[erlang-questions] Differences between erlang:system_profile and erlang:trace

Björn-Egil Dahlberg egil@REDACTED
Tue Jan 3 10:06:17 CET 2012

 From what I remember (trying to recall something written five years 
ago) system_profile used to respect this policy but from looking at the 
code it seems like it doesn't now. The tracing and scheduler queues has 
been rewritten several times since then and it is possible that 
something has been lost.

I will have a look at it. Thank you for reporting this.


On 2012-01-03 01:37, Magnus Klaar wrote:
> Hi!
> I'm seeing a strange behavior when using erlang:system_info/2 to trace 
> the running processes on an erlang node. I know it's flagged as 
> experimental so I will assume it's actually a feature even if the 
> documentation does not agree. When a process is used to receive the 
> profile messages from erlang:system_info/2 the receiving process is 
> also profiled. The effect of this is that the process receiving the 
> profile messages will receive an infinite sequence of 'inactive' ... 
> 'active' ... 'inactive' .... messages from the runtime system. The 
> manpage states that the "The receiver is excluded from all 
> profiling.". I've compared this with using the erlang:trace/3 function 
> to trace running processes, when this function is used the tracer 
> processes never receives a message when the tracer process scheduled.
> Two eunit tests for showing the difference: 
> https://gist.github.com/1552673
> Running "erl -noshell -s system_profile test -s init stop" on my 
> system yields the following result:
> system_profile.erl:30:<0.35.0>: Total: 525169, For tracer: 524824, For 
> others: 345
> system_profile:13: system_profile_test_...*failed*
> ::{assertion_failed,[{module,system_profile},
>                    {line,32},
>                    {expression,"SelfCount =:= 0"},
>                    {expected,true},
>                    {value,false}]}
> system_profile.erl:58:<0.123.0>: Total: 338, For tracer: 0, For 
> others: 338
> =======================================================
>   Failed: 1.  Skipped: 0.  Passed: 1.
> My questions are: Am I using it wrong? If so, how should it be used? 
> If not, Is it a bug?
> MVH Magnus
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