1. Choosing a pool class
This section contains a simple procedure for choosing a pool class based on the properties of the data you plan to store in it. The MPS works well if you can segregate your data into a variety of pools, choosing the most appropriate pool class for each.
Note
Pool classes can differ in many ways not considered here: speed, vulnerability to fragmentation, control overhead, and so on. This procedure gives you a decent recommendation, but an expert in the MPS might be able to make a better recommendation. And if no pool class in the open source MPS exactly matches your needs, then it is possible to develop new pool classes. See Writing a new pool class.
First, do you need the MPS to automatically reclaim unreachable blocks? If so, you need an automatically managed (garbage collected) pool class and you should consult Choosing an automatic pool class below. Otherwise, you need a manually managed pool class and you should consult Choosing a manual pool class below.
1.1. Choosing an automatic pool class
Answer these questions about your data:
Is it acceptable for the MPS to move blocks in memory and to place barriers (1) on blocks? (For example, it might not be acceptable to move a block if it has been passed to foreign code that remembered its location.)
Do your blocks contain references to blocks stored in automatically managed pools (including references to other blocks in the same pool, if it’s automatically managed)? And if so, are these references exact or weak?
Second, look up your answers in this table to find the recommended pool class to use:
Movable & protectable? |
References? |
Use this pool class |
---|---|---|
yes |
none |
|
yes |
exact |
|
yes |
weak |
|
no |
none |
|
no |
exact |
|
no |
weak |
nothing suitable |
1.2. Choosing a manual pool class
Answer these questions about your data:
Are the blocks fixed in size? If so, use MFS (Manual Fixed Small).
Are the lifetimes of blocks predictable? If so, use MVT (Manual Variable Temporal), and arrange that objects that are predicted to die at about the same time are allocated from the same allocation point.
Otherwise, use MVFF (Manual Variable First Fit).
2. Pool class properties
This table summarizes the properties of each pool class provided by the open source MPS. For “block” properties, “yes” means that the property holds for all blocks allocated from the pool. An entry “—” indicates that a property makes no sense for a pool class: for example, if blocks in a pool may not contain references, it makes no sense to ask whether they may contain weak references (1).
Property |
|||||||||
---|---|---|---|---|---|---|---|---|---|
Supports |
no |
no |
no |
no |
no |
yes |
yes |
no |
no |
Supports |
no |
no |
no |
no |
no |
yes |
yes |
yes |
no |
Supports allocation points? |
yes |
yes |
yes |
yes |
yes |
no |
yes |
yes |
yes |
Manages memory using allocation frames? |
no |
no |
no |
no |
no |
no |
no |
no |
yes |
Supports segregated allocation caches? |
no |
no |
no |
no |
no |
yes |
yes |
no |
no |
Timing of collections? [2] |
auto |
auto |
auto |
auto |
auto |
— |
— |
— |
— |
May contain references? [3] |
yes |
no |
yes |
yes |
no |
no |
no |
no |
yes |
May contain exact references? [4] |
yes |
— |
yes |
yes |
— |
— |
— |
— |
yes |
May contain ambiguous references? [4] |
no |
— |
no |
no |
— |
— |
— |
— |
no |
May contain weak references? [4] |
no |
— |
no |
yes |
— |
— |
— |
— |
no |
Allocations fixed or variable in size? |
var |
var |
var |
var |
var |
fixed |
var |
var |
var |
Alignment? [5] |
conf |
conf |
conf |
conf |
conf |
conf |
|||
Dependent objects? [8] |
no |
— |
no |
yes |
— |
— |
— |
— |
no |
May use remote references? [9] |
no |
— |
no |
no |
— |
— |
— |
— |
no |
Blocks are automatically managed? [10] |
yes |
yes |
yes |
yes |
yes |
no |
no |
no |
no |
Blocks are promoted between generations |
yes |
yes |
no |
no |
no |
— |
— |
— |
— |
Blocks are manually managed? [10] |
no |
no |
no |
no |
no |
yes |
yes |
yes |
yes |
Blocks are scanned? [11] |
yes |
no |
yes |
yes |
no |
no |
no |
no |
yes |
Blocks support base pointers only? [12] |
no |
no |
yes |
yes |
yes |
— |
— |
— |
yes |
Blocks support internal pointers? [12] |
yes |
yes |
no |
no |
no |
— |
— |
— |
no |
Blocks may be protected by barriers? |
yes |
no |
yes |
yes |
yes |
no |
no |
no |
yes |
Blocks may move? |
yes |
yes |
no |
no |
no |
no |
no |
no |
no |
Blocks may be finalized? |
yes |
yes |
yes |
yes |
yes |
no |
no |
no |
no |
Blocks must be formatted? [11] |
yes |
yes |
yes |
yes |
yes |
no |
no |
no |
yes |
Blocks may use in-band headers? |
yes |
yes |
yes |
yes |
yes |
— |
— |
— |
no |
Notes
3. Writing a new pool class
If none of the pool classes supplied with the MPS are quite right for your application, don’t despair: the MPS is designed to be extensible with new pool classes, and designed so that the properties of pools are as orthogonal as possible. So if you need a pool containing objects that are scannable but unformatted, or movable objects which are manually managed, or a pool all of whose objects are roots, there is no technical reason why it should not be possible to write it.
If you’d be interested in our developing new pool classes for your requirements, or if you’ve started writing a new pool class yourself, we’d love to hear from you.