21. AMS pool class

21.1. Introduction

.intro: This is the design of the AMS pool class.

.readership: MM developers.

.source: design.mps.buffer, design.mps.trace, design.mps.scan, design.mps.action and design.mps.class-interface [none of these were actually used – pekka 1998-04-21]. No requirements doc [we need a req.mps that captures the commonalities between the products – pekka 1998-01-27].

21.2. Overview

.overview: This document describes the design of the AMS (Automatic Mark-and-Sweep) pool class. The AMS pool is a proof-of-concept design for a mark-sweep pool in the MPS. It’s not meant to be efficient, but it could serve as a model for an implementation of a more advanced pool (such as EPVM).

21.3. Requirements

.req.mark-sweep: The pool must use a mark-and-sweep GC algorithm.

.req.colour: The colour representation should be as efficient as possible.

.req.incremental: The pool must support incremental GC.

.req.ambiguous: The pool must support ambiguous references to objects in it (but ambiguous references into the middle of an object do not preserve the object).

.req.format: The pool must be formatted, for generality.

.req.correct: The design and the implementation should be simple enough to be seen to be correct.

.req.simple: Features not related to mark-and-sweep GC should initially be implemented as simply as possible, in order to save development effort.

.not-req.grey: We haven’t figured out how buffers ought to work with a grey mutator, so we use .req.correct to allow us to design a pool that doesn’t work in that phase. This is acceptable as long as we haven’t actually implemented grey mutator collection.

21.4. Architecture

21.4.1. Subclassing

.subclass: Since we expect to have many mark-and-sweep pools, we build in some protocol for subclasses to modify various aspects of the behaviour. Notably there’s a subclassable segment class, and a protocol for performing iteration.

21.4.2. Allocation

.align: We divide the segments in grains, each the size of the format alignment. .alloc-bit-table: We keep track of allocated grains using a bit table. This allows a simple implementation of allocation and freeing using the bit table operators, satisfying .req.simple, and can simplify the GC routines. Eventually, this should use some sophisticated allocation technique suitable for non-moving automatic pools.

.buffer: We use buffered allocation, satisfying .req.incremental. The AMC buffer technique is reused, although it is not suitable for non-moving pools, but req.simple allows us to do that for now.

.extend: If there’s no space in any existing segment, a new segment is allocated. The actual class is allowed to decide the size of the new segment.

.no-alloc: Do not support PoolAlloc(), because we can’t support one-phase allocation for a scannable pool (unless we disallow incremental collection). For exact details, see design.mps.buffer.

.no-free: Do not support PoolFree(), because automatic pools don’t need explicit free and having it encourages clients to use it (and therefore to have dangling pointers, double frees, and other memory management errors.)

21.4.3. Colours

.colour: Objects in a segment which is not condemned (for some trace) take their colour (for this trace) from the segment.

.colour.object: Since we need to implement a non-copying GC, we keep track of the colour of each object in a condemned segment separately. For this, we use bit tables with a bit for each grain. This format is fast to access, has better locality than mark bits in the objects themselves, and allows cheap interoperation with the allocation bit table.

.colour.encoding: As to the details, we follow analysis.non-moving-colour(3), implementing both the alloc-white sharing option described in analysis.non-moving-colour.constraint.reclaim.white-free-bit and the vanilla three-table option, because the former cannot work with interior pointers. However, the colour encoding in both is the same.

.ambiguous.middle: We will allow ambiguous references into the middle of an object (as required by .req.ambiguous), using the trick in analysis.non-moving-colour.interior.ambiguous-only to speed up scanning.

.interior-pointer: Note that non-ambiguous interior pointers are outlawed.

.colour.alloc: Objects are allocated black. This is the most efficient alternative for traces in the black mutator phase, and .not-req.grey means that’s sufficient.

Note

Some day, we need to think about allocating grey or white during the grey mutator phase.

21.4.4. Scanning

.scan.segment: The tracer protocol requires (for segment barrier hits) that there is a method for scanning a segment and turning all grey objects on it black. This cannot be achieved with a single sequential sweep over the segment, since objects that the sweep has already passed may become grey as later objects are scanned.

.scan.graph: For a non-moving GC, it is more efficient to trace along the reference graph than segment by segment. It also allows passing type information from fix to scan. Currently, the tracer doesn’t offer this option when it’s polling for work.

.scan.stack: Tracing along the reference graph cannot be done by recursive descent, because we can’t guarantee that the stack won’t overflow. We can, however, maintain an explicit stack of things to trace, and fall back on iterative methods (.scan.iter) when it overflows and can’t be extended.

.scan.iter: As discussed in .scan.segment, when scanning a segment, we need to ensure that there are no grey objects in the segment when the scan method returns. We can do this by iterating a sequential scan over the segment until nothing is grey (see .marked.scan for details).

.scan.iter.only: Some iterative method is needed as a fallback for the more advanced methods, and as this is the simplest way of implementing the current tracer protocol, we will start by implementing it as the only scanning method.

.scan.buffer: We do not scan between ScanLimit and Limit of a buffer (see .iteration.buffer), as usual.

Note

design.mps.buffer should explain why this works, but doesn’t. Pekka P. Pirinen, 1998-02-11.

.fix.to-black: When fixing a reference to a white object, if the segment does not refer to the white set, the object cannot refer to the white set, and can therefore be marked as black immediately (rather than grey).

21.5. Implementation

21.5.1. Colour

.colour.determine: Following the plan in .colour, if SegWhite(seg) includes the trace, the colour of an object is given by the bit tables. Otherwise if SegGrey(seg) includes the trace, all the objects are grey. Otherwise all the objects are black.

.colour.bits: As we only have searches for runs of zero bits, we use two bit tables, the non-grey and non-white tables, but this is hidden beneath a layer of macros talking about grey and white in positive terms.

.colour.single: We have only implemented a single set of mark and scan tables, so we can only condemn a segment for one trace at a time. This is checked for in condemnation. If we want to do overlapping white sets, each trace needs its own set of tables.

.colour.check: The grey-and-non-white state is illegal, and free objects must be white as explained in analysis.non-moving-colour.contraint.reclaim.

21.5.2. Iteration

.iteration: Scan, reclaim and other operations need to iterate over all objects in a segment. We abstract this into a single iteration function, even though we no longer use it for reclaiming and rarely for scanning.

.iteration.buffer: Iteration skips directly from ScanLimit to Limit of a buffer. This is because this area may contain partially-initialized and uninitialized data, which cannot be processed. Since the iteration skips the buffer, callers need to take the appropriate action, if any, on it.

Note

ScanLimit is used for reasons which are not documented in design.mps.buffer.

21.5.3. Scanning Algorithm

.marked: Each segment has a marksChanged flag, indicating whether anything in it has been made grey since the last scan iteration (.scan.iter) started. This flag only concerns the colour of objects with respect to the trace for which the segment is condemned, as this is the only trace for which objects in the segment are being made grey by fixing. Note that this flag doesn’t imply that there are grey objects in the segment, because the grey objects might have been subsequently scanned and blackened.

.marked.fix: The marksChanged flag is set TRUE by AMSFix() when an object is made grey.

.marked.scan: AMSScan() must blacken all grey objects on the segment, so it must iterate over the segment until all grey objects have been seen. Scanning an object in the segment might grey another one (.marked.fix), so the scanner iterates until this flag is FALSE, setting it to FALSE before each scan. It is safe to scan the segment even if it contains nothing grey.

.marked.scan.fail: If the format scanner returns failure (see protocol.mps.scanning), we abort the scan in the middle of a segment. So in this case the marksChanged flag is set back to TRUE, because we may not have blackened all grey objects.

Note

Is that the best reference for the format scanner?

.marked.unused: The marksChanged flag is meaningless unless the segment is condemned. We make it FALSE in these circumstances.

.marked.condemn: Condemnation makes all objects in a segment either black or white, leaving nothing grey, so it doesn’t need to set the marksChanged flag which must already be FALSE.

.marked.reclaim: When a segment is reclaimed, it can contain nothing marked as grey, so the marksChanged flag must already be FALSE.

.marked.blacken: When the tracer decides not to scan, but to call PoolBlacken(), we know that any greyness can be removed. AMSBlacken() does this and resets the marksChanged flag, if it finds that the segment has been condemned.

.marked.clever: AMS could be clever about not setting the marksChanged flag, if the fixed object is ahead of the current scan pointer. It could also keep low- and high-water marks of grey objects, but we don’t need to implement these improvements at first.

21.5.4. Allocation

.buffer-init: We take one init arg to set the Rank on the buffer, just to see how it’s done.

.no-bit: As an optimization, we won’t use the alloc bit table until the first reclaim on the segment. Before that, we just keep a high-water mark.

.fill: AMSBufferFill() takes the simplest approach: it iterates over the segments in the pool, looking for one which can be used to refill the buffer.

.fill.colour: The objects allocated from the new buffer must be black for all traces (.colour.alloc), so putting it on a black segment (meaning one where neither SegWhite(seg) nor SegGrey(seg) include the trace, see .colour.determine) is obviously OK. White segments (where SegWhite(seg) includes the trace) are also fine, as we can use the colour tables to make it black. At first glance, it seems we can’t put it on a segment that is grey but not white for some trace (one where SegWhite(seg) doesn’t include the trace, but SegGrey(seg) does), because the new objects would become grey as the buffer’s ScanLimit advanced. However, in many configurations, the mutator would hit a barrier as soon as it started initializing the object, which would flip the buffer. In fact, the current (2002-01) implementation of buffers assumes buffers are black, so they’d better.

.fill.colour.reclaim: In fact, putting a buffer on a condemned segment will screw up the accounting in AMCReclaim(), so it’s disallowed.

.fill.slow: AMSBufferFill() gets progressively slower as more segments fill up, as it laboriously checks whether the buffer can be refilled from each segment, by inspecting the allocation bit map. This is helped a bit by keeping count of free grains in each segment, but it still spends a lot of time iterating over all the full segments checking the free size. Obviously, this can be much improved (we could keep track of the largest free block in the segment and in the pool, or we could keep the segments in some more efficient structure, or we could have a real free list structure).

.fill.extend: If there’s no space in any existing segment, the segSize method is called to decide the size of the new segment to allocate. If that fails, the code tries to allocate a segment that’s just large enough to satisfy the request.

.empty: AMSBufferEmpty() makes the unused space free, since there’s no reason not to. We have to adjust the colour tables as well, since these grains were black and now they need to be white (or at least encoded -G and W).

.reclaim.empty.buffer: Segments which after reclaim only contain a buffer could be destroyed by trapping the buffer, but there’s no point to this.

21.5.5. Initialization

.init: The initialization method AMSInit() takes three additional arguments: the format of objects allocated in the pool, the chain that controls GC timing, and a flag for supporting ambiguous references.

.init.share: If support for ambiguity is required, the shareAllocTable flag is reset to indicate the pool uses three separate bit tables, otherwise it is set and the pool shares a table for non-white and alloc (see .colour.encoding).

.init.align: The pool alignment is set equal to the format alignment (see design.mps.align).

.init.internal: Subclasses call AMSInitInternal() to avoid the problems of sharing va_list and emitting a superfluous PoolInitAMS event.

21.5.6. Condemnation

.condemn.buffer: Buffers are not condemned, instead they are coloured black, to make sure that the objects allocated will be black, following .colour.alloc (or, if you wish, because buffers are ignored like free space, so need the same encoding).

21.5.7. Reclaim

.reclaim: Reclaim uses either of analysis.non-moving-colour .constraint.reclaim.white-free-bit (just reuse the non-white table as the alloc table) or analysis.non-moving-colour.constraint.reclaim.free-bit (copy it), depending on the shareAllocTable flag (as set by .init.share). However, bit table still has to be iterated over to count the free grains. Also, in a debug pool, each white block has to be splatted.

21.5.8. Segment merging and splitting

.split-merge: We provide methods for splitting and merging AMS segments. The pool implementation doesn’t cause segments to be split or merged – but a subclass might want to do this (see .stress.split-merge). The methods serve as an example of how to implement this facility.

.split-merge.constrain: There are some additional constraints on what segments may be split or merged:

• .split-merge.constrain.align: Segments may only be split or merged at an address which is aligned to the pool alignment as well as to the arena grain size.

  .split-merge.constrain.align.justify: This constraint is implied by the design of allocation and colour tables, which cannot represent segments starting at unaligned addresses. The constraint only arises if the pool alignment is larger than the arena alignment. There’s no requirement to split segments at unaligned addresses.

• .split-merge.constrain.empty: The higher segment must be empty. That is, the higher segment passed to SegMerge() must be empty, and the higher segment returned by SegSplit() must be empty.

  .split-merge.constrain.empty.justify: This constraint makes the code significantly simpler. There’s no requirement for a more complex solution at the moment (as the purpose is primarily pedagogic).

.split-merge.fail: The split and merge methods are not proper anti-methods for each other (see design.mps.seg.split-merge.fail.anti.no). Methods will not reverse the side-effects of their counterparts if the allocation of the colour and allocation bit tables should fail. Client methods which over-ride split and merge should not be written in such a way that they might detect failure after calling the next method, unless they have reason to know that the bit table allocations will not fail.

21.6. Testing

.stress: There’s a stress test, MMsrc!amsss.c, that does 800 kB of allocation, enough for about three GCs. It uses a modified Dylan format, and checks for corruption by the GC. Both ambiguous and exact roots are tested.

.stress.split-merge: There’s also a stress test for segment splitting and merging, MMsrc!segsmss.c. This is similar to amsss.c – but it defines a subclass of AMS, and causes segments to be split and merged. Both buffered and non-buffered segments are split / merged.

21.7. Notes

.addr-index.slow: Translating from an address to and from a grain index in a segment uses macros such as AMS_INDEX and AMS_INDEX_ADDR. These are slow because they call SegBase() on every translation – we could cache that.

.grey-mutator: To enforce the restriction set in .not-req.grey we check that all the traces are flipped in AMSScan(). It would be good to check in AMSFix() as well, but we can’t do that, because it’s called during the flip, and we can’t tell the difference between the flip and the grey mutator phases with the current tracer interface.