[erlang-questions] enif_send() and overrun heap

[Daniel Goertzen]

I wonder if running under a dirty scheduler counts as "running from a
created thread" in which case the first parameter to enif_send() should be
NULL.

On Wed, 20 Jun 2018 at 08:30 Guilherme Andrade <g@REDACTED> wrote:

> Hello list,
>
> I'm bumping into a weird issue in OTP 20.3 (macOS) whereby calling
> enif_send() a few dozen times a second from a dirty scheduler (CPU bound),
> with msg_env=NULL, results in heap overrun.
>
> These is the flow that sooner or later results in heap overrun:
>
> 1) Single Erlang process makes a specific NIF call ~25 times per second
> (dirty scheduler, CPU bound)
> 2) This call will receive a few network packets (non-blocking)
> 3) Each of these packets gets wrapped in a tuple (allocated in process env)
> 4) For each of these wrapped packets, a lookup is made in a map, passed as
> a NIF argument, for a process dedicated to handling this particular packet
> 5.a) If the lookup succeeds, enif_send() is called to dispatch the wrapped
> packet to said dedicated process (with msg_env=NULL) - this is what happens
> to most packets
> 5.b) If the lookup fails, the wrapped packet is accumulated and later
> returned to the NIF caller
>
> Now, when total packets per second increase to a few dozen, sooner or
> later (sometimes as soon as after ~10 seconds) the VM stops abruptly with
> this error message:
>
> > hend=0x0000000013655fa0
> > stop=0x0000000013655ef8
> > htop=0x00000000136562c8
> > heap=0x0000000013652db0
> > beam/erl_gc.c, line 708: <0.506.0>: Overrun stack and heap
>
> (The pid mentioned above corresponds to the NIF caller.)
>
> I tried three things (independently) that prevent the overrun from
> happening under this load:
> A) Increasing the NIF caller heap size from the default (233 words) to
> 23300 words
> B) Not running the NIF under a dirty scheduler
> C) Not calling enif_send
>
> Any ideas on why the overrun happens? Am I missing some very obvious
> transgression in the way enif_send() or dirty schedulers are supposed to be
> used?
>
>
> --
> Guilherme
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