An Erlang process is lightweight compared to threads and processes in operating systems.
A newly spawned Erlang process uses 309 words of memory in the non-SMP emulator without HiPE support. (SMP support and HiPE support both add to this size.) The size can be found as follows:
Erlang (BEAM) emulator version 5.6 [async-threads:0] [kernel-poll:false]
Eshell V5.6 (abort with ^G)
1> Fun = fun() -> receive after infinity -> ok end end.
#Fun<...>
2> {_,Bytes} = process_info(spawn(Fun), memory).
{memory,1232}
3> Bytes div erlang:system_info(wordsize).
309
The size includes 233 words for the heap area (which includes the stack). The garbage collector increases the heap as needed.
The main (outer) loop for a process must be tail-recursive. Otherwise, the stack grows until the process terminates.
DO NOT
loop() ->
receive
{sys, Msg} ->
handle_sys_msg(Msg),
loop();
{From, Msg} ->
Reply = handle_msg(Msg),
From ! Reply,
loop()
end,
io:format("Message is processed~n", []).
The call to
DO
loop() ->
receive
{sys, Msg} ->
handle_sys_msg(Msg),
loop();
{From, Msg} ->
Reply = handle_msg(Msg),
From ! Reply,
loop()
end.
The default initial heap size of 233 words is quite conservative to support Erlang systems with hundreds of thousands or even millions of processes. The garbage collector grows and shrinks the heap as needed.
In a system that use comparatively few processes, performance
might be improved by increasing the minimum heap size
using either the
The gain is twofold:
The emulator probably uses more memory, and because garbage collections occur less frequently, huge binaries can be kept much longer.
In systems with many processes, computation tasks that run for a short time can be spawned off into a new process with a higher minimum heap size. When the process is done, it sends the result of the computation to another process and terminates. If the minimum heap size is calculated properly, the process might not have to do any garbage collections at all. This optimization is not to be attempted without proper measurements.
All data in messages between Erlang processes is copied,
except for
When a message is sent to a process on another Erlang node, it is first encoded to the Erlang External Format before being sent through a TCP/IP socket. The receiving Erlang node decodes the message and distributes it to the correct process.
Constant Erlang terms (also called literals) are kept in constant pools; each loaded module has its own pool. The following function does not build the tuple every time it is called (only to have it discarded the next time the garbage collector was run), but the tuple is located in the module's constant pool:
DO
days_in_month(M) ->
element(M, {31,28,31,30,31,30,31,31,30,31,30,31}).
But if a constant is sent to another process (or stored in an Ets table), it is copied. The reason is that the runtime system must be able to keep track of all references to constants to unload code containing constants properly. (When the code is unloaded, the constants are copied to the heap of the processes that refer to them.) The copying of constants might be eliminated in a future Erlang/OTP release.
Shared subterms are not preserved in the following cases:
That is an optimization. Most applications do not send messages with shared subterms.
The following example shows how a shared subterm can be created:
kilo_byte() ->
kilo_byte(10, [42]).
kilo_byte(0, Acc) ->
Acc;
kilo_byte(N, Acc) ->
kilo_byte(N-1, [Acc|Acc]).
1> byte_size(list_to_binary(efficiency_guide:kilo_byte())). 1024
Using the
2> erts_debug:size(efficiency_guide:kilo_byte()). 22
Using the
3> erts_debug:flat_size(efficiency_guide:kilo_byte()). 4094
It can be verified that sharing will be lost if the data is inserted into an Ets table:
4> T = ets:new(tab, []).
17
5> ets:insert(T, {key,efficiency_guide:kilo_byte()}).
true
6> erts_debug:size(element(2, hd(ets:lookup(T, key)))).
4094
7> erts_debug:flat_size(element(2, hd(ets:lookup(T, key)))).
4094
When the data has passed through an Ets table,
In a future Erlang/OTP release, it might be implemented a way to (optionally) preserve sharing.
The SMP emulator (introduced in R11B) takes advantage of a multi-core or multi-CPU computer by running several Erlang scheduler threads (typically, the same as the number of cores). Each scheduler thread schedules Erlang processes in the same way as the Erlang scheduler in the non-SMP emulator.
To gain performance by using the SMP emulator, your application must have more than one runnable Erlang process most of the time. Otherwise, the Erlang emulator can still only run one Erlang process at the time, but you must still pay the overhead for locking. Although Erlang/OTP tries to reduce the locking overhead as much as possible, it will never become exactly zero.
Benchmarks that appear to be concurrent are often sequential.
The estone benchmark, for example, is entirely sequential. So is
the most common implementation of the "ring benchmark"; usually one process
is active, while the others wait in a
The