[−][src]Crate dyn_cache
Caches for storing the results of repeated function calls. The cache types available use minimal dynamic dispatch to allow storing arbitrarily many types of query results in a single parent store.
There are two main flavors of cache available for use in this crate:
| Shared type | Synchronized? |
|---|---|
sync::SharedSendCache | Mutex |
local::SharedLocalCache | RefCell |
These variants are used by calling sync::SharedSendCache::cache_with or
local::SharedLocalCache::cache.
The shared cache types above are implemented by wrapping these "inner" types:
| Mutable type | Requires Send? |
|---|---|
sync::SendCache | yes |
local::LocalCache | no |
These "inner" caches require mutable access to call their functions like
local::LocalCache::get which returns either a reference or a
CacheMiss that can be passed back to the cache in
local::LocalCache::store to initialize a value in the cache.
See sync::SendCache::get and sync::SendCache::store for the
thread-safe equivalents.
The shared variants are defined by wrapping these inner cache types in reference counting and synchronized mutability.
Query types
Each query type maps to a typed "namespace" within the unityped cache storage, each query having a distinct type each for its scope, input, and output.
Scopes
The scope of a query is its identifier within cache storage.
Scopes must implement Eq and Hash so that results can be
efficiently and uniquely indexed within a namespace.
Each scope identifies 0-1 (Input, Output) pairs in each namespace. The
same type of scope can be used in multiple queries without collision if
the types of inputs, outputs, or both differ.
Inputs
The input to a query determines when it is (re-)run. If a given query has been run before, then the previous input is compared to the current input before potentially running the query. If the input hasn't changed, the query can be skipped and its previously-stored output is returned.
Outputs
The only constraint on query outputs is that they are owned (Output: 'static). This imposes the inconvenient requirement that all access to
stored values occurs during the scope of a closure (similar to thread-locals
in the standard library).
The most common way to work around this requirement is to choose output
types that cheaply implement std::clone::Clone.
Allocations
In order to store distinct query results in the same container, allocations and indirection are required.
Borrowed query parameters
All of the cache functions accept a reference to a type Key: ToOwned<Owned=Scope> so that the scope is only cloned on the first
insertion to its storage and all subsequent lookups can be with a borrowed
type.
Like the query scope, functions to get cache values accept a borrowed version of the input and only clone it when the input has changed.
Causes
There are three situations where these caches allocate:
- caching new types which haven't been seen by that cache instance yet
- storing the results of a new query
- updating the results of a stored query
There are several types of allocations performed by the caches in this crate:
| Allocation | Causes |
|---|---|
| box a new, empty namespace | (1) |
| resize a cache's map of namespaces | (1) |
call .to_owned() on a scope/key | (2) |
| resize a namespace's storage | (2) |
call .to_owned() on an input/arg | (2), (3) |
| update an output's dependents | (1), (2), (3) |
Outside of these, only user-defined functions should perform any allocation.
Garbage Collection
All of the caches have a gc() method which retains only used values. A
value is used if it or a value which depends on it has been used/rooted
since the last call to gc().
Rooting
When a cache read fails, we expect that the value will be populated immediately after and a new node in the dependency graph is created. The new dependency node is marked as an incoming dependent on any cache values which are accessed during the initialization of the new value. The new node is then marked as a "root" for the garbage collector once it has been initialized and the cache populated. If in subsequent epochs the rooted value is accessed again it will be re-rooted and its dependents will be marked as live even if they were not directly accessed in that epoch.
When a cache read succeeds, its dependency node is marked as being depended upon by the node (if any) which was being initialized during the read, linking the two dependencies together.
Modules
| local | A cache for types which are not thread-safe ( |
| sync | A thread-safe cache which requires stored types implement |
Structs
| CacheEntry | A fully-initialized input/output pair, ready to be written to the store. |
| CacheMiss | The result of a failed attempt to retrieve a value from the cache.
Initialize a full |