst-case STW time to under 50µs, and this approach may make it practical to eliminate STW mark termination altogether.
Eliminating stack re-scanning will in turn simplify and eliminate many other parts of the garbage collector that exist solely to improve the performance of stack re-scanning. This includes stack barriers (which introduce significant complexity in many parts of the runtime) and maintenance of the re-scan list. Hence, in addition to substantially improving STW time, the hybrid write barrier should also reduce the overall complexity of the garbage collector.
The Go garbage collector is a tricolor concurrent collector [Dijkstra '78]. Every object is shaded either white, grey, or black. At the beginning of a GC cycle, all objects are white, and it is the goal of the garbage collector to mark all reachable objects black and then free all white objects. The garbage collector achieves this by shading GC roots (stacks and globals, primarily) grey and then endeavoring to turn all grey objects black while satisfying the strong tricolor invariant:
No black object may contain a pointer to a white object.
Ensuring the tricolor invariant in the presence of concurrent pointer updates requires barriers on either pointer reads or pointer writes (or both). There are many flavors of barrier [Pirinen '98]. Go 1.7 uses a coarsened Dijkstra write barrier [Dijkstra '78], where pointer writes are implemented as follows:
writePointer(slot, ptr): shade(ptr) *slot = ptr
shade(ptr) marks the object at
ptr grey if it is not already grey
or black. This ensures the strong tricolor invariant by conservatively
*slot may be in a black object, and ensuring
cannot be white before installing it in
The Dijkstra barrier has several advantages over other types of barriers. It does not require any special handling of pointer reads, which has performance advantages since pointer reads tend to outweigh pointer writes by an order of magnitude or more. It also ensures forward progress; unlike, for example, the Steele write barrier [Steele '75], objects transition monotonically from white to grey to black, so the total work is bounded by the heap size.
However, it also has disadvantages. In particular, it presents a trade-off for pointers on stacks: either writes to pointers on the stack must have write barriers, which is prohibitively expensive, or stacks must be permagrey. Go chooses the later, which means that many stacks must be re-scanned during STW. The garbage collector first scans all stacks at the beginning of the GC cycle to collect roots. However, without stack write barriers, we can't ensure that the stack won't later contain a reference to a white object, so a scanned stack is only black until its goroutine executes again, at which point it conservatively reverts to grey. Thus, at the end of the cycle, the garbage collector must re-scan grey stacks to blacken them and finish marking any remaining heap pointers. Since it must ensure the stacks don't continue to change during this, the whole re-scan process happens with the world stopped.
Re-scanning the stacks can take 10's to 100's of milliseconds in an application with a large number of active goroutines.
We propose to eliminate stack re-scanning and replace Go's write barrier with a hybrid write barrier that combines a Yuasa-style deletion write barrier [Yuasa '90] with a Dijkstra-style insertion write barrier [Dijkstra '78]. The hybrid write barrier is implemented as follows:
writePointer(slot, ptr): shade(*slot) if current stack is grey: shade(ptr) *slot = ptr
That is, the write barrier shades the object whose reference is being overwritten, and, if the current goroutine's stack has not yet been scanned, also shades the reference being installed.
The hybrid barrier makes stack re-scanning unnecessary; once a stack has been scanned and blackened, it remains black. Hence, it eliminates the need for stack re-scanning and the mechanisms that exist to support stack re-scanning, including stack barriers and the re-scan list.
The hybrid barrier requires that objects be allocated black (allocate-white is a common policy, but incompatible with this barrier). However, while not required by Go's current write barrier, Go already allocates black for other reasons, so no change to allocation is necessary.
To our knowledge, the hybrid barrier is a novel approach to concurrent GC.
A full proof of the hybrid write barrier is given at the end of this proposal. Here we give the high-level intuition behind the barrier.
Unlike the Dijkstra write barrier, the hybrid barrier does not satisfy the strong tricolor invariant: for example, a black goroutine (a goroutine whose stack has been scanned) can write a pointer to a white object into a black object without shading the white object. However, it does satisfy the weak tricolor invariant [Pirinen '98]:
Any white object pointed to by a black object is reachable from a grey object via a chain of white pointers (it is grey-protected).
The weak tricolor invariant observes that it's okay for a black object to point to a white object, as long as some path ensures the garbage collector will get around to marking that white object.
Any write barrier has to prohibit a mutator from "hiding" an object; that is, rearranging the heap graph to violate the weak tricolor invariant so the garbage collector fails to mark a reachable object. For example, in a sense, the Dijkstra barrier allows a mutator to hide a white object by moving the sole pointer to it to a stack that has already been scanned. The Dijkstra barrier addresses this by making stacks permagray and re-scanning them during STW.
In the hybrid barrier, the two shades and the condition work together to prevent a mutator from hiding an object:
shade(*slot)prevents a mutator from hiding an object by moving the sole pointer to it from the heap to its stack. If it attempts to unlink an object from the heap, this will shade it.
shade(ptr)prevents a mutator from hiding an object by moving the sole pointer to it from its stack into a black object in the heap. If it attempts to install the pointer into a black object, this will shade it.
Once a goroutine's stack is black, the
shade(ptr)prevents hiding an object by moving it from the stack to the heap, but this requires first having a pointer hidden on the stack. Immediately after a stack is scanned, it only points to shaded objects, so it's not hiding anything, and the
shade(*slot)prevents it from hiding any other pointers on its stack.
The hybrid barrier combines the best of the Dijkstra barrier and the Yuasa barrier. The Yuasa barrier requires a STW at the beginning of marking to either scan or snapshot stacks, but does not require a re-scan at the end of marking. The Dijkstra barrier lets concurrent marking start right away, but requires a STW at the end of marking to re-scan stacks (though more sophisticated non-STW approaches are possible [Hudson '97]). The hybrid barrier inherits the best properties of both, allowing stacks to be concurrently scanned at the beginning of the mark phase, while also keeping stacks black after this initial scan.
The advantage of the hybrid barrier is that it lets a stack scan permanently blacken a stack (without a STW and without write barriers to the stack), which entirely eliminates the need for stack re-scanning, in turn eliminating the need for stack barriers and re-scan lists. Stack barriers in particular introduce significant complexity throughout the runtime, as well as interfering with stack walks from external tools such as GDB and kernel-based profilers.
Also, like the Dijkstra-style write barrier, the hybrid barrier does not require a read barrier, so pointer reads are regular memory reads; and it ensures progress, since objects progress monotonically from white to grey to black.
The disadvantages of the hybrid barrier are minor. It may result in more floating garbage, since it retains everything reachable from roots (other than stacks) at any point during the mark phase. However, in practice it's likely that the current Dijkstra barrier is retaining nearly as much. The hybrid barrier also prohibits certain optimizations: in particular, the Go compiler currently omits a write barrier if it can statically show that the pointer is nil, but the hybrid barrier requires a write barrier in this case. This may slightly increase binary size.
Alternative barrier approaches
There are several variations on the proposed barrier that would also work, but we believe the proposed barrier represents the best set of trade-offs.
A basic variation is to make the Dijkstra-style aspect of the barrier unconditional:
writePointer(slot, ptr): shade(*slot) shade(ptr) *slot = ptr
The main advantage of this barrier is that it's easier to reason about. It directly ensures there are no black-to-white pointers in the heap, so the only source of black-to-white pointers can be scanned stacks. But once a stack is scanned, the only way it can get a white pointer is by traversing reachable objects, and any white object that can be reached by a goroutine with a black stack is grey-protected by a heap object.
The disadvantage of this barrier is that it's effectively twice as expensive as the proposed barrier for most of the mark phase.
Similarly, we could simply coarsen the stack condition:
writePointer(slot, ptr): shade(*slot) if any stack is grey: shade(ptr) *slot = ptr
This has the advantage of making cross-stack writes such as those allowed by channels safe without any special handling, but prolongs when the second shade is enabled, which slows down pointer writes.
A different approach would be to require that all stacks be blackened before any heap objects are blackened, which would enable a pure Yuasa-style deletion barrier:
writePointer(slot, ptr): shade(*slot) *slot = ptr
As originally proposed, the Yuasa barrier takes a complete snapshot of the stack before proceeding with marking. Yuasa argued that this was reasonable on hardware that could perform bulk memory copies very quickly. However, Yuasa's proposal was in the context of a single-threaded system with a comparatively small stack, while Go programs regularly have thousands of stacks that can total to a large amount of memory.
However, this complete snapshot isn't necessary. It's sufficient to ensure all stacks are black before scanning any heap objects. This allows stack scanning to proceed concurrently, but has the downside that it introduces a bottleneck to the parallelism of the mark phase between stack scanning and heap scanning. This bottleneck has downstream effects on goroutine availability, since allocation is paced against marking progress.
Finally, there are other types of black mutator barrier techniques. However, as shown by Pirinen, all possible black mutator barriers other than the Yuasa barrier require a read barrier [Pirinen '98]. Given the relative frequency of pointer reads to writes, we consider this unacceptable for application performance.
Alternative approaches to re-scanning
Going further afield, it's also possible to make stack re-scanning concurrent without eliminating it [Hudson '97]. This does not require changes to the write barrier, but does introduce significant additional complexity into stack re-scanning. Proposal #17505 gives a detailed design for how to do concurrent stack re-scanning in Go.
Channel operations and go statements
The hybrid barrier assumes a goroutine cannot write to another
goroutine's stack. This is true in Go except for two operations:
channel sends and starting goroutines, which can copy values directly
from one stack to another. For channel operations, the
necessary if either the source stack or the destination stack is
grey. For starting a goroutine, the destination stack is always black,
shade(ptr) is necessary if the source stack is grey.
In a racy program, two goroutines may store to the same pointer simultaneously and invoke the write barrier concurrently on the same slot. The hazard is that this may cause the barrier to fail to shade some object that it would have shaded in a sequential execution, particularly given a relaxed memory model. While racy Go programs are generally undefined, we have so far maintained that a racy program cannot trivially defeat the soundness of the garbage collector (since a racy program can defeat the type system, it can technically do anything, but we try to keep the garbage collector working as long as the program stays within the type system).
Suppose optr is the value of the slot before either write to the slot and ptr1 and ptr2 are the two pointers being written to the slot. "Before" is well-defined here because all architectures that Go supports have coherency, which means there is a total order over all reads and writes of a single memory location. If the goroutine's respective stacks have been scanned, then ptr1 and ptr2 will clearly be shaded, since those shades don't read from memory. Hence, the difficult case is if the goroutine's stacks have been scanned. In this case, the barriers reduce to:
|Goroutine G1||Goroutine G2|
optr1 = *slot shade(optr1) *slot = ptr1
optr2 = *slot shade(optr2) *slot = ptr2
Given that we're only dealing with one memory location, the property
of coherence means we can reason about this execution as if it were
sequentially consistent. Given this, concurrent execution of the write
barriers permits one outcome that is not permitted by sequential
execution: if both barriers read
*slot before assigning to it, then
only optr will be shaded, and neither ptr1 nor ptr2 will be
shaded by the barrier. For example:
|Goroutine G1||Goroutine G2|
optr1 = *slot shade(optr1)
optr2 = *slot
We assert that this is safe. Suppose ptr1 is written first. This execution is nearly indistinguishable from an execution that simply skips the write of ptr1. The only way to distinguish it is if a read from another goroutine G3 observes slot between the two writes. However, given our assumption that stacks have already been scanned, either ptr1 is already shaded, or it must be reachable from some other place in the heap anyway (and will be shaded eventually), so concurrently observing ptr1 doesn't affect the marking or reachability of ptr1.
The hybrid barrier could be a problem if C code overwrites a Go
pointer in Go memory with either nil or a C pointer. Currently, this
operation does not require a barrier, but with any sort of deletion
barrier, this does require the barrier. However, a program that does
this would violate the cgo pointer passing rules, since Go code is not
allowed to pass memory to a C function that contains Go pointers.
Furthermore, this is one of the "cheap dynamic checks" enabled by the
default setting of
GODEBUG=cgocheck=1, so any program that violates
this rule will panic unless the checks have been explicitly disabled.
Write barrier omission
The current write barrier can be omitted by the compiler in cases where the compiler knows the pointer being written is permanently shaded, such as nil pointers, pointers to globals, and pointers to static data. These optimizations are generally unsafe with the hybrid barrier. However, if the compiler can show that both the current value of the slot and the value being written are permanently shaded, then it can still safely omit the write barrier. This optimization is aided by the fact that newly allocated objects are zeroed, so all pointer slots start out pointing to nil, which is permanently shaded.
Low-pause stack scans
Currently the garbage collector pauses a goroutine while scanning its stack. If goroutines have large stacks, this can introduce significant tail latency effects. The hybrid barrier and the removal of the existing stack barrier mechanism would make it feasible to perform stack scans with only brief goroutine pauses.
In this design, scanning a stack pauses the goroutine briefly while it scans the active frame. It then installs a blocking stack barrier over the return to the next frame and lets the goroutine resume. Stack scanning then continues toward the outer frames, moving the stack barrier up the stack as it goes. If the goroutine does return as far as the stack barrier, before it can return to an unscanned frame, the stack barrier blocks until scanning can scan that frame and move the barrier further up the stack.
One complication is that a running goroutine could attempt to grow its stack during the stack scan. The simplest solution is to block the goroutine if this happens until the scan is done.
Like the current stack barriers, this depends on write barriers when writing through pointers to other frames. For example, in a partially scanned stack, an active frame could use an up-pointer to move a pointer to a white object out of an unscanned frame and into the active frame. Without a write barrier on the write that removes the pointer from the unscanned frame, this could hide the white object from the garbage collector.
However, with write barriers on up-pointers, this is safe. Rather than arguing about "partially black" stacks, the write barrier on up-pointers lets us view the stack as a sequence of separate frames, where unscanned frames are treated as part of the heap. Writes without write barriers can only happen to the active frame, so we only have to view the active frame as the stack.
This design is technically possible now, but the complexity of composing it with the existing stack barrier mechanism makes it unappealing. With the existing stack barriers gone, the implementation of this approach becomes relatively straightforward. It's also generally simpler than the existing stack barriers in many dimensions, since there are at most two stack barriers per goroutine at a time, and they are present only during the stack scan.
Strictly bounded mark termination
This proposal goes a long way toward strictly bounding the time spent in STW mark termination, but there are some other known causes of longer mark termination pauses. The primary cause is a race that can trigger mark termination while there is still remaining heap mark work. This race and how to resolve it are detailed in the "Mark completion race" appendix.
Concurrent mark termination
With stack re-scanning out of mark termination, it may become practical to make the remaining tasks in mark termination concurrent and eliminate the mark termination STW entirely. On the other hand, the hybrid barrier may reduce STW so much that completely eliminating it is not a practical concern.
The following is a probably incomplete list of remaining mark
termination tasks and how to address them. Worker stack scans can be
eliminated by having workers self-scan during mark. Scanning the
finalizer queue can be eliminated by adding explicit barriers to
queuefinalizer. Without these two scans (and with the fix for the
mark completion race detailed in the appendix), mark termination will
produce no mark work, so finishing the work queue drain also becomes
mcaches can be flushed lazily at the beginning of the
sweep phase using rolling synchronization. Flushing the heap profile
can be done immediately at the beginning of sweep (this is already
concurrent-safe, in fact). Finally, updating global statistics can be
done using atomics and possibly a global memory barrier.
Concurrent sweep termination
Likewise, it may be practical to eliminate the STW for sweep
termination. This is slightly complicated by the fact that the hybrid
barrier requires a global memory fence at the beginning of the mark
phase to enable the write barrier and ensure all pointer writes prior
to enabling the write barrier are visible to the write barrier.
Currently, the STW for sweep termination and setting up the mark phase
accomplishes this. If we were to make sweep termination concurrent, we
could instead use a ragged barrier to accomplish the global memory
fence, or the
on recent Linux kernels.
This proposal does not affect the language or any APIs and hence satisfies the Go 1 compatibility guidelines.
Austin plans to implement the hybrid barrier during the Go 1.8
development cycle. For 1.8, we will leave stack re-scanning support in
the runtime fro debugging purposes, but disable it by default using a
GODEBUG variable. Assuming things go smoothly, we will remove stack
re-scanning support when the tree opens for Go 1.9 development.
The planned implementation approach is:
Fix places that do unusual or "clever" things with memory containing pointers and make sure they cooperate with the hybrid barrier. We'll presumably find more of these as we debug in later steps, but we'll have to make at least the following changes:
Ensure barriers on stack-to-stack copies for channel sends and starting goroutines.
Check all places where we clear memory since the hybrid barrier requires distinguishing between clearing for initialization and clearing already-initialized memory. This will require a barrier-aware
memclrand disabling the
duffzerooptimization for pointers with types.
Check all uses of unmanaged memory in the runtime to make sure it is initialized properly. This is particularly important for pools of unmanaged memory such as the fixalloc allocator that may reuse memory.
Implement concurrent scanning of background mark worker stacks. Currently these are placed on the rescan list and only scanned during mark termination, but we're going to disable the rescan list. We could arrange for background mark workers to scan their own stacks, or explicitly keep track of heap pointers on background mark worker stacks.
Modify the write barrier to implement the hybrid write barrier and the compiler to disable write barrier elision optimizations that aren't valid for the hybrid barrier.
Disable stack re-scanning by making rescan enqueuing a no-op unless a
GODEBUGvariable is set. Likewise, disable stack barrier insertion unless this variable is set.
Use checkmark mode and stress testing to verify that no objects are missed.
Wait for the Go 1.9 development cycle.
Remove stack re-scanning, the rescan list, stack barriers, and the
GODEBUGvariable to enable re-scanning. Possibly, switch to low-pause stack scans, which can reuse some of the stack barrier mechanism.
Appendix: Mark completion race
Currently, because of a race in the mark completion condition, the garbage collector can begin mark termination when there is still available mark work. This is safe because mark termination will finish draining this work, but it makes mark termination unbounded. This also interferes with possible further optimizations that remove all mark work from mark termination.
Specifically, the following interleaving starts mark termination without draining all mark work:
workers is zero and there is one buffer on the full list.
|Thread 1||Thread 2|
inc(&workers) gcDrain(&work) => acquires only full buffer => adds more pointer to work work.dispose() => returns buffer to full list n := dec(&workers) [n=0] if n == 0 && [true]
In this example, a race between observing the
workers count and
observing the state of the full list causes thread 1 to start mark
termination prematurely. Simply checking
workers exhibits a similar race.
To fix this race, we propose introducing a single atomic non-zero indicator for the number of non-empty work buffers. Specifically, this will count the number of work caches that are caching a non-empty work buffer plus one for a non-empty full list. Many buffer list operations can be done without modifying this count, so we believe it will not be highly contended. If this does prove to be a scalability issue, there are well-known techniques for scalable non-zero indicators [Ellen '07].
Appendix: Proof of soundness, completeness, and boundedness
This section argues that the hybrid write barrier satisfies the weak tricolor invariant, and hence is sound in the sense that it does not collect reachable objects; that it terminates in a bounded number of steps; and that it eventually collects all unreachable objects, and hence is complete. We have also further verified these properties using a randomized stateless model.
The following proofs consider global objects to be a subset of the heap objects. This is valid because the write barrier applies equally to global objects. Similarly, we omit explicit discussion of nil pointers, since the nil pointer can be considered an always-black heap object of zero size.
The hybrid write barrier satisfies the weak tricolor invariant [Pirinen '98]. However, rather than directly proving this, we prove that it satisfies the following modified tricolor invariant:
Any white object pointed to by a black object is grey-protected by a heap object (reachable via a chain of white pointers from the grey heap object). That is, for every B -> W edge, there is a path G -> W₁ -> ... -> Wₙ -> W where G is a heap object.
This is identical to the weak tricolor invariant, except that it requires that the grey-protector is a heap object. This trivially implies the weak tricolor invariant, but gives us a stronger basis for induction in the proof.
Lemma 1 establishes a simple property of paths we'll use several times.
Lemma 1. In a path O₁ -> ... -> Oₙ where O₁ is a heap object, all Oᵢ must be heap objects.
Proof. Since O₁ is a heap object and heap objects can only point to other heap objects, by induction, all Oᵢ must be heap objects. ∎
In particular, if some object is grey-protected by a heap object, every object in the grey-protecting path must be a heap object.
Lemma 2 extends the modified tricolor invariant to white objects that are indirectly reachable from black objects.
Lemma 2. If the object graph satisfies the modified tricolor invariant, then every white object reachable (directly or indirectly) from a black object is grey-protected by a heap object.
Proof. Let W be a white object reachable from black object B via simple path B -> O₁ -> ... -> Oₙ -> W. Note that W and all Oᵢ and must be heap objects because stacks can only point to themselves (in which case it would not be a simple path) or heap objects, so O₁ must be a heap object, and by lemma 1, the rest of the path must be heap objects. Without loss of generality, we can assume none of Oᵢ are black; otherwise, we can simply reconsider using the shortest path suffix that starts with a black object.
If there are no Oᵢ, B points directly to W and the modified tricolor invariant directly implies that W is grey-protected by a heap object.
If any Oᵢ is grey, then W is grey-protected by the last grey object in the path.
Otherwise, all Oᵢ are white. Since O₁ is a white object pointed to by a black object, O₁ is grey-protected by some path G -> W₁ -> ... -> Wₙ -> O₁ where G is a heap object. Thus, W is grey-protected by G -> W₁ -> ... -> Wₙ -> O₁ -> ... -> Oₙ -> W. ∎
Lemma 3 builds on lemma 2 to establish properties of objects reachable by goroutines.
Lemma 3. If the object graph satisfies the modified tricolor invariant, then every white object reachable by a black goroutine (a goroutine whose stack has been scanned) is grey-protected by a heap object.
Proof. Let W be a white object reachable by a black goroutine. If W is reachable from the goroutine's stack, then by lemma 2 W is grey-protected by a heap object. Otherwise, W must be reachable from a global X. Let O be the last non-white object in the path from X to W (O must exist because X itself is either grey or black). If O is grey, then O is a heap object that grey-protects W. Otherwise, O is black and by lemma 2, W is grey-protected by some heap object. ∎
Now we're ready to prove that the hybrid write barrier satisfies the weak tricolor invariant, which implies it is sound (it marks all reachable objects).
Theorem 1. The hybrid write barrier satisfies the weak tricolor invariant.
Proof. We first show that the hybrid write barrier satisfies the modified tricolor invariant. The proof follows by induction over the operations that affect the object graph or its coloring.
Base case. Initially there are no black objects, so the invariant holds trivially.
Write pointer in the heap. Let obj.slot := ptr denote the write, where obj is in the heap, and let optr denote the value of obj.slot prior to the write.
Let W be a white object pointed to by a black object B after the heap write. There are two cases:
B ≠ obj: W was pointed to by them same black object B before the write, and, by assumption, W was grey-protected by a path G -> W₁ -> ... -> Wₙ -> W, where G is a heap object. If none of these edges are obj.slot, then W is still protected by G. Otherwise, the path must have included the edge obj -> optr and, since the write barrier shades optr, W is grey-protected by optr after the heap write.
B = obj: We first establish that W was grey-protected before the write, which breaks down into two cases:
W = ptr: The goroutine must be black, because otherwise the write barrier shades ptr, so it is not white. ptr must have been reachable by the goroutine for it to write it, so by lemma 3, ptr was grey-protected by some heap object G prior to the write.
W ≠ ptr: B pointed to W before the write and, by assumption, W was grey-protected by some heap object G before the write.
Because obj was black before the write, it could not be in the grey-protecting path from G to W, so this write did not affect this path, so W is still grey-protected by G.
Write pointer in a stack. Let stk.slot := ptr denote the write.
Let W be a white object pointed to by a black object B after the stack write. We first establish that W was grey-protected before the stack write, which breaks down into two cases:
B = stk and W = ptr: W may not have been pointed to by a black object prior to the stack write (that is, the write may create a new B -> W edge). However, ptr must have been reachable by the goroutine, which is black (because B = stk), so by lemma 3, W was grey-protected by some heap object G prior to the write.
Otherwise: W was pointed to by the same black object B prior to the stack write, so, by assumption, W was grey-protected by some heap object G prior to the write.
By lemma 1, none of the objects in the grey-protecting path from heap object G to W can be a stack, so the stack write does not modify this path. Hence, W is still grey-protected after the stack write by G.
Scan heap object. Let obj denote the scanned object. Let W be an object pointed to by a black object B after the scan. B cannot be obj because immediately after the scan, obj does not point to any white objects. Thus, B must have been black and pointed to W before the scan as well, so, by assumption, W was grey-protected by a path G -> W₁ -> ... -> Wₙ -> W, where G is a heap object. If some Wᵢ was an object pointed to by obj, then W is grey-protected by Wᵢ after the scan. Otherwise, W is still grey-protected by G.
Stack scan. This case is symmetric with scanning a heap object.
Allocate an object. Since new objects are allocated black and point to nothing, the invariant trivially holds across allocation.
Create a stack. This case is symmetric with object allocation because new stacks start out empty and hence are trivially black.
This completes the induction cases and shows that the hybrid write barrier satisfies the modified tricolor invariant. Since the modified tricolor invariant trivially implies the weak tricolor invariant, the hybrid write barrier satisfies the weak tricolor invariant. ∎
The garbage collector is also bounded, meaning it eventually terminates.
Theorem 2. A garbage collector using the hybrid write barrier terminates in a finite number of marking steps.
Proof. We observe that objects progress strictly from white to grey to black and, because new objects (including stacks) are allocated black, the total marking work is bounded by the number of objects at the beginning of garbage collection, which is finite. ∎
Finally, the garbage collector is also complete, in the sense that it eventually collects all unreachable objects. This is trivial from the fact that the garbage collector cannot mark any objects that are not reachable when the mark phase starts.
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