[erlang-questions] Why intra-node messages are copied and not refcounted, rationale?
Thu Jun 4 23:04:39 CEST 2015
This may help you in your studies:
On Sat, May 9, 2015 at 4:34 AM, Nahuel Greco <ngreco@REDACTED> wrote:
> 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
> Nahuel Greco.
> On Thu, May 7, 2015 at 11:27 AM, Jesper Louis Andersen <
> jesper.louis.andersen@REDACTED> wrote:
>> On Thu, May 7, 2015 at 3:07 PM, Nahuel Greco <ngreco@REDACTED> 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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