Design of the Manually-Managed Variable-Size First-Fit Pool
===========================================================
:author: Gavin Matthews
:date: 1998-09-09
:organization: Harlequin
:status: incomplete doc
:revision: $Id$
Introduction
------------
_`.intro`: The pool was created in a response to a belief that the
ScriptWorks EPDL/EPDR's first fit policy is beneficial for some classes
of client behaviour, but the performance of a linear free list was
unacceptable. This pool implements a first (or last) fit policy for
variable-sized manually-managed objects, with control over first/last,
segment preference high/low, and slot fit low/high.
Document History
................
_`.hist.0`: GavinM wrote a list of methods and function plus some notes 1998-09-09.
_`.hist.1`: Added overview, removed bogus ArenaEnter design, and described
buffered allocation. pekka 1999-01-06
_`.hist.2`: Modified for the "Sunset On Segments" redesign of segments. Buffered
allocation is no longer limited to segment boundaries.
Overview
--------
_`.over`: This pool implements certain variants of the address-ordered first-fit
policy. The implementation allows allocation across segment boundaries.
_`.over.buffer`: Buffered allocation is also supported, but in that case, the
buffer-filling policy is worst-fit. Buffered and unbuffered allocation can be
used at the same time, but in that case, the first ap must be created before
any allocations.
_`.over.buffer.class`: The pool uses the simplest buffer class,
BufferClass. This is appropriate since these buffers don't attach to segments,
and hence don't constrain buffered regions to lie within segment boundaries.
_`.over.segments`: The pool uses the simplest segment class (SegClass). There's no
need for anything more complex.
Methods
-------
_`.method`: The MVFF pool supports the following methods:
_`.method.init`: ``Res MVFFInit(Pool pool, va_list arg)``
This takes six `keyword arguments`_:
``MPS_KEY_EXTEND_BY``
the segment size
``MPS_KEY_MEAN_SIZE``
the average object size
``MPS_KEY_ALIGN``
the alignment of allocations and frees (must be at least sizeof(void*))
``MPS_KEY_MVFF_SLOT_HIGH``
whether to allocate objects at the end of free blocks found, as
opposed to at the start (for unbuffered allocation)
``MPS_KEY_MVFF_ARENA_HIGH``
whether to express SegPrefHIGH to the arena, as opposed to
SegPrefLOW
``MPS_KEY_MVFF_FIRST_FIT``
whether to use the suitable block of lowest address, as opposed
to the highest (for unbuffered allocation)
.. _`keyword arguments`: keyword-arguments.rst
_`.method.init.epdl`: To simulate the EPDL pool, specify extendBy, avgSize, and
maxSize as normal, and use slotHigh=FALSE, arenaHigh=FALSE, firstFit=TRUE.
_`.method.init.epdr`: To simulate the EPDR pool, specify extendBy, avgSize, and
maxSize as normal, and use slotHigh=TRUE, arenaHigh=TRUE, firstFit=TRUE.
_`.method.init.other`: The performance characteristics of other combinations are
unknown.
_`.method.finish`: The PoolFinish method,
_`.method.alloc`: Alloc and Free methods are supported, implementing the policy
set by the pool params (see `.method.init`_).
_`.method.describe`: The usual describe method.
_`.method.buffer`: The buffer methods implement a worst-fit fill strategy.
External Functions
------------------
_`.function`: MVFF supports the following external functions:
_`.function.free-size`: ``size_t mps_mvff_free_size(mps_pool_t pool)``
This function returns the total size of free space in segments allocated to
the MVFF pool instance.
_`.function.size`: ``size_t mps_mvff_size(mps_pool_t pool)``
This function returns the total memory used by pool segments, whether free or
allocated.
_`.function.class`: ``mps_class_t mps_class_mvff(void)``
This function returns the class object for the pool class, to be used in pool
creation.
Implementation
--------------
_`.impl.free-list`: The pool stores its free list in a CBS (see
`design.mps.cbs `_). It uses the CBS's mayUseInline facility to
avoid running out of memory to store the free this. This is the reason
for the alignment restriction above.
Details
-------
_`.design.seg-size`: When adding a segment, we use extendBy as the segment size
unless the object won't fit, in which case we use the object size (in both
cases we align up).
_`.design.seg-fail`: If allocating a segment fails, we try again with a segment
size just large enough for the object we're allocating. This is in response to
request.mps.170186.
A. References
-------------
B. Document History
-------------------
========== ====== ================================================
2002-06-07 RB_ Converted from MMInfo database design document.
2013-05-10 RB_ Converted to reStructuredText.
Updated to document keyword arguments, replacing
varargs.
========== ====== ================================================
.. _RB: mailto:rb@ravenbrook.com
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