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Module Netmcore_mempool

module Netmcore_mempool: sig .. end
Memory pools

A memory pool is a block of shared memory that is set aside for storing shared data structures. The pool needs to be created before the worker processes are forked off that are going to use the pool. The worker processes do not map the pool to some random address, but rather the processes inherit the pool from the common master process which ensures that all processes will see the pool at the same address.

In order to allow inheritance, the function Netmcore.start for starting workers needs to get an additional argument ~inherit_resources. The resource ID of the pool must be put into this list - otherwise the worker does not get access to the pool.

It is not possible to enlarge the pool later (because of the inheritance method for making the pool accessible). It is advised to make the pool large enough for all possible data cases, and to let the user configure this size.

exception Out_of_pool_memory
val create_mempool : ?alloc_really:bool -> int -> Netmcore.res_id
Creates the memory pool as shared memory object of the passed size (rounded up to the next multiple of pages) and returns the resource ID.

Note that the process calling this function cannot use the pool, but only worker processes that are forked later. It is possible to call create_mempool before running Netmcore.startup.

Option alloc_really: On some operating systems (namely Linux) shared memory is not fully included into the memory bookkeeping as long as nothing is written into it (so-called overcommitment). This means that the memory is not reserved, and when something is written for the first time, it might happen that the system cannot grant the request. The consequence is a bus error. By setting alloc_really to true, all allocated memory pages are immediately written to, and thus the problem is avoided (or better, if memory is really tight, you get the bus error now immediately, at least).

Note that memory pools have kernel persistence! They are not automatically deleted when the process finishes. Call unlink_mempool to delete memory pools.

val alloc_mem : Netmcore.res_id -> int -> Netsys_mem.memory
Allocate memory in this pool. The passed int the size of the returned memory object. The size is rounded up to the next multiple of pages.

Blocks are actually allocated in units of pages.

Raises Out_of_pool_memory if there is not enough contiguous space in the pool.

val size_mem : Netmcore.res_id -> Netsys_mem.memory -> int
Returns the size of this block, or raises Not_found
val size_mem_at_addr : Netmcore.res_id -> nativeint -> int
Returns the size of the block at this address, or raises Not_found
val free_mem : Netmcore.res_id -> Netsys_mem.memory -> unit
Frees this allocated block
val stats : Netmcore.res_id -> int * int * int
Returns (total, free, contiguous) where
  • total is the total size of the pool
  • free is the number of free bytes
  • contiguous is the size of the largest contiguous free block

val debug_info : Netmcore.res_id -> string
Returns a string describing the allocations etc.
val shm_name : Netmcore.res_id -> string
Returns the name of the shared memory object
val sem_container : Netmcore.res_id -> Netsys_sem.container
Returns the semaphore container
val unlink_mempool : Netmcore.res_id -> unit
Deletes the persistent objects of this pool
module Debug: sig .. end
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