Does erlang record when a process started?

Question

I'm working with monitoring an Erlang application and I'm currently trying to determine how long a specific PID has been running. Absolute timestamp or duration would work for me, but I do not see either of those bits of data in process_info or via the sys module. Is there a way to get this information from within the Erlang VM?

I can get the start time of the overall VM from the ps command, but that doesn't have any visibility of individual Erlang processes.

Edit:

I've noticed that when the VM crashes, the erl_crash.dump contains a started timestamp for each process, so I know it's in there!

Answer 1

I was looking in toolbar:start(). the process monitor and don't look time alive or time init, but it is easy to make. In dictionary of process you can save a value when init and that this process can respond to other process about this value.

For read value in dictionary process you can use get/1 and for save use put/2 (first key and second value).

Respond to other process about this value is same that others responses, etc.

Answer 2

Write a process which manages this. Since you can find the PID of the given process, you can also set a monitor on the process. Then, if the process errors out, you will get a message in your mailbox.

I would guess this could form the basis for a solution for you.

Answer 3

tl;dr: yes, but you can't get to it.

If you dig into the OTP source code at https://github.com/erlang/otp, you'll find (by searching for "erl_crash") that the file responsible for writing the crash dump is called erts/emulator/beam/break.c.

Searching that file for "started" (it's both a good guess, and it's what appears in the crash dump) will get you to lines 248-249 (all line numbers based on the OTP-18.3.1 tag), which look like this:

approx_started = (time_t) p->approx_started;
erts_print(to, to_arg, "Started: %s", ctime(&approx_started));

Searching the rest of the source code for approx_started shows it being declared in erts/emulator/beam/erl_process.h as a member of struct process. It is written in erts/emulator/beam/erl_process.c. The only place it is read is in break.c, when writing the crash dump.

So, yes, Erlang does record the (approximate) time that a process was started. But, no, you can't get to it.

I have no idea why it's "approximate".