General data structures

The fundamental structure of generational garbage collection is the Chain, which describes a sequence of generations.

A chain specifies the "capacity" and "mortality" for each generation. When creating an automatically collected pool, the client code may specify the chain which will control collections for that pool. The same chain may be used for multiple pools. If no chain is specified, the pool uses the arena's default generation chain.

Each generation in a chain has a GenDesc structure, allocated in an array pointed to from the chain. In addition to the generations in the chains, the arena has a unique GenDesc structure, named topGen and described in comments as "the dynamic generation" (misleadingly: in fact it is the least dynamic generation).

Each automatically collected pool has a set of PoolGen structures, one for each generation that it can allocate or promote into. The PoolGen structures for each generation point to the GenDesc for that generation, and are linked together in a ring on the GenDesc. These structures are used to gather accounting information for strategy decisions.

The non-moving automatic pool classes (AMS, AWL and LO) do not support generational collection, so they allocate into a single generation. The moving automatic pool classes (AMC and AMCZ) have one pool generations for each generation in the chain, plus one pool generation for the arena's "top generation".

AMC data structures

An AMC pool creates an array of pool generation structures of type amcGen (a subclass of PoolGen). Each pool generation points to the forwarding buffer for that generation: this is the buffer that surviving objects are copied into.

AMC segments point to the AMC pool generation that the segment belongs to, and AMC buffers point to the AMC pool generation that the buffer will be allocating into.

The forwarding buffers are set up during AMC pool creation. Each generation forwards into the next higher generation in the chain, except for the top generation, which forwards to itself. Thus, objects are "promoted" up the chain of generations until they end up in the top generations, which is shared between all generational pools.

Collections

Collections in the MPS start in one of two ways:

1. A collection of the world starts via traceCondemnAll(). This simply condemns all segments in all automatic pools.

2. A collection of some set of generations starts via TracePoll(). This calls ChainDeferral() for each chain; this function indicates if the chain needs collecting, and if so, how urgent it is to collect that chain. The most urgent chain in need of collection (if any) is then condemned by calling ChainCondemnAuto().

   This function chooses the set of generations to condemn, computes the zoneset corresponding to the union those generations, and condemns those zones by calling TraceCondemnZones().

   Note that the condemnation is of every segment in an automatic pool in any zone in the zoneset. It is not limited to the segments actually associated with the condemned generations.

Zones

Each generation in each chain has a zoneset associated with it (gen->zones); the condemned zoneset is the union of some number of generation's zonesets.

An attempt is made to use distinct zonesets for different generations. Segments in automatic pools are allocated using PoolGenAlloc() which creates a SegPref using the zoneset from the generation's GenDesc. The zoneset for each generation starts out empty. If the zoneset is empty, an attempt is made to allocate from a free zone. The GenDesc zoneset is augmented with whichever zones the new segment occupies.

Note that this zoneset can never shrink.

Parameters

.param.intro: A generation has two parameters, capacity and mortality, specified by the client program.

.param.capacity: The capacity of a generation is the amount of new allocation in that generation (that is, allocation since the last time the generation was condemned) that will cause the generation to be collected by TracePoll().

.param.capacity.misnamed: The name capacity is unfortunate since it suggests that the total amount of memory in the generation will not exceed this value. But that will only be the case for pool classes that always promote survivors to another generation. When there is old allocation in the generation (that is, prior to the last time the generation was condemned), as there is in the case of non-moving pool classes, the size of a generation is unrelated to its capacity.

.param.mortality: The mortality of a generation is the proportion (between 0 and 1) of memory in the generation that is expected to be dead when the generation is collected. It is used in TraceStart() to estimate the amount of data that will have to be scanned in order to complete the trace.

Accounting

.accounting.intro: Pool generations maintain the sizes of various categories of data allocated in that generation for that pool. This accounting information is reported via the event system, but also used in two places:

.accounting.poll: ChainDeferral() uses the new size of each generation to determine which generations in the chain are over capacity and so might need to be collected via TracePoll().

.accounting.condemn: ChainCondemnAuto() uses the new size of each generation to determine which generations in the chain will be collected; it also uses the total size of the generation to compute the mortality.

.accounting.check: Computing the new size for a pool generation is far from straightforward: see job003772 for some (former) errors in this code. In order to assist with checking that this has been computed correctly, the locus module uses a double-entry book-keeping system to account for every byte in each pool generation. This uses six accounts:

.account.total: Memory acquired from the arena.

.account.total.negated: From the point of view of the double-entry system, the total should be negative as it is owing to the arena, but it is inconvenient to represent negative sizes, and so the positive value is stored instead.

.account.total.negated.justification: We don't have a type for signed sizes; but if we represented it in two's complement using the unsigned Size type then Clang's unsigned integer overflow detector would complain.

.account.free: Memory that is not in use (free or lost to fragmentation).

.account.new: Memory in use by the client program, allocated since the last time the generation was condemned.

.account.old: Memory in use by the client program, allocated prior to the last time the generation was condemned.

.account.newDeferred: Memory in use by the client program, allocated since the last time the generation was condemned, but which should not cause collections via TracePoll(). (Due to ramping; see below.)

.account.oldDeferred: Memory in use by the client program, allocated prior to the last time the generation was condemned, but which should not cause collections via TracePoll(). (Due to ramping; see below.)

.accounting.op: The following operations are provided:

.accounting.op.alloc: Allocate a segment in a pool generation. Debit total, credit free. (But see .account.total.negated.)

.accounting.op.free: Free a segment. First, ensure that the contents of the segment are accounted as free, by artificially ageing any memory accounted as new or newDeferred (see .accounting.op.age) and then artifically reclaiming any memory accounted as old or oldDeferred (see .accounting.op.reclaim). Finally, debit free, credit total. (But see .account.total.negated.)

.accounting.op.fill: Allocate memory, for example by filling a buffer. Debit free, credit new or newDeferred.

.accounting.op.empty: Deallocate memory, for example by emptying the unused portion of a buffer. Debit new or newDeferred, credit free.

.accounting.op.age: Condemn memory. Debit new or newDeferred, credit old or oldDeferred.

.accounting.op.reclaim: Reclaim dead memory. Debit old or oldDeferred, credit free.

.accounting.op.undefer: Stop deferring the accounting of memory. Debit oldDeferred, credit old. Debit newDeferred, credit new.

Ramps

The intended semantics of ramping are pretty simple. It allows the client to advise us of periods of large short-lived allocation on a particular AP. Stuff allocated using that AP during its "ramp" will probably be dead when the ramp finishes. How the MPS makes use of this advice is up to us, but for instance we might segregate those objects, collect them less enthusiastically during the ramp and then more enthusiastically soon after the ramp finishes. Ramps can nest.

A ramp is entered by calling:

mps_ap_alloc_pattern_begin(ap, mps_alloc_pattern_ramp())

or similar, and left in a similar way.

This is implemented on a per-pool basis, for AMC only (it's ignored by the other automatic pools). PoolAMC throws away the identity of the AP specified by the client. The implementation is intended to work by changing the generational forwarding behaviour, so that there is a "ramp generation" - one of the regular AMC generations - which forwards to itself if collected during a ramp (instead of promoting to an older generation). It also tweaks the strategy calculation code, in a way with consequences I am documenting elsewhere.

Right now, the code sets this ramp generation to the last generation specified in the pool's "chain": it ordinarily forwards to the "after-ramp" generation, which is the "dynamic generation" (i.e. the least dynamic generation, i.e. the arena-wide "top generation"). My recollection, and some mentions in design/poolamc, suggests that the ramp generation used to be chosen differently from this.

So far, it doesn't sound too ghastly, I guess, although the subversion of the generational system seems a little daft. Read on....

An AMC pool has a rampMode (which is really a state of a state machine), taking one of five values: OUTSIDE, BEGIN, RAMPING, FINISH, and COLLECTING (actually the enum values are called RampX for these X). We initialize in OUTSIDE. The pool also has a rampCount, which is the ramp nesting depth and is used to allow us to ignore ramp transitions other than the outermost. According to design/poolamc, there's an invariant (in BEGIN or RAMPING, rampCount > 0; in COLLECTING or OUTSIDE, rampCount == 0), but this isn't checked in AMCCheck() and in fact is false for COLLECTING (see below).

There is a small set of events causing state machine transitions:

• entering an outermost ramp;
• leaving an outermost ramp;
• condemning any segment of a ramp generation (detected in AMCWhiten);
• reclaiming any AMC segment.

Here's pseudo-code for all the transition events:

Entering an outermost ramp:

if not FINISH, go to BEGIN.

Leaving an outermost ramp:

if RAMPING, go to FINISH. Otherwise, go to OUTSIDE.

Condemning a ramp generation segment:

If BEGIN, go to RAMPING and make the ramp generation forward to itself (detach the forwarding buffer and reset its generation). If FINISH, go to COLLECTING and make the ramp generation forward to the after-ramp generation.

Reclaiming any AMC segment:

If COLLECTING:

if rampCount > 0, go to BEGIN. Otherwise go to OUTSIDE.

Now, some deductions:

1. When OUTSIDE, the count is always zero, because (a) it starts that way, and the only ways to go OUTSIDE are (b) by leaving an outermost ramp (count goes to zero) or (c) by reclaiming when the count is zero.
2. When BEGIN, the count is never zero (consider the transitions to BEGIN and the transition to zero).
3. When RAMPING, the count is never zero (again consider transitions to RAMPING and the transition to zero).
4. When FINISH, the count can be anything (the transition to FINISH has zero count, but the Enter transition when FINISH can change that and then it can increment to any value).
5. When COLLECTING, the count can be anything (from the previous fact, and the transition to COLLECTING).
6. This is a bug!! The ramp generation is not always reset (to forward to the after-ramp generation). If we get into FINISH and then see another ramp before the next condemnation of the ramp generation, we will Enter followed by Leave. The Enter will keep us in FINISH, and the Leave will take us back to OUTSIDE, skipping the transition to the COLLECTING state which is what resets the ramp generation forwarding buffer. [TODO: check whether I made an issue and/or fixed it; NB 2013-06-04]

The simplest change to fix this is to change the behaviour of the Leave transition, which should only take us OUTSIDE if we are in BEGIN or COLLECTING. We should also update design/poolamc to tell the truth, and check the invariants, which will be these:

OUTSIDE => zero BEGIN => non-zero RAMPING => non-zero

A cleverer change might radically rearrange the state machine (e.g. reduce the number of states to three) but that would require closer design thought and should probably be postponed until we have a clearer overall strategy plan.

While I'm writing pseudo-code versions of ramp-related code, I should mention this other snippet, which is the only other code relating to ramping (these notes are useful when thinking about the broader strategy code):

In AMCBufferFill(), if we're RAMPING, and filling the forwarding buffer of the ramp generation, and the ramp generation is the forwarding buffer's generation, set amcSeg->new to FALSE. Otherwise, add the segment size to poolGen.newSize.

And since I've now mentioned the amcSeg->new flag, here are the only other uses of that:

• it initializes as TRUE.
• When leaving an outermost ramp, go through all the segments in the pool. Any non-white segment in the rampGen with new set to FALSE has its size added to poolGen->newSize and gets new set to TRUE.
• in AMCWhiten(), if new is TRUE, the segment size is deducted from poolGen.newSize and new is set to FALSE.

Starting a Trace

TODO: Why do we start a trace? How do we choose what to condemn?

Trace Progress

TODO: When do we do some tracing work? How much tracing work do we do?