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Module Netmcore_heap

module Netmcore_heap: sig .. end
Shared heaps of structured values

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 heap where the type of the root element is 'a
type 'a descr 
A descriptor ("address") pointing to the heap. Descriptors can be marshalled.
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):

  • Function types
  • Lazy values
  • Objects
  • Custom blocks (including bigarrays, but excluding int32, int64, nativeint which are specially supported)
  • I/O references like in_channel and out_channel
  • empty arrays are only partially supported (in particular, comparisons x = [| |] do not work)

val minimum_size : 'a -> int
Returns the size value one must pass to create_heap at minimum to put this root element onto the heap.
val root : 'a heap -> 'a
Returns the root element
val descr_of_heap : 'a heap -> 'a descr
Get the descriptor of a heap
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 
Mutators allow it to push new values onto the heap.

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 -> 'r) -> 'r
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
Pushes a new value onto the heap
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
Creates a deep copy of the input value, and stores the duplicate in normal process memory.
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 -> 'z) -> 'z
val with_value_3 : 'a heap -> (unit -> 'b * 'c * 'd) -> ('b * 'c * 'd -> 'z) -> 'z
val with_value_4 : 'a heap ->
(unit -> 'b * 'c * 'd * 'e) -> ('b * 'c * 'd * 'e -> 'z) -> 'z
val with_value_5 : 'a heap ->
(unit -> 'b * 'c * 'd * 'e * 'f) -> ('b * 'c * 'd * 'e * 'f -> 'z) -> 'z
Same as with_value, but a tuple of values can be passed down
val with_value_n : 'a heap -> (unit -> 'b list) -> ('b list -> 'c) -> 'c
Same as with_value, but a list of values can be passed down
val destroy : 'a heap -> unit
Destroys the heap and gives the memory back to the pool
val gc : 'a heap -> unit
Lock the heap and do a GC pass
val pool : 'a heap -> Netmcore.res_id
Return the pool ID
val mut_pool : mutator -> Netmcore.res_id
Return the pool ID
val sem_container : 'a heap -> Netsys_sem.container
Return the semaphore container
val mut_sem_container : mutator -> Netsys_sem.container
Return the semaphore container
val debug_info : 'a heap -> string
Returns a multi-line debug string

Example: Mutable Variable

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

    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
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