DESIGN OF THE MANUALLY-MANAGED VARIABLE-SIZE FIRST-FIT POOL
                          design.mps.poolmvff
                             incomplete doc
                           gavinm 1998-09-09

INTRODUCTION

.intro: The pool was created in a response to a belief that 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 vararg parameters:
  - extendBy -- the segment size;
  - avgSize -- the average object size;
  - alignment -- the alignment of allocations and frees (must be at least 
sizeof(void*));
  - slotHigh -- whether to allocate objects at the end of free blocks found, as 
opposed to at the start (for unbuffered allocation);
  - arenaHigh -- whether to express SegPrefHIGH to the arena, as opposed to 
SegPrefLOW;
  - firstFit -- whether to use the suitable block of lowest address, as opposed 
to the highest (for unbuffered allocation).
.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 EPDL 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.