[erlang-questions] erl_tracer, trace_status and pids

[Loïc Hoguin]

Thanks for the replies!

On 05/29/2017 01:26 PM, Lukas Larsson wrote:
>     * when you use enabled_*/3 and trace_*/5 functions for everything,
>     the trace/5 function will never be called
> 
> hmm, that should be true in the current implementation. That may change 
> in the future though.

I'll change it in the future then. :-)

>     * trace_status is always called before any other enabled_*/3 so I
>     should be fine just doing nothing in enabled_*/3, right? I'm only
>     checking that the tracer processes are still alive anyway.
> 
> 
> no, trace_status is not always called before enabled_*. The only event 
> that I know of where it is called is enabled_call/3, there may be a 
> couple of more but not all of them.

Alright. I'm a bit confused still. Would it make sense to only perform 
some checks for trace_status, for example checking that the tracer 
process exists? Then for other enabled_*/3 calls I would just reject 
events I don't need.

>     Basically if you could shed light on the circumstances around
>     trace_status that would help a lot for optimizing this. :-)
> 
> 
> the check for trace_status call is made in various places in the vm code 
> where the VM finds it convenient to do a check and see if the tracer is 
> still alive. There are no specific rules about when it is called.

It sounds like the answer to my question is a yes. :-)

>     Another question, I have a pool of tracer processes and want to send
>     the events to one of them depending on the pid (all events of a pid
>     gets to the same tracer process). Is this a valid solution?
> 
>     nth = (tracee.pid >> 4) % len;
> 
>     This gives me the number of the tracer I will send the message to.
> 
> 
> It is one way to do it, but you are relying on the internal 
> representation of a PID, which may change, and I don't think you get a 
> very good hash.
> 
> In OTP-20 there is a new nif called enif_hash/3 that can be used to get 
> a better hash value. 
> (http://erlang.org/documentation/doc-9.0-rc1/erts-9.0/doc/html/erl_nif.html#enif_hash)

Thanks, I will use it on OTP 20+ then.

-- 
Loïc Hoguin
https://ninenines.eu