.. highlight:: none


.. index::
   pair: MVT pool class; design
   single: pool class; MVT design

.. _design-poolmvt:



Manual Variable Temporal (MVT) pool design
==========================================

.. mps:prefix:: design.mps.poolmvt
   pair: MVT pool class; design
   single: pool class; MVT design


Introduction
------------

:mps:tag:`intro` This is a second-generation design for a pool that manually
manages variable-sized objects. It is intended as a replacement for
poolmv (except in its control pool role) and poolepdl, and it is
intended to satisfy the requirements of the Dylan "misc" pool and the
product malloc/new drop-in replacement.

:mps:tag:`readership` MM developers

:mps:tag:`source` req.dylan(6), req.epcore(16), req.product(2)

:mps:tag:`background` design.mps.poolmv, design.mps.poolepdl(0),
design.product.soft.drop(0), paper.wil95(1), paper.vo96(0),
paper.grun92(1), paper.beck82(0), `mail.ptw.1998-02-25.22-18`_.

.. _mail.ptw.1998-02-25.22-18: https://info.ravenbrook.com/project/mps/mail/1998/02/25/22-18/0.txt


Definitions
-----------

:mps:tag:`def.alignment` Alignment is a constraint on an object's address,
typically to be a power of 2 (see also, glossary.alignment )

:mps:tag:`def.bit-map` A bitmap is a boolean-valued vector (see also,
glossary.bitmap ).

:mps:tag:`def.block` A block is a contiguous extent of memory. In this
document, block is used to mean a contiguous extent of memory managed
by the pool for the pool client, typically a subset of a segment
(compare with :mps:ref:`.def.segment`).

:mps:tag:`def.cartesian-tree` A cartesian tree is a binary tree ordered by
two keys (paper.stephenson83(0)).

:mps:tag:`def.crossing-map` A mechanism that supports finding the start of
an object from any address within the object, typically only required
on untagged architectures (see also, glossary.crossing.map ).

:mps:tag:`def.footer` A block of descriptive information describing and
immediately following another block of memory (see also
:mps:ref:`.def.header`).

:mps:tag:`def.fragmentation` Fragmented memory is memory reserved to the
program but not usable by the program because of the arrangement of
memory already in use (see also, glossary.fragmentation ).

:mps:tag:`def.header` A block of descriptive information describing and
immediately preceding another block of memory (see also,
glossary.in-band.header ).

:mps:tag:`def.in-band` From "in band signalling", when descriptive
information about a data structure is stored in the data structure
itself (see also, glossary.in-band.header ).

:mps:tag:`def.out-of-band` When descriptive information about a data
structure is stored separately from the structure itself (see also,
glossary.out-of-band.header ).

:mps:tag:`def.refcount` A refcount is a count of the number of users of an
object (see also, glossary.reference.count ).

:mps:tag:`def.segment` A segment is a contiguous extent of memory. In this
document, segment is used to mean a contiguous extent of memory
managed by the MPS arena (design.mps.arena_) and subdivided by the
pool to provide blocks (see :mps:ref:`.def.block`) to its clients.

.. _design.mps.arena: arena.html

:mps:tag:`def.splay-tree` A splay tree is a self-adjusting binary tree
(paper.st85(0), paper.sleator96(0)).

:mps:tag:`def.splinter` A splinter is a fragment of memory that is too small
to be useful (see also, glossary.splinter )

:mps:tag:`def.subblock` A subblock is a contiguous extent of memory. In this
document, subblock is used to mean a contiguous extent of memory
manage by the client for its own use, typically a subset of a block
(compare with :mps:ref:`.def.block`).


Abbreviations
-------------

:mps:tag:`abbr.abq` ABQ = Available Block Queue

:mps:tag:`abbr.ap` AP = Allocation Point

:mps:tag:`abbr.cbs` CBS = Coalescing Block Structure

:mps:tag:`abbr.mps` MPS = Memory Pool System

:mps:tag:`abbr.ps` PS = PostScript



Overview
--------

:mps:tag:`overview` MVT is intended to satisfy the requirements of the
clients that need manual-variable pools, improving on the performance
of the existing manual-variable pool implementations, and reducing the
duplication of code that currently exists. The expected clients of MVT
are: Dylan (currently for its misc pool), EP (particularly the dl
pool, but all pools other than the PS object pool), and Product
(initially the malloc/new pool, but also other manual pool classes).


Requirements
------------

:mps:tag:`req.cat` Requirements are categorized per guide.req(2).

:mps:tag:`req.risk` req.epcore(16) is known to be obsolete, but the revised
document has not yet been accepted.


Critical requirements
.....................

:mps:tag:`req.fun.man-var` The pool class must support manual allocation and
freeing of variable-sized blocks (source: req.dylan.fun.misc.alloc,
req.epcore.fun.{dl,gen,tmp,stat,cache,trap}.{alloc,free},
req.product.fun.{malloc,new,man.man}).

:mps:tag:`non-req.fun.gc` There is not a requirement that the pool class
support formatted objects, scanning, or collection objects; but it
should not be arbitrarily precluded.

:mps:tag:`req.fun.align` The pool class must support aligned allocations to
client-specified alignments. An individual instance need only support
a single alignment; multiple instances may be used to support more
than one alignment (source: req.epcore.attr.align).

:mps:tag:`req.fun.reallocate` The pool class must support resizing of
allocated blocks (source req.epcore.fun.dl.promise.free,
req.product.dc.env.{ansi-c,cpp}).

:mps:tag:`non-req.fun.reallocate.in-place` There is not a requirement blocks
must be resized in place (where possible); but it seems like a good
idea.

:mps:tag:`req.fun.thread` Each instance of the pool class must support
multiple threads of allocation (source req.epcore.fun.dl.multi,
req.product.dc.env.{ansi-c,cpp}).

:mps:tag:`req.attr.performance` The pool class must meet or exceed
performance of "competitive" allocators (source:
rec.epcore.attr.{run-time,tp}, req.product.attr.{mkt.eval, perform}).
[Dylan does not seem to have any requirement that storage be allocated
with a particular response time or throughput, just so long as we
don't block for too long. Clearly there is a missing requirement.]

:mps:tag:`req.attr.performance.time` By inference, the time overhead must be
competetive.

:mps:tag:`req.attr.performance.space` By inference, the space overhead must
be competetive.

:mps:tag:`req.attr.reliability` The pool class must have "rock-solid
reliability" (source: req.dylan.attr.rel.mtbf, req.epcore.attr.rel,
req.product.attr.rel).

:mps:tag:`req.fun.range` The pool class must be able to manage blocks
ranging in size from 1 byte to all of addressable memory
(req.epcore.attr.{dl,gen,tmp,stat,cache,trap}.obj.{min,max}. The range
requirement may be satisfied by multiple instances each managing a
particular client-specified subrange of sizes. [Dylan has requirements
req.dylan.attr.{capacity,obj.max}, but no requirement that such
objects reside in a manual pool.]

:mps:tag:`req.fun.debug` The pool class must support debugging erroneous
usage by client programs (source: req.epcore.fun.{dc.variety,
debug.support}, req.product.attr.{mkt.eval,perform}). Debugging is
permitted to incur additional overhead.

:mps:tag:`req.fun.debug.boundaries` The pool class must support checking for
accesses outside the boundaries of live objects.

:mps:tag:`req.fun.debug.log` The pool class must support logging of all
allocations and deallocations.

:mps:tag:`req.fun.debug.enumerate` The pool class must support examining all
allocated objects.

:mps:tag:`req.fun.debug.free` The pool class must support detecting
incorrect, overlapping, and double frees.

:mps:tag:`req.fun.tolerant` The pool class must support tolerance of
erroneous usage (source req.product.attr.use.level.1).


Essential requirements
......................

:mps:tag:`req.fun.profile` The pool class should support memory usage
profiling (source: req.product.attr.{mkt.eval, perform}).

:mps:tag:`req.attr.flex` The pool class should be flexible so that it can be
tuned to specific allocation and freeing patterns (source:
req.product.attr.flex,req.epcore.attr.{dl,cache,trap}.typ). The
flexibility requirement may be satisfied by multiple instances each
optimizing a specific pattern.

:mps:tag:`req.attr.adapt` The pool class should be adaptive so that it can
accommodate changing allocation and freeing patterns (source:
req.epcore.fun.{tmp,stat}.policy,
req.product.attr.{mkt.eval,perform}).


Nice requirements
.................

:mps:tag:`req.fun.suballocate` The pool class may support freeing of any
aligned, contiguous subset of an allocated block (source
req.epcore.fun.dl.free.any, req.product.attr.{mkt.eval,perform}).


Architecture
------------

:mps:tag:`arch.overview` The pool has several layers: client allocation is
by Allocation Points (APs).

:mps:tag:`arch.overview.ap` APs acquire storage from the pool
available-block queue (ABQ).

:mps:tag:`arch.overview.abq` The ABQ holds blocks of a minimum configurable
size: "reuse size".

:mps:tag:`arch.overview.storage` The ABQ acquires storage from the arena,
and from its internal free block managers.

:mps:tag:`arch.overview.storage.contiguous` The arena storage is requested
to be contiguous to maximize opportunities for coalescing (Loci will
be used when available).

:mps:tag:`arch.overview.cbs` The free block managers hold blocks freed by
the client until, through coalescing, they have reached the reuse
size, at which point they are made available on the ABQ.

:mps:tag:`arch.ap` The pool will use allocation points as the allocation
interface to the client.

:mps:tag:`arch.ap.two-phase` Allocation points will request blocks from the
pool and suballocate those blocks (using the existing AP, compare and
increment, 2-phase mechanism) to satisfy client requests.

:mps:tag:`arch.ap.fill` The pool will have a configurable "fill size" that
will be the preferred size block used to fill the allocation point.

:mps:tag:`arch.ap.fill.size` The fill size should be chosen to amortize the
cost of refill over a number of typical reserve/commit operations, but
not so large as to exceed the typical object population of the pool.

:mps:tag:`arch.ap.no-fit` When an allocation does not fit in the remaining
space of the allocation point, there may be a remaining fragment.

:mps:tag:`arch.ap.no-fit.sawdust` If the fragment is below a configurable
threshold (minimum size), it will be left unused (but returned to the
free block managers so it will be reclaimed when adjacent objects are
freed).

:mps:tag:`arch.ap.no-fit.splinter` otherwise, the remaining fragment will be
(effectively) returned to the head of the available-block queue, so
that it will be used as soon as possible (that is, by objects of similar
birthdate).

:mps:tag:`arch.ap.no-fit.oversize` If the requested allocation exceeds the
fill size it is treated exceptionally (this may indicate the client
has either misconfigured or misused the pool and should either change
the pool configuration or create a separate pool for these exceptional
objects for best performance).

:mps:tag:`arch.ap.no-fit.oversize.policy` Oversize blocks are assumed to
have exceptional lifetimes, hence are allocated to one side and do not
participate in the normal storage recycling of the pool.

:mps:tag:`arch.ap.refill.overhead` If reuse size is small, or becomes small
due to :mps:ref:`.arch.adapt`, all allocations will effectively be treated
exceptionally (the AP will trip and a oldest-fit block will be chosen
on each allocation). This mode will be within a constant factor in
overhead of an unbuffered pool.

:mps:tag:`arch.abq` The available block queue holds blocks that have
coalesced sufficiently to reach reuse size.

:mps:tag:`arch.abq.reuse.size` A multiple of the quantum of virtual memory
is used as the reuse size (:mps:ref:`.analysis.policy.size`).

:mps:tag:`arch.abq.fifo` It is a FIFO queue (recently coalesced blocks go to
the tail of the queue, blocks are taken from the head of the queue for
reuse).

:mps:tag:`arch.abq.delay-reuse` By thus delaying reuse, coalescing
opportunities are greater.

:mps:tag:`arch.abq.high-water` It has a configurable high water mark, which
when reached will cause blocks at the head of the queue to be returned
to the arena, rather than reused.

:mps:tag:`arch.abq.return` When the MPS supports it, the pool will be able
to return free blocks from the ABQ to the arena on demand.

:mps:tag:`arch.abq.return.segment` :mps:ref:`.arch.abq.return` can be guaranteed to
be able to return a segment by setting reuse size to twice the size of
the segments the pool requests from the arena.

:mps:tag:`arch.cbs` The coalescing block structure holds blocks that have
been freed by the client.

:mps:tag:`arch.cbs.optimize` The data structure is optimized for coalescing.

:mps:tag:`arch.cbs.abq` When a block reaches reuse size, it is added to the
ABQ.

:mps:tag:`arch.cbs.data-structure` The data structures are organized so that
a block can be both in the free block managers and on the ABQ
simultaneously to permit additional coalescing, up until the time the
block is removed from the ABQ and assigned to an AP.

:mps:tag:`arch.fragmentation.internal` Internal fragmentation results from
The pool will request large segments from the arena to minimize the
internal fragmentation due to objects not crossing segment boundaries.

:mps:tag:`arch.modular` The architecture will be modular, to allow building
variations on the pool by assembling different parts.

:mps:tag:`arch.modular.example` For example, it should be possible to build
pools with any of the freelist mechanisms, with in-band or out-of-band
storage (where applicable), that do or do not support derived object
descriptions, etc.

:mps:tag:`arch.modular.initial` The initial architecture will use
:mps:ref:`.sol.mech.free-list` for the free block managers,
:mps:ref:`.sol.mech.storage.out-of-band`, :mps:ref:`.sol.mech.desc.derived`, and
:mps:ref:`.sol.mech.allocate.buffer`.

:mps:tag:`arch.segregate` The architecture will support segregated
allocation through the use of multiple allocation points. The client
will choose the appropriate allocation point either at run time, or
when possible, at compile time.

:mps:tag:`arch.segregate.initial` The initial architecture will segregate
allocations into two classes: large and small. This will be
implemented by creating two pools with different parameters.

:mps:tag:`arch.segregate.initial.choice` The initial architecture will
provide glue code to choose which pool to allocate from at run time.
If possible this glue code will be written in a way that a good
compiler can optimize the selection of pool at compile time.
Eventually this glue code should be subsumed by the client or
generated automatically by a tool.

:mps:tag:`arch.debug` Debugging features such as tags, fenceposts, types,
creators will be implemented in a layer above the pool and APs. A
generic pool debugging interface will be developed to support
debugging in this outer layer.

:mps:tag:`arch.debug.initial` The initial architecture will have counters
for objects/bytes allocated/freed and support for detecting
overlapping frees.

:mps:tag:`arch.dependency.loci` The architecture depends on the arena being
able to efficiently provide segments of varying sizes without
excessive fragmentation. The locus mechanism should satisfy this
dependency. (See :mps:ref:`.analysis.strategy.risk`.)

:mps:tag:`arch.dependency.mfs` The architecture internal data structures
depend on efficient manual management of small, fixed-sized objects (2
different sizes). The MFS pool should satisfy this dependency.

:mps:tag:`arch.contingency` Since the strategy we propose is new, it may not
work.

:mps:tag:`arch.contingency.pathological` In particular, pathological
allocation patterns could result in fragmentation such that no blocks
recycle from the free bock managers to the ABQ.

:mps:tag:`arch.contingency.fallback` As a fallback, there will be a pool
creation parameter for a high water mark for the free space.

:mps:tag:`arch.contingency.fragmentation-limit` When the free space as a
percentage of all the memory managed by the pool (a measure of
fragmentation) reaches that high water mark, the free block managers
will be searched oldest-fit before requesting additional segments from
the arena.

:mps:tag:`arch.contingency.alternative` We also plan to implement
:mps:ref:`.sol.mech.free-list.cartesian-tree` as an alternative free block
manager, which would permit more efficient searching of the free
blocks.

:mps:tag:`arch.parameters` The architecture supports several parameters so
that multiple pools may be instantiated and tuned to support different
object cohorts. The important parameters are: reuse size, minimum
size, fill size, ABQ high water mark, free block fragmentation limit
(see :mps:ref:`.arch.contingency.fragmentation-limit`).

:mps:tag:`arch.parameters.client-visible` The client-visible parameters of
the pool are the minimum object size, the mean object size, the
maximum object size, the reserve depth and fragmentation limit. The
minimum object size determines when a splinter is kept on the head of
the ABQ (:mps:ref:`.arch.ap.no-fit.splinter`). The maximum object size
determines the fill size (:mps:ref:`.arch.ap.fill.size`) and hence when a
block is allocated exceptionally (:mps:ref:`.arch.ap.no-fit.oversize`). The
mean object size is the most likely object size. The reserve depth is
a measure of the hysteresis of the object population. The mean object
size, reserve depth and, maximum object size are used to determine the
size of the ABQ (:mps:ref:`.arch.abq.high-water`). The fragmentation limit is
used to determine when contingency mode is used to satisfy an
allocation request (:mps:ref:`.arch.contingency`).

:mps:tag:`arch.adapt` We believe that an important adaptation to explore is
tying the reuse size inversely to the fragmentation (as measured in
:mps:ref:`.arch.contingency.fragmentation-limit`).

:mps:tag:`arch.adapt.reuse` By setting reuse size low when fragmentation is
high, smaller blocks will be available for reuse, so fragmentation
should diminish.

:mps:tag:`arch.adapt.overhead` This will result in higher overhead as the AP
will need to be refilled more often, so reuse size should be raised
again as fragmentation diminishes.

:mps:tag:`arch.adapt.oldest-fit` In the limit, if reuse size goes to zero,
the pool will implement a "oldest-fit" policy: the oldest free block
of sufficient size will be used for each allocation.

:mps:tag:`arch.adapt.risk` This adaptation is an experimental policy and
should not be delivered to clients until thoroughly tested.


Analysis
--------

:mps:tag:`analysis.discard` We have discarded many traditional solutions based
on experience and analysis in paper.wil95(1). In particular, managing
the free list as a linear list arranged by address or size and basing
policy on searching such a linear list in a particular direction, from
a particular starting point, using fit and/or immediacy as criteria.
We believe that none of these solutions is derived from considering
the root of the problem to be solved (as described in
:mps:ref:`.analysis.strategy`), although their behavior as analyzed by Wilson
gives several insights.

:mps:tag:`analysis.strategy` For any program to run in the minimum required
memory (with minimal overhead -- we discard solutions such as
compression for now), fragmentation must be eliminated. To eliminate
fragmentation, simply place blocks in memory so that they die "in
order" and can be immediately coalesced. This ideal is not achievable,
but we believe we can find object attributes that correlate with
deathtime and exploit them to approximate the ideal. Initially we
believe birth time and type (as approximated by size) will be useful
attributes to explore.

:mps:tag:`analysis.strategy.perform` To meet :mps:ref:`.req.attr.performance`, the
implementation of :mps:ref:`.sol.strategy` must be competitive in both time
and space.

:mps:tag:`analysis.strategy.risk` The current MPS segment substrate can cause
internal fragmentation which an individual pool can do nothing about.
We expect that request.epcore.170193.sugg.loci_ will be implemented to
remove this risk.

.. _`request.epcore.170193.sugg.loci`: https://info.ravenbrook.com/project/mps/import/2001-11-05/mmprevol/request/epcore/170193/

:mps:tag:`analysis.policy` Deferred coalescing, when taken to the extreme will
not minimize the memory consumption of a program, as no memory would
ever be reused. Eager reuse appears to lead to more fragmentation,
whereas delayed reuse appears to reduce fragmentation
(paper.wil95(1)). The systems studied by Wilson did not directly
address deferring reuse. Our proposed policy is to reuse blocks when
they reach a (configurable) size. We believe that this policy along
with the policy of segregating allocations by death time, will greatly
reduce fragmentation.

:mps:tag:`analysis.policy.risk` This policy could lead to pathological behavior
if allocations cannot be successfully segregated.

:mps:tag:`analysis.policy.allocate.segregate` This policy has some similarities
to CustomAlloc (paper.grun92(1)). CustomAlloc segregates objects by
size classes, and then within those classes chooses a different
allocator depending on whether that size class has a stable or
unstable population. Classes with stable population recycle storage
within the class, whereas classes with unstable populations return
their storage to the general allocation pool for possible reuse by
another class. CustomAlloc, however, requires profiling the
application and tuning the allocator according to those profiles.
Although we intend to support such tuning, we do not want to require
it.

:mps:tag:`analysis.policy.reallocate` For reallocation, :mps:ref:`.req.fun.suballocate`
can be used to free the remainder if a block is made smaller. Doing so
will cause the freed block to obey :mps:ref:`.sol.policy.allocate` (that is,
the freed block will not be treated specially, it will be subject to
the normal policy on reuse). Copying can be used if a block is made
larger. paper.vo96(0) reports success in over-allocating a block the
first time it is resized larger, presumably because blocks that are
resized once tend to be resized again and over-allocating may avoid a
subsequent copy. If each object that will be reallocated can be given
its own allocation point until its final reallocation, the allocation
point can be used to hold released or spare storage.

:mps:tag:`analysis.policy.size` We believe that this will take advantage of the
underlying virtual memory system's ability to compact the physical
memory footprint of the program by discarding free fragments that
align with the virtual memory quantum. (In a VM system one can
approximate compaction by sparse mapping. If every other page of a
segment is unused, the unused pages can be unmapped, freeing up
physical memory that can be mapped to a new contiguous vm range.)

:mps:tag:`analysis.mech.free-list` The literature (paper.grun92(1),
paper.vo96(0)) indicate that :mps:ref:`.sol.mech.free-list.cartesian-tree`
provides a space-efficient implementation at some cost in speed.
:mps:ref:`.sol.mech.free-list.splay-tree` is faster but less space-efficient.
:mps:ref:`.sol.mech.free-list.bitmap` is unstudied. Many of the faster
allocators maintain caches of free blocks by size to speed allocation
of "popular" sizes. We intend to initially explore not doing so, as we
believe that policy ultimately leads to fragmentation by mixing
objects of varying death times. Instead we intend to use a free list
mechanism to support fast coalescing, deferring reuse of blocks until
a minimum size has been reached.

:mps:tag:`analysis.mech.allocate.optimize-small` Wilson (paper.wil95(1)) notes
that small blocks typically have short lifetimes and that overall
performance is improved if you optimize the management of small
blocks, e.g., :mps:ref:`.sol.mech.allocate.lookup-table` for all small blocks.
We believe that :mps:ref:`.sol.mech.allocate.buffer` does exactly that.

:mps:tag:`analysis.mech.allocate.optimize-new` Wilson (paper.wil95(1)) reports
some benefit from "preserving wilderness", that is, when a block of
memory must be requested from the system to satisfy an allocation,
only the minimum amount of that block is used, the remainder is
preserved (effectively by putting it at the tail of the free list).
This mechanism may or may not implement :mps:ref:`.sol.policy.allocate`. We
believe a better mechanism is to choose to preserve or not, based on
:mps:ref:`.sol.policy.allocate`.


Ideas
-----

:mps:tag:`sol` Many solution ideas for manual management of variable-sized
memory blocks are enumerated by paper.wil95(1). Here we list the most
promising, and some of our own.


Strategy
........

:mps:tag:`sol.strategy` To run a program in the minimal required memory,
with minimal overhead, utilize memory efficiently. Memory becomes
unusable when fragmented. Strategy is to minimize fragmentation. So
place blocks where they won't cause fragmentation later.

:mps:tag:`sol.strategy.death` Objects that will die together (in time)
should be allocated together (in space); thus they will coalesce,
reducing fragmentation.

:mps:tag:`sol.strategy.death.birth` Assume objects allocated near each other
in time will have similar deathtimes (paper.beck82(0)).

:mps:tag:`sol.strategy.death.type` Assume objects of different type may have
different deathtimes, even if born together.

:mps:tag:`sol.strategy.death.predict` Find and use program features to predict deathtimes.

:mps:tag:`sol.strategy.reallocate` Reallocation implies rebirth, or at least
a change in lifetime

:mps:tag:`sol.strategy.debug` As much of the debugging functionality as
possible should be implemented as a generally available MPS utility;
the pool will provide support for debugging that would be expensive or
impossible to allocate outside the pool.


Policy
......

Policy is an implementable decision procedure, hopefully approximating
the strategy.

:mps:tag:`sol.policy.reuse` Defer reusing blocks, to encourage coalescing.

:mps:tag:`sol.policy.split` When a block is split to satisfy an allocation,
use the remainder as soon as possible.

:mps:tag:`sol.policy.size` Prevent :mps:ref:`.sol.policy.reuse` from consuming all
of memory by choosing a (coalesced) block for reuse when it reaches a
minimum size.

:mps:tag:`sol.policy.size.fixed` Use the quantum of virtual memory (e.g.,
one page) as minimum size.

:mps:tag:`sol.policy.size.tune` Allow tuning minimum size.

:mps:tag:`sol.policy.size.adapt` Adaptively change minimum size.

:mps:tag:`sol.policy.allocate` Allocate objects with similar birthdate and
lifetime together.

:mps:tag:`sol.policy.allocate.segregate` Segregate allocations by type.

:mps:tag:`sol.policy.allocate.segregate.size` Use size as a substitute for type.

:mps:tag:`sol.policy.allocate.segregate.tune` Permit tuning of segregation.

:mps:tag:`sol.policy.allocate.segregate.adapt` Adaptively segregate allocations.

:mps:tag:`sol.policy.reallocate` Implement reallocation in a central
mechanism outside of the pool, create a generic pool interface in
support of same.

:mps:tag:`sol.policy.debug` Implement a pool debugging interface.

:mps:tag:`sol.policy.debug.counters` Implement debugging counters in the
pool that are queried with a generic interface.

:mps:tag:`sol.policy.debug.verify` Implement debugging error returns on
overlapping frees.


Mechanism
.........

Mechanisms are algorithms or data structures used to implement policy.

:mps:tag:`sol.mech.free-list` Mechanisms that can be used to describe the
free list.

:mps:tag:`sol.mech.free-list.cartesian-tree` Using address and size as keys
supports fast coalescing of adjacent blocks and fast searching for
optimal-sized blocks. Unfortunately, because the shape of the tree is
constrained by the second key, it can become unbalanced. This data
structure is used in the SunOS 4.1 malloc (paper.grun92(1)).

:mps:tag:`sol.mech.free-list.splay-tree` The amortized cost of a splay tree
is competitive with balanced binary trees in the worst case, but can
be significantly better for regular patterns of access because
recently-accessed keys are moved to the root of the tree and hence can
be re-accessed quickly. This data structure is used in the System Vr4
malloc (paper.vo96(0)). (For a complete analysis of the splay tree
algorithm time bounds see paper.st85(0).)

:mps:tag:`sol.mech.free-list.bitmap` Using address as an index and
fix-sized blocks, the booleans can represent whether a block is free
or not. Adjacent blocks can be used to construct larger blocks.
Efficient algorithms for searching for runs in a vector are known.
This data structure is used in many file system disk block managers.

:mps:tag:`sol.mech.free-list.refcount` A count of the number of allocated
but not freed subblocks of a block can be used to determine when a
block is available for reuse. This is an extremely compact data
structure, but does not support subblock reuse.

:mps:tag:`sol.mech.free-list.hybrid` Bitmaps appear suited particularly to
managing small, contiguous blocks. The tree structures appear suited
particularly to managing varying-sized, discontiguous blocks. A
refcount can be very efficient if objects can be placed accurately
according to death time. A hybrid mechanism may offer better
performance for a wider range of situations.

:mps:tag:`sol.mech.free-list.linked` An address-ordered singly linked free
list using space in each free block to store the block's size and a
pointer to the next block.

:mps:tag:`sol.mech.storage` Methods that can be used to store the free list description.

:mps:tag:`sol.mech.storage.in-band` The tree data structures (and
:mps:ref:`.sol.mech.free-list.linked`) are amenable to being stored in the
free blocks themselves, minimizing the space overhead of management.
To do so imposes a minimum size on free blocks and reduces the
locality of the data structure.

:mps:tag:`sol.mech.storage.out-of-band` The bit-map data structure must be
stored separately.

:mps:tag:`sol.mech.desc` for an allocated block to be freed, its base and
bound must be known

:mps:tag:`sol.mech.desc.derived` Most clients can supply the base of the
block. Some clients can supply the bound.

:mps:tag:`sol.mech.desc.in-band` When the bound cannot be supplied, it can
be stored as an in-band "header". If neither the base nor bound can be
supplied (e.g., the client may only have an interior pointer to the
block), a header and footer may be required.

:mps:tag:`sol.mech.desc.out-of-band` In un-tagged architectures, it may be
necessary to store the header and footer out-of-band to distinguish
them from client data. Out-of-band storage can improve locality and
reliability. Any of the free-list structures can also be used to
describe allocated blocks out-of-band.

:mps:tag:`sol.mech.desc.crossing-map` An alternative for untagged
architectures is to store a "crossing map" which records an encoding
of the start of objects and then store the descriptive information
in-band.

:mps:tag:`sol.mech.allocate` Mechanisms that can be used to allocate blocks
(these typically sit on top of a more general free-list manager).

:mps:tag:`sol.mech.allocate.lookup-table` Use a table of popular sizes to
cache free blocks of those sizes.

:mps:tag:`sol.mech.allocate.buffer` Allocate from contiguous blocks using
compare and increment.

:mps:tag:`sol.mech.allocate.optimize-small` Use a combination of techniques
to ensure the time spent managing a block is small relative to the
block's lifetime; assume small blocks typically have short lifetimes.

:mps:tag:`sol.mech.allocate.optimize-new` When "virgin" memory is acquired
from the operating system to satisfy a request, try to preserve it
(that is, use only what is necessary).

:mps:tag:`sol.mech.allocate.segregate.size` Use size as a substitute for
type.

:mps:tag:`sol.mech.reallocate` use :mps:ref:`.req.fun.suballocate` to return unused
memory when a block shrinks, but differentiate this from an erroneous
overlapping free by using separate interfaces.



Implementation
--------------

The implementation consists of the following separable modules:


Splay Tree
..........

:mps:tag:`impl.c.splay` The implementation of
:mps:ref:`.sol.mech.free-list.splay-tree`. See design.mps.splay_.

.. _design.mps.splay: splay.txt.html


Coalescing Block Structure
..........................

:mps:tag:`impl.c.cbs` The initial implementation will use
:mps:ref:`.sol.mech.free-list.splay-tree` and
:mps:ref:`.sol.mech.storage.out-of-band`. For locality, this storage should be
managed as a linked free list of splay nodes suballocated from blocks
acquired from a pool shared by all CBS's. Must support creation and
destruction of an empty tree. Must support search, insert and delete
by key of type Addr. Must support finding left and right neighbors of
a failed search for a key. Must support iterating over the elements of
the tree with reasonable efficiency. Must support storing and
retrieving a value of type Size associated with the key. Standard
checking and description should be provided. See design.mps.cbs_.

.. _design.mps.cbs: cbs.txt.html


Fail-over to address-ordered free list
......................................

:mps:tag:`impl.c.freelist` Because the CBS uses out-of-band storage, it may
be unable to handle insert (design.mps.cbs.function.cbs.insert.fail_)
and delete (design.mps.cbs.function.cbs.delete.fail_) operations. When
this happen MVT fails over to an address-ordered singly linked free
list. This uses in-band storage and so cannot run out of memory, but
it has much worse performance than the CBS. Therefore MVT eagerly
attempts to flush blocks from the free list back to the CBS. See
design.mps.freelist_ for the design and implementation of the free
list.

.. _design.mps.cbs.function.cbs.delete.fail: cbs.html#design.mps.cbs.function.cbs.delete.fail
.. _design.mps.cbs.function.cbs.insert.fail: cbs.html#design.mps.cbs.function.cbs.insert.fail
.. _design.mps.freelist: freelist.html


Available Block Queue
.....................

:mps:tag:`impl.c.abq` The initial implementation will be a queue of fixed
size (determined at pool creation time from the high water mark). Must
support creation and destruction of an empty queue. Must support
insertion at the head or tail of the queue (failing if full), peeking
at the head of the queue, and removal of the head (failing if empty)
or any element of the queue (found by a search). Standard checking and
description should be provided. See design.mps.abq_.

.. _design.mps.abq: abq.txt.html


Pool implementation
...................

:mps:tag:`impl.c` The initial implementation will use the above modules to
implement a buffered pool. Must support creation and destruction of
the pool. Creation takes parameters: minimum size, mean size, maximum
size, reserve depth and fragmentation limit. Minimum, mean, and
maximum size are used to calculate the internal fill and reuse sizes.
Reserve depth and mean size are used to calculate the ABQ high water
mark. Fragmentation limit is used to set the contingency mode. Must
support buffer initialization, filling and emptying. Must support
freeing. Standard checking and description should be provided.
[Eventually, it should support scanning, so it can be used with
collected pools, but no manual pool currently does.]

:mps:tag:`impl.c.future` The implementation should not preclude "buffered
free" (`mail.ptw.1997-12-05.19-07`_) being added in the future.

.. _mail.ptw.1997-12-05.19-07: https://info.ravenbrook.com/project/mps/mail/1997/12/05/19-07/0.txt

:mps:tag:`impl.c.parameters` The pool parameters are calculated as follows
from the input parameters: minimum, mean, and maximum size are taked
directly from the parameters.

:mps:tag:`impl.c.parameter.fill-size` The fill size is set to the maximum
size times the reciprocal of the fragmentation limit, rounded up to
the arena grain size.

:mps:tag:`imple.c.parameter.reuse-size` The reuse size is set to twice the
fill size (see :mps:ref:`.arch.abq.return.segment`,
:mps:ref:`.impl.c.free.merge.segment`).

:mps:tag:`impl.c.parameter.abq-limit` The ABQ high-water limit is set to the
reserve depth times the mean size (that is, the queue should hold as
many reuse blocks as would take to cover the population hysteresis if
the population consisted solely of mean-sized blocks, see
:mps:ref:`.arch.abq.high-water`).

:mps:tag:`impl.c.parameter.avail-limit` The free block high-water limit is
implemented by comparing the available free space to an "available
limit". The available limit is updated each time a segment is
allocated from or returned to the arena by setting it to the total
size of the pool times the fragmentation limit divide vy 100 (see
:mps:ref:`.arch.contingency.fallback`).

:mps:tag:`impl.c.ap.fill` An AP fill request will be handled as follows:

- If the request is larger than fill size, attempt to request a
  segment from the arena sufficient to satisfy the request.

- Use any previously returned splinter (from :mps:ref:`.impl.c.ap.empty`), if
  large enough.

- Attempt to retrieve a free block from the head of the ABQ (removing
  it from ABQ and the free block managers if found).

- If above fragmentation limit, attempt to find a block in the free
  block managers, using oldest-fit search.

- Attempt to request a segment of fill size from the arena.

- Attempt to find a block in the free block managers, using oldest-fit
  search.

- Otherwise, fail.

:mps:tag:`impl.c.ap.empty` An AP empty request will be handled as follows:

- If remaining free is less than min size, return it to the free block
  managers.

- If the remaining free is larger than any previous splinter, return
  that splinter to the free block managers and save this one for use
  by a subsequent fill.

- Otherwise return the remaining block to the free block managers.

:mps:tag:`impl.c.free` When blocks are returned to the free block managers
they may be merged with adjacent blocks. If a merge occurs with a
block on the ABQ, the ABQ must be adjusted to reflect the merge.

:mps:tag:`impl.c.free.exception` Exceptional blocks are returned directly to
the arena.

:mps:tag:`impl.c.free.merge` If a merge occurs and the merged block is
larger than reuse size:

- If the ABQ is full, remove the block at the head of the ABQ from the
  ABQ and the free block managers and return it to the arena(*).

- Insert the newly merged block at the tail of the ABQ, leaving it in
  the free block managers for further merging.

:mps:tag:`impl.c.free.merge.segment` (*) Merged blocks may not align with
arena segments. If necessary, return the interior segments of a block
to the arena and return the splinters to the free block managers.

:mps:tag:`impl.c.free.merge.segment.reuse` If the reuse size (the size at
which blocks recycle from the free block managers to the ABQ) is at
least twice the fill size (the size of segments the pool allocates
from the arena), we can guarantee that there will always be a
returnable segment in every ABQ block.

:mps:tag:`impl.c.free.merge.segment.overflow` If the reuse size is set
smaller (see :mps:ref:`.arch.adapt`), there may not be a returnable segment in
an ABQ block, in which case the ABQ has "overflowed". Whenever this
occurs, the ABQ will be refilled by searching the free block managers
for dropped reusable blocks when needed.

:mps:tag:`impl.c.free.merge.segment.risk` The current segment structure does
not really support what we would like to do. Loci should do better:
support reserving contiguous address space and mapping/unmapping any
portion of that address space.

:mps:tag:`impl.c.free.merge.alternative` Alternatively, if the MPS segment
substrate permitted mapping/unmapping of pages, the pool could use
very large segments and map/unmap pages as needed.


AP Dispatch
...........

:mps:tag:`impl.c.multiap` The initial implementation will be a glue layer
that selects among several AP's for allocation according to the
predicted deathtime (as approximated by size) of the requested
allocation. Each AP will be filled from a pool instance tuned to the
range of object sizes expected to be allocated from that AP. [For
bonus points provide an interface that creates a batch of pools and
AP's according to some set of expected object sizes. Eventually expand
to understand object lifetimes and general lifetime prediction keys.]

:mps:tag:`impl.c.multiap.sample-code` This glue code is not properly part of
the pool or MPS interface. It is a layer on top of the MPS interface,
intended as sample code for unsophisticated clients. Sophisticated
clients will likely want to choose among multiple AP's more directly.


Testing
-------

:mps:tag:`test.component` Components :mps:ref:`.impl.c.splay`, :mps:ref:`.impl.c.cbs`, and
:mps:ref:`.impl.c.abq` will be subjected to individual component tests to
verify their functionality.

:mps:tag:`test.qa` Once poolmvt is integrated into the MPS, the standard MPS
QA tests will be applied to poolmvt prior to each release.

:mps:tag:`test.customer` Customer acceptance tests will be performed on a
per-customer basis before release to that customer (cf.
proc.release.epcore(2).test)


Text
----

Possible tweaks (from `mail.pekka.1998-04-15.13-10`_):

.. _mail.pekka.1998-04-15.13-10: https://info.ravenbrook.com/project/mps/mail/1998/04/15/13-10/0.txt

- Try to coalesce splinters returned from AP's with the front (or any)
  block on the ABQ.

- Sort ABQ in some other way to minimize splitting/splinters. For
  example, proximity to recently allocated blocks.