29. MVFF pool class¶
29.1. Introduction¶
.intro: This is the design of the MVFF (Manual Variable First-Fit) pool class. 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.
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.
29.2. 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.
29.3. Methods¶
.method: The MVFF pool supports the following methods:
.method.init: This takes six keyword arguments:
Keyword argument |
Description |
|---|---|
|
The segment size. |
|
The average object size. |
|
The alignment of allocations and frees.
Must be at least |
|
Whether to allocate objects at the end of free blocks found, as opposed to at the start (for unbuffered allocation). |
|
Whether to express |
|
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 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: PoolAlloc() and PoolFree() 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.
29.4. External Functions¶
.function: MVFF supports the following external functions:
.function.free-size: mps_mvff_free_size() returns the total
size of free space in segments allocated to the MVFF pool instance.
.function.size: mps_mvff_size() returns the total memory used
by pool segments, whether free or allocated.
.function.class: mps_class_mvff() returns the class object for
the pool class, to be used in pool creation.
29.5. Implementation¶
.impl.free-list: The pool stores its free list in a CBS (see //gdr-peewit/info.ravenbrook.com/project/mps/branch/2013-05-17/emergency/design/poolmvff.txt design.mps.cbs), failing over in emergencies to a Freelist (see design.mps.freelist) when the CBS cannot allocate new control structures. This is the reason for the alignment restriction above.
29.6. 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.
