[erlang-questions] Proposal: process_info(Pid, start_time)

[Vladimir Gordeev]

Good feature.

Currently I'm trying to build a trace visualization tool, and such
information would be quite useful.
It would allow to explore in what order different parts of the system were
started.

Also +1 for monotonic timestamp, that would allow to consistently put it
along other trace timestamps.


On Mon, Oct 9, 2017 at 5:20 PM, Danil Zagoskin <z@REDACTED> wrote:

> Hi!
>
> TL;DR: Would you like to know when any process was started in little cost
> of memory usage on 32-bit systems and crashdump generation speed?
>
> Sometimes we have to inspect long-running systems.
> And if a complex enough system runs for a month, it's hard to answer these
> questions:
>  - Process A should run at most 1 hour. Did it stall a week ago?
>  - Process B is supervised and it's not good when it restarts. Did it
> restart last night?
>  - Did process C exist when process D was started?
>  - Was pid E reused by a new process since it was stored in ETS?
>
> To answer that questions we need a process start time to be stored
> somewhere.
> Given that almost every project uses third-party libraries with their own
> processes, the universal solution could be having process start time
> available in process_info.
>
> It turned out that ERTS already have a process start time, used by a
> crashdump writer.
>
> I proposed a PR exposing that value in process_info: https://github.
> com/erlang/otp/pull/1597
> It's very simple — just return a value we already have.
>
> But as Rickard Green noticed, it's better to have this time as Erlang
> monotonic time instead of system seconds in current implementation.
> This will slightly increase memory consumption on 32-bit systems and
> slightly slow down crashdump generation.
>
> The other option Rickard proposed is to remove that timestamp completely
> and save some memory.
>
>
> So, the questions are:
>
>  - Do you find process_info(Pid, start_time) useful and worth 4
> bytes/process on 32 bit systems?
>  - Do you find process start time in a crashdump useful?
>
> --
> Danil Zagoskin | z@REDACTED
>
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