module Netmcore_heap:Shared heaps of structured valuessig
..end
These heaps live in Netmcore_mempool
-type shared memory pools,
and can contain an arbitrary number of Ocaml values. These values
can be mutable, but special care has to be taken when modifying them.
The first value pushed onto the heap is called the root element.
All other values must be (directly or indirectly) reachable from the
root element.
Heaps are created with a certain initial size. The heaps remain connected with the memory pool, and they are enlarged if necessary by allocating more blocks in the pool.
As the heaps are shared between processes, it must be taken care
that no reference is made from shared heaps to normal process-local
memory. These references would only be valid in the process creating
them, and following such references from other processes would probably
crash the program (or at least return wrong values). In order to ensure
this, it is strictly forbidden to directly manipulate mutable
data structures. The Netmcore_heap.modify
function has to be used,
as this function makes it possible to copy more values to the heap.
Unfortunately, there is nothing in the type system that would prevent
direct mutation. so this can only be ensured by the discipline of the
programmer.
The values of the heap are also garbage-collected: If all allocated space is used and more values need to be added, it is first tried to get rid of old unreachable values. The garbarge collection is done by the process that happens to initiate the addition of the value that does no more fit onto the heap. During garbage collection, no other additions can be done, but read accesses are not prevented. The garbage collector does not move values (addresses remain unchanged).
The garabage collector only considers values as reachable that are
reachable via the root element. It is not sufficient when a value
is only reachable via a process-specific reference.
type 'a
heap
'a
type 'a
descr
val create_heap : Netmcore.res_id -> int -> 'a -> 'a heap
create_heap pool_id size root
: Creates a new heap with size
bytes in the pool identified by pool_id
. This ID must refer
to a Netmcore_mempool
-managed pool.
The value root
is copied to the new heap. This is done by
deeply duplicating root
and all values pointed to by root
,
and storing these duplicates in the heap.
The possible types of value root
are restricted. In particular,
the following types are not supported (as 'a
or as a component
of 'a
):
int32
, int64
, nativeint
which are specially supported)in_channel
and out_channel
x = [| |]
do not work)val minimum_size : 'a -> int
size
value one must pass to create_heap
at minimum
to put this root element onto the heap.val root : 'a heap -> 'a
val descr_of_heap : 'a heap -> 'a descr
val heap_of_descr : Netmcore.res_id -> 'a descr -> 'a heap
heap_of_descr pool d
:
Get the heap for a descriptor. This assumes that the heap still
exists.type
mutator
Caveat: pushed values are not considered as roots, and thus they
need immediately be attached to the existing data structure.
Otherwise, the next push might trigger a garbage collection, and
the new value is deleted. If this is not possible, one can
call pin
instead (see below).
val modify : 'a heap -> (mutator -> 'b) -> 'b
modify h mutate
: This function locks the heap so that this process
has exclusive write access to it for the duration of the mutate
function. The mutate
function is immediately called back, and
the argument of mutate
is the mutator m
that allows one to push
values onto the heap.
By calling add m x
from the body of mutate
one can create a copy
of x
that is stored in the heap.
val add : mutator -> 'a -> 'a
val add_uniform_array : mutator -> int -> 'a -> 'a array
add_uniform_array m n x
: Pushes a new value with n elements onto
the heap. Each index position of the array is initialized with
the same copy of x
.val add_init_array : mutator -> int -> (int -> 'a) -> 'a array
add_init_array m n f
: Pushes a new value with n element onto
the heap. The index position k
is inititialized by running
f k
and pushing the copy of this onto the heap.val add_some : mutator -> 'a -> 'a option
add_some mut x
: Returns Some x
where the O'Caml value representing
Some
is allocated in the heap using mut
. It is assumed that x
is
already a resident of the heap. This means x
is not copied!val pin : mutator -> 'a -> unit
pin m x
: Pins a shared value x
so it cannot be deleted by
the garbage collector. The value remains pinned for the lifetime
of the mutator m
(i.e. the runtime of the modify
function).
Pinning is relatively expensive if done in masses, and should be
avoided if possible.
val copy : 'a -> 'a
val with_value : 'a heap -> (unit -> 'b) -> ('b -> 'c) -> 'c
with_value h find process
: Logically, this runs
process (find ())
and returns the result. While find
is being
executed, the heap is write-locked. This returns a value x
.
While process
is being executed, the value x
is temporarily
added to the set of reachable values, so that a parallely running
garbage collection will not delete it.
Note that x
must reside in the heap!
Calling modify
from find
will cause a deadlock. Calling
it from process
is allowed.
val with_value_2 : 'a heap -> (unit -> 'b * 'c) -> ('b * 'c -> 'd) -> 'd
val with_value_3 : 'a heap -> (unit -> 'b * 'c * 'd) -> ('b * 'c * 'd -> 'e) -> 'e
val with_value_4 : 'a heap ->
(unit -> 'b * 'c * 'd * 'e) -> ('b * 'c * 'd * 'e -> 'f) -> 'f
val with_value_5 : 'a heap ->
(unit -> 'b * 'c * 'd * 'e * 'f) -> ('b * 'c * 'd * 'e * 'f -> 'g) -> 'g
with_value
, but a tuple of values can be passed downval with_value_n : 'a heap -> (unit -> 'b list) -> ('b list -> 'c) -> 'c
with_value
, but a list of values can be passed downval destroy : 'a heap -> unit
val gc : 'a heap -> unit
val debug_info : 'a heap -> string
This example creates a heap that stores a single value. (This is
available as Netmcore_ref
.)
let shared_ref x =
(* The shm version of [ref x] *)
let r = ref x in
let init_size = minimum_size r in
let hp = create_heap pool_id init_size r in
hp
let deref sref =
(* The shm version of [!] *)
!(root sref)
let assign sref x =
(* The shm version of [:=] - however, a copy of x is done *)
modify sref
(fun add ->
(root sref) := add x
)
module Debug:sig
..end