[erlang-questions] Why intra-node messages are copied and not refcounted, rationale?
Sat May 9 05:34:39 CEST 2015
Thanks Jesper for your crystal clear explanation. I wrongly assumed when
you appended two Refc binaries (via process local ProcBin's) a new
null-data Refc binary were created in the shared heap pointing to the
previous two. Imagining BEAM having the binaries stored in a persistent
data structure schema on a shared refcounted heap, my next thought was "why
not use it for all data structures?". Need to dive more in the BEAM
On Thu, May 7, 2015 at 11:27 AM, Jesper Louis Andersen <
> On Thu, May 7, 2015 at 3:07 PM, Nahuel Greco <> wrote:
>> What's the rationale of accepting a copying cpu/memory overhead? Why a
>> refcounting mechanism like the used for binaries is not desirable for all
>> the data structures?
> Really good questions!
> Why copy and not pass by reference? It is implementation specific, as the
> Erlang language allows for both semantics, and there were a variant of the
> VM which used varying reference-passing tricks in place of the current
> copying solution. The biggest disadvantage is that if you send very large
> messages, then the copying overhead is large and costs performance. But
> there are also some important advantages to copying. Each process can have
> its own heap arena for instance, so this breaks up the memory space into
> many small chunks which can be individually garbage collected. Erlang is a
> soft realtime system, so you can't afford long garbage collection pauses.
> This is usually really good for GC pause times as they can be quite small.
> The second big advantage is that the semantics are the same for local
> processes as well as distributed processes. You *have* to copy if the
> process lives on another node anyway. The third advantage is that data
> becomes local to a process, and thus local to a processing core. There are
> advantages to this model on a system where memory access is highly
> non-uniform, because you can in theory pick memory close to the processing
> As an example, I have been brewing on some latency benchmarks for
> webservers. A sneak peak is the median response time of this test:
> out.go: 50.000% 1.72ms
> out.undertow: 50.000% 1.53ms
> out.cowboy: 50.000% 6.33ms
> where clearly, Erlang is far slower than a Go or Undertow, a Java-based
> webserver. But once we look at the 99.99th percentile, things are different:
> out.go: 99.990% 58.75ms
> out.undertow: 99.990% 47.90ms
> out.cowboy: 99.990% 38.62ms
> and at the 99.999th percentile it gets really funny:
> out.go: 99.999% 216.45ms
> out.undertow: 99.999% 64.61ms
> out.cowboy: 99.999% 45.09ms
> what you see here is garbage collection overhead because you have such
> large heaps to traverse, or that there are some other factor imposing
> additional latency for a few of the calls.
> Why not use refcounting? One price of refcounting is unpredictable
> performance. If you ref-count a large list and free it up, then the GC has
> to get rid of this list, and it will take quite some time. Many refcounting
> GC's runs this as a background job to handle this. In fact, Refcounting GCs
> are dual to Mark&Sweep GCs. In the binary heap, a binary can contain no
> pointers, which means reclamation of memory is always constant.
> Furthermore, without optimizations, refcounting GCs tend to perform badly.
> With optimizations, they start to look like Mark&Sweep collectors, as
> written in .
> In other words, both decisions are design decisions which tend to yield
> good soft realtime performance on current hardware, and predictable
> performance, especially in the worst case. You may not want a system that
> stalls for at least 216ms every 100.000th call. But there are no rules in
> the Erlang semantics which constrains us from coming up with another
> message passing scheme, should we invent one that is even better. It is
> just that the current behavior is highly desirable in the systems where
> Erlang tend to get used.
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