[erlang-bugs] Dirty schedulers and '-smp disable'

[Steve Vinoski]

On Tue, Jun 30, 2015 at 10:39 AM, Knut Nesheim <knutin@REDACTED> wrote:

> Dear list,
>
> I ran into unexpected behaviour in the following situation:
>
>  * OTP 18.0, compiled from the git tag with dirty schedulers enabled
>  * NIF with the ERL_NIF_DIRTY_JOB_CPU_BOUND flag
>  * Small machine with only one core (AWS t1.micro)
>  * The first log line from startup with no explicit flags looks like
> this: Erlang/OTP 18 [erts-7.0] [source] [64-bit] [async-threads:10]
> [hipe] [kernel-poll:false]
>
> When I call the NIF, the calling process hangs forever. When I call it
> from the shell, I'm unable to interrupt the process (C-g, i 1 does
> nothing useful).
>
> If I explicitly use '-smp enable' as arguments to erl, the NIF runs
> fine. In that case the first log line looks like this: Erlang/OTP 18
> [erts-7.0] [source] [64-bit] [smp:1:1] [ds:1:1:10] [async-threads:10]
> [hipe] [kernel-poll:false]
>
> This behaviour got me a bit confused, as there is no indication what
> is happening except "something somewhere got stuck". It's not a common
> case for me, as most machines have multiple cores except tiny cloud
> instances or virtual machines.
>

The short answer is that currently, dirty schedulers always require SMP.

The longer answer is that configure should raise an error if this
configuration is attempted. I can't recall for sure but I think it behaved
like this at one point, but a lot changed for Erlang 18 and so perhaps this
config check got lost along the way.

--steve
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