package offheap
Install
Dune Dependency
Authors
Maintainers
Sources
sha256=c733401498a155c5665ecb7a776946934237af52d8496717f9b242ec4faaf942
md5=0278b913dc2cdb74231d0ca3e4196e36
Description
A copy of OCaml objects is moved to memory managed by malloc and free, out of the reach of the GC. If the objects are very large, performance can be improved as the traversal of large live objects does not slow the GC down.
Published: 21 Sep 2018
README
Offheap
offheap
moves copies of OCaml values off the heap into memory managed by malloc
and free
, allowing them to be manually managed and reclaimed. Moving large objects off the heap can potentially improve performance as they are not reachable by the garbage collector, avoiding expensive traversals.
Interface
val copy : ?alloc:alloc -> 'a -> 'a t
Creates a copy of an object, returning a handle to it. The handle is required to free the object. The old value is still usable.
val get : 'a t -> 'a
Returns a reference to the copied object from the handle.
val free : 'a t -> unit
Frees the off-heap memory. All references to the copied object are invalid from this point onwards.
Custom allocators
By default, a malloc-based allocator is used. Custom allocators can be provided, however they must be implemented in C: copying cannot call back into OCaml code. Callbacks could potentially access the copied object or trigger a heap sweep which would fail since at the point of the call the headers of blocks in the old object are left in an inconsistent state. The custom allocator must implement the following two functions:
void *malloc(value allocator, size_t size)
void free(value allocator)
They must be stored in the first two fields of an Abstract_tag
block. Additional data may be stored in other fields of the same block, which is passed to the allocator functions.
Limitations
The object to be copied cannot reference non-Ocaml values (Custom_tag
or Abstract_tag
).
The builtin polymorphic comparison and hash functions do not work with off-heap objects anymore since they do not traverse off-heap objects.
Acknowledgement
The implementation was inspired by the iterative traversal implemented in the Obj
module of the OCaml runtime. This library was inspired by the existing ancient
library, which is unable to handle large objects because of recursive traversals.
License
The code is published under the MIT License.