Erlang Memory Mapped Files

Version: 6953f07

Erlang MMAP `emmap`

This Erlang library provides a wrapper that allows you to memory map files into the Erlang memory space.

Authors

Supported Platforms

Linux, MacOS

NOTE: On MacOS emmap:resize/2 is not supported, it will return {error, fixed_size}.

Basic Usage

The basic usage is

{ok, Mem, _Info} = emmap:open("filename", [read, shared, direct]),
{ok, Binary}     = file:pread(Mem, 100, 40),
...
ok = file:close(Mem).

The open options is a list containing zero or more options.

From this point, Mem can be used either with the file or with the emmap functions interchangeably:

{ok, Binary} = file:pread(Mem, Position, Length) read Length bytes at Position in the file.
ok = file:pwrite(Mem, Position, Binary) writes to the given position.
{ok, Binary} = file:read(Mem, Length) read 1..Length bytes from current position, or return eof if pointer is at end of file.
{ok, Pos} = file:position(Mem, Where) see file:position/2 documentation.
ok = file:close(Mem)

A memory map can be closed either by calling emmap:close/1 or file:close/1. When using the direct option, and emmap:close/1 is called, the memory map is not immediately closed, but will get automatically closed when all binaries that reference this memory map are garbage collected.

Atomic operations on the memory mapped file

The emmap application offers a way to do atomic add, sub, xchg, cas as well as bitwise and, or, xor operations using emmap:patomic_*/3 and emmap:patomic_cas/4 functions.

Effectively this directly changes the content of the underlying memory, is thread-safe, and persistent.

{ok, OldValue} = emmap:patomic_add(Mem, Position, 1).

This approach allows to implement persistent atomic counters that survive node restarts.

Atomic persistent counters

The emmap application allows a user to maintain atomic persistent counters. This could be useful for continuous numbering of some events in the system which could be efficiently shared among processes in a thread-safe way and at the same time being persistent. This is a very light-weight approach compared to using mnesia or other form of persistent storage.

Here is an example:

F  = emmap:open_counters("/tmp/mem.bin", 2),
N1 = emmap:inc_counter(F, 0),
N2 = emmap:inc_counter(F, 0, 5),
N3 = emmap:inc_counter(F, 0),
N4 = emmap:inc_counter(F, 0, 2),
N5 = emmap:set_counter(F, 0, 15),
N6 = emmap:read_counter(F, 0),
emmap:close_counters(F),
io_format("N1=~w, N2=~w, N3=~w, N4=~w, N5=~w, N6=~w\n",
          [N1, N2, N3, N4, N5, N6]).  % Prints: N1=0, N2=1, N3=6, N4=7, N5=9, N6=15

Shared memory and using mutable binaries

While Erlang goes at length to achieve immutability, sometimes applications might need to have access to mutable memory. This can be accomplished by using the direct shared access to the memory mapped file.

Example:

shell1> {ok, MM, _Info} = emmap:open("/tmp/mem.data", 0, 8, [create, direct, read, write, shared, nolock]).
shell2> {ok, MM, _Info} = emmap:open("/tmp/mem.data", 0, 8, [create, direct, read, write, shared, nolock]).
shell1> emmap:pwrite(MM, 0, <<"test1">>).
shell2> {ok, Bin} = emmap:pread(MM, 0, 5).
{ok, <<"test1">>}
shell1> emmap:pwrite(MM, 0, <<"test2">>).
shell2> Bin.
<<"test2">>
shell1> emmap:pwrite(MM, 0, <<"test3">>).
shell2> Bin.
<<"test3">>

Though this may seem odd that a bound Bin variable returns a different value when we printed it in the shell2 the second time, it is the result of opening memory mapped file using the direct option. In this case the binaries read from memory map point to the actual memory in that map rather than being copies of that memory. For some applications, such as when using that memory to store atomic counters, this property can be very valuable.

Using the option direct has the effect that the mmap file is not closed until all references to binaries coming out of read/pread have been garbage collected. This is a consequence of that such binaries are referring directly to the mmap'ed memory.

When passing auto_unlink option to emmap:open/4, the memory mapped file will be automatically deleted when it is closed.

Persistent FIFO used as a container or guarded by a process.

The emmap_queue module implements a persistent FIFO queue based on a memory-mapped file. This means that in-memory operations of enqueuing items are automatically persisted on disk.

A queue used as a container will persistent messages stored in queue on disk, and has constant time complexity of the push and pop operations. The open/3 is given an initial storage in bytes, which will automatically grow unless the fixed_size option is provided, in which case when the queue becomes full, a push/2 call will return {error, full}. In the example below we are using auto_unlink option which automatically deletes the memory mapped file at the end of the test case (something you might not want in other cases):

{ok, Q} = emmap_queue:open(Filename, 1024, [auto_unlink]),
ok = emmap_queue:push(Q, a),
ok = emmap_queue:push(Q, {b,1}),
ok = emmap_queue:push(Q, {c,d}),
a     = emmap_queue:pop(Q),
{b,1} = emmap_queue:pop(Q),
{c,d} = emmap_queue:pop(Q),
nil   = emmap_queue:pop_and_purge(Q).

Use emmap_queue:pop_and_purge/1 to reclaim the space in memory when the queue becomes empty.

When a queue is wrapped in a gen_server, it is suitable for use in a multi-process use cases. This is implemented using emmap_queue:start_link/4,emmap_queue:enqueue/2, and emmap_queue:dequeue/1 functions. In the example below we are using the auto_unlink option which automatically deletes the memory mapped file at the end of the test case (something you might not want in other cases):

{ok, Pid} = emmap_queue:start_link(?MODULE, Filename, 1024, [auto_unlink]),
ok = emmap_queue:enqueue(Pid, a),
ok = emmap_queue:enqueue(Pid, {b,1}),
ok = emmap_queue:enqueue(Pid, {c,d}),
a     = emmap_queue:dequeue(Pid),
{b,1} = emmap_queue:dequeue(Pid),
{c,d} = emmap_queue:dequeue(Pid),
nil   = emmap_queue:dequeue(Pid).