36. MPS Strategy¶
36.1. Introduction¶
.intro This is the design of collection strategy for the MPS.
.readership MPS developers.
36.2. Overview¶
.overview The MPS uses “strategy” code to make three decisions:
when to start a collection trace;
what to condemn;
how to schedule tracing work.
This document describes the current strategy, identifies some weaknesses in it, and outlines some possible future development directions.
36.3. Requirements¶
[TODO: source some from req.dylan, or do an up-to-date requirements analysis – NB 2013-03-25]
Garbage collection is a trade-off between time and space: it consumes some [CPU] time in order to save some [memory] space. Strategy shifts the balance point. A better strategy will take less time to produce more space. Examples of good strategy might include:
choosing segments to condemn which contain high proportions of dead objects;
starting a trace when a large number of objects have just died;
doing enough collection soon enough that the client program never suffers low-memory problems;
using otherwise-idle CPU resources for tracing.
Conversely, it would be bad strategy to do the reverse of each of these (condemning live objects; tracing when there’s very little garbage; not collecting enough; tracing when the client program is busy).
Abstracting from these notions, requirements on strategy would relate to:
Maximum pause time and other utilization metrics (for example, bounded mutator utilization, minimum mutator utilization, total MPS CPU usage);
Collecting enough garbage (for example: overall heap size; low-memory requirements).
Allowing client control (for example, client recommendations for collection timing or condemnation).
There are other possible strategy considerations which are so far outside the scope of current strategy and MPS design that this document disregards them. For example, either inferring or allowing the client to specify preferred relative object locations (“this object should be kept in the same cache line as that one”), to improve cache locality.
36.4. Generations¶
The largest part of the current MPS strategy implementation is the support for generational garbage collections.
36.4.1. 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”.
36.4.2. 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.
36.4.3. Collections¶
Collections in the MPS start in one of two ways:
A collection of the world starts via
TraceStartCollectAll(). This simply condemns all segments in all automatic pools.A collection of some set of generations starts via
PolicyStartTrace(). See .policy.start.
36.4.4. 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 LocusPref 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.
36.4.5. 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.
36.4.6. 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 by PolicyStartTrace().
.accounting.condemn: PolicyStartTrace() 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.
36.4.7. 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:
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.
When BEGIN, the count is never zero (consider the transitions to BEGIN and the transition to zero).
When RAMPING, the count is never zero (again consider transitions to RAMPING and the transition to zero).
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).
When COLLECTING, the count can be anything (from the previous fact, and the transition to COLLECTING).
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, setamcSeg->newto FALSE. Otherwise, add the segment size topoolGen.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->newSizeand gets new set to TRUE.in
AMCWhiten(), if new is TRUE, the segment size is deducted frompoolGen.newSizeand new is set to FALSE.
36.5. Policy¶
.policy: Functions that make decisions about what action to take are collected into the policy module (policy.c). The purpose of doing so is to make it easier to understand this set of decisions and how they interact, and to make it easier to maintain and update the policy.
36.5.1. Assignment of zones¶
.policy.alloc: Allocate size bytes of memory on behalf of
pool, based on the preferences described by pref. If
successful, update *tractReturn to point to the first tract in the
allocated memory and return ResOK. Otherwise, return a result code
describing the problem, for example ResCOMMIT_LIMIT.
.policy.alloc.impl: This tries various methods in succession until one succeeds. First, it tries to allocate from the arena’s free land in the requested zones. Second, it tries allocating from free zones. Third, it tries extending the arena and then trying the first two methods again. Fourth, it tries allocating from any zone that is not blacklisted. Fifth, it tries allocating from any zone at all.
.policy.alloc.issue: This plan performs poorly under stress. See for example job003898.
36.5.2. Deciding whether to collect the world¶
.policy.world: Determine whether now is a good time for
mps_arena_step() to start a collection of the world. Return
TRUE if so, FALSE if not. The availableTime argument is an
estimate of the time that’s available for the collection, now is
the current time as returned by ClockNow(), and clocks_per_sec
is the result of calling ClocksPerSec().
.policy.world.impl: There are two conditions: the estimate of the
available time must be enough to complete the collection, and the last
collection of the world must be long enough in the past that the
mps_arena_step() won’t be spending more than a certain fraction of
runtime in collections. (This fraction is given by the
ARENA_MAX_COLLECT_FRACTION configuration parameter.)
36.5.3. Starting a trace¶
.policy.start: Consider starting a trace. If a trace was started,
update *traceReturn to point to the trace and return TRUE.
Otherwise, leave *traceReturn unchanged and return FALSE.
.policy.start.impl: This uses the “Lisp Machine” strategy, which tries to schedule collections of the world so that the collector just keeps pace with the mutator: that is, it starts a collection when the predicted completion time of the collection is around the time when the mutator is predicted to reach the current memory limit. See [Pirinen].
.policy.start.chain: If it is not yet time to schedule a collection
of the world, PolicyStartTrace() considers collecting a set of
zones corresponding to a set of generations on a chain.
It picks these generations by calling 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
policyCondemnChain(), which 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 resulting condemned set includes 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.
36.5.4. Trace progress¶
.policy.poll: Return TRUE if the MPS should do some tracing work; FALSE if it should return to the mutator.
.policy.poll.again: Return TRUE if the MPS should do another unit
of work; FALSE if it should return to the mutator. start is the
clock time when the MPS was entered; moreWork and tracedWork
are the results of the last call to TracePoll().
.policy.poll.impl: The implementation keep doing work until either
the maximum pause time is exceeded (see design.mps.arena.pause-time_),
or there is no more work to do. Then it schedules the next collection
so that there is approximately one call to TracePoll() for every
ArenaPollALLOCTIME bytes of allocation.
36.6. References¶
- Pirinen
“The Lisp Machine Strategy”; Pekka Pirinin; 1998-04-27; <https://info.ravenbrook.com/project/mps/doc/2002-06-18/obsolete-mminfo/mminfo/strategy/lisp-machine/>
