While tracing my modules using dbg, I encountered with the problem how to collect messages such as spawn, exit, register, unregister, link, unlink, getting_linked, getting_unlinked, which are allowed by erlang:trace, but only for those processes which were spawned from my modules directly?
As an examle I don't need to know which processes io module create, when i call io:format in some module function. Does anybody know how to solve this problem?
Short answer:
one way is to look at call messages followed by spawn messages.
Long answer:
I'm not an expert on dbg. The reason is that I've been using an (imho much better, safer and even handier) alternative: pan , from https://gist.github.com/gebi/jungerl/tree/master/lib/pan
The API is summarized in the html doc.
With pan:start you can trace specifying a callback module that receives all the trace messages. Then your callback module can process them, e.g. keep track of processes in ETS or a state data that is passed into every call.
The format of the trace messages is specified under pan:scan.
For examples of callback modules, you may look at src/cb_*.erl.
Now to your question:
With pan you can trace on process handling and calls in your favourit module like this:
pan:start({ip, CallbackModule}, Node, all, [procs,call], {Module}).
where Module is the name of your module (in this case: sptest)
Then the callback module (in this case: cb_write) can look at the spawn messages that follow a call message within the same process, e.g.:
32 - {call,{<6761.194.0>,{'fun',{shell,<node>}}},{sptest,run,[[97,97,97]]},{1332,247999,200771}}
33 - {spawn,{<6761.194.0>,{'fun',{shell,<node>}}},{{<6761.197.0>,{io,fwrite,2}},{io,fwrite,[[77,101,115,115,97,103,101,58,32,126,115,126,110],[[97,97,97]]]}},{1332,247999,200805}}
As pan is also using the same tracing back end as dbg, the trace messages (and the information) can be collected using the Erlang trace BIF-s as well, but pan is much more secure.
Related
From Using trace and dbg in Erlang, I'm aware that one can trace calls to specified functions from all functions using
1>dbg:p(all, c).
However, how does one trace calls to all functions from all functions? e.g:
1>dbg:foo().
*ALL Erlang function calls will be traced from here on out. Prepare yourself.*
2>lists:append("abc", "def").
*trace*
Tracing all calls to all functions is not something you want to do, as that would easily swamp your output and make your shell unusable. After all, the shell calls functions to perform its duties too, as does dbg, so you'd end up seeing endless traces of calls to io functions involved in generating and displaying the trace.
Instead, take a look at the various dbg:tp and dbg:tpl functions. Apply them after your call to dbg:p(all, c). They allow you to trace specific modules and specific functions. Start by tracing a particular function or module, and then based on the trace you see, widen your trace to callers of that function. You can also use dbg:ctp and dbg:ctpl to turn off tracing for specific functions or modules once they're no longer relevant to your debugging efforts. With this approach, you can use dbg to iteratively zero in on whatever it is you're looking for.
I am having trouble using pthreads in my MPI program. My program runs fine without involving pthreads. But I then decided to execute a time-consuming operation in parallel and hence I create a pthread that does the following (MPI_Probe, MPI_Get_count, and MPI_Recv). My program fails at MPI_Probe and no error code is returned. This is how I initialize the MPI environment
MPI_Init_thread(&argc, &argv, MPI_THREAD_MULTIPLE, &provided_threading_support);
The provided threading support is '3' which I assume is MPI_THREAD_SERIALIZED. Any ideas on how I can solve this problem?
The provided threading support is '3' which I assume is MPI_THREAD_SERIALIZED.
The MPI standard defines thread support levels as named constants and only requires that their values are monotonic, i.e. MPI_THREAD_SINGLE < MPI_THREAD_FUNNELED < MPI_THREAD_SERIALIZED < MPI_THREAD_MULTIPLE. The actual numeric values are implementation-specific and should never be used or compared against.
MPI communication calls by default never return error codes other than MPI_SUCCESS. The reason for that is, MPI calls the communicator's error handler before an MPI call returns and all communicators are initially created with MPI_ERRORS_ARE_FATAL installed as their error handler. That error handler terminates the program and usually prints some debugging information, e.g. the reason for the failure. Both MPICH (and its countless variants) and Open MPI produce quite elaborate reports on what led to the termination.
To enable user error handling on communicator comm, you should make the following call:
MPI_Comm_set_errhandler(comm, MPI_ERRORS_RETURN);
Watch out for the error codes returned - their numerical values are also implementation-specific.
If your MPI implementation isn't willing to give you MPI_THREAD_MULTIPLE, there's three things you can do:
Get a new MPI implementation.
Protect MPI calls with a critical section.
Cut it out with the threading thing.
I would suggest #3. The whole point of MPI is parallelism -- if you find yourself creating multiple threads for a single MPI subprocess, you should consider whether those threads should have been independent subprocesses to begin with.
Particularly with MPI_THREAD_MULTIPLE. I could maybe see a use for MPI_THREAD_SERIALIZED, if your threads are sub-subprocess workers for the main subprocess thread... but MULTIPLE implies that you're tossing data around all over the place. That loses you the primary convenience offered by MPI, namely synchronization. You'll find yourself essentially reimplementing MPI on top of MPI.
Okay, now that you've read all that, the punchline: 3 is MPI_THREAD_MULTIPLE. But seriously. Reconsider your architecture.
I was playing with the process dictionary inside a gen_server module, i called get() function and i get something like this.
{'$ancestors',[main_server,<0.30.0>]},
{'$initial_call',{child_server,init,1}}]
what happen if i erased the process dictionary, what would go wrong ?
i erased it and every thing worked fine, even
calling a function that generates an exception in the child_server the main_server still can get the exit signal.
$ancestors is used only in the initialization stage, to get the parent's PID, which is used to catch the EXIT message coming from the parent, so that the terminate stuff can get executed. Erasing this key when the server is up and running makes no difference.
$initial_call, on the other hand, is used in the crash report by proc_lib to dump the MFA info.
A quick grep in the OTP source tree can certainly help.
I think some debug functions may use process dictionary, for example erlang:process_info/2
In this example, the author avoids a deadlock situation by doing:
self() ! {start_worker_supervisor, Sup, MFA}
in his gen_server's init function. I did something similar in one of my projects and was told this method was frowned upon, and that it was better to cause an immediate timeout instead. What is the accepted pattern?
Update for Erlang 19+
Consider using the new gen_statem behaviour. This behaviour supports generating of events internal to the FSM:
The state function can insert events using the action() next_event and such an event is inserted as the next to present to the state function. That is, as if it is the oldest incoming event. A dedicated event_type() internal can be used for such events making them impossible to mistake for external events.
Inserting an event replaces the trick of calling your own state handling functions that you often would have to resort to in, for example, gen_fsm to force processing an inserted event before others.
Using the action functionality in that module, you can ensure your event is generated in init and always handled before any external events, specifically by creating a next_event action in your init function.
Example:
...
callback_mode() -> state_functions.
init(_Args) ->
{ok, my_state, #data{}, [{next_event, internal, do_the_thing}]}
my_state(internal, do_the_thing, Data) ->
the_thing(),
{keep_state, Data);
my_state({call, From}, Call, Data) ->
...
...
Old answer
When designing a gen_server you generally have the choice to perform actions in three different states:
When starting up, in init/1
When running, in any handle_* function
When stopping, in terminate/2
A good rule of thumb is to execute things in the handling functions when acting upon an event (call, cast, message etc). The stuff that gets executed in init should not wait for events, that's what the handle callbacks are for.
So, in this particular case, a kind of "fake" event is generated. I'd say it seems that the gen_server always wants to initiate the starting of the supervisor. Why not just do it directly in init/1? Is there really a requirement to be able to handle another message in-between (the effect of doing it in handle_info/2 instead)? That windown is so incredibly small (the time between start of the gen_server and the sending of the message to self()) so it's highly unlikely to happen at all.
As for the deadlock, I would really advise against calling your own supervisor in your init function. That's just bad practice. A good design pattern for starting worker process would be one top level supervisor, with a manager and a worker supervisor beneath. The manager starts workers by calling the worker supervisor:
[top_sup]
| \
| \
| \
man [work_sup]
/ | \
/ | \
/ | \
w1 ... wN
Just to complement what has already been said about splitting a servers initialisation into two parts, the first in the init/1 function and the second in either handle_cast/2 or handle_info/2. There is really only one reason to do this and that is if the initialisation is expected to take a long time. Then splitting it up will allow the gen_server:start_link to return faster which can be important for servers started by supervisors as they "hang" while starting their children and one slow starting child can delay the whole supervisor startup.
In this case I don't think it is bad style to split the server initialisation.
It is important to be careful with errors. An error in init/1 will cause the supervisor to terminate while an error in the second part as they will cause the supervisor to try and restart that child.
I personally think it is better style for the server to send a message to itself, either with an explicit ! or a gen_server:cast, as with a good descriptive message, for example init_phase_2, it will be easier to see what is going on, rather than a more anonymous timeout. Especially if timeouts are used elsewhere as well.
Calling your own supervisor sure does seem like a bad idea, but I do something similar all the time.
init(...) ->
gen_server:cast(self(), startup),
{ok, ...}.
handle_cast(startup, State) ->
slow_initialisation_task_reading_from_disk_fetching_data_from_network_etc(),
{noreply, State}.
I think this is clearer than using timeout and handle_info, it's pretty much guaranteed that no message can get ahead of the startup message (no one else has our pid until after we've sent that message), and it doesn't get in the way if I need to use timeouts for something else.
This may be very efficient and simple solution, but I think it is not good erlang style.
I am using timer:apply_after, which is better and does not make impression of interacting with external module/gen_*.
I think that the best way would be to use state machines (gen_fsm). Most of our gen_srvers are really state machine, however because initial work effort to set up get_fsm I think we end up with gen_srv.
To conclude, I would use timer:apply_after to make code clear and efficient or gen_fsm to be pure Erlang style (even faster).
I have just read code snippets, but example itself is somehow broken -- I do not understand this construct of gen_srv manipulating supervisor. Even if it is manager of some pool of future children, this is even more important reason to do it explicitly, without counting on processes' mailbox magic. Debugging this would be also hell in some bigger system.
Frankly, I don't see a point in splitting initialization. Doing heavy lifting in init does hang supervisor, but using timeout/handle_info, sending message to self() or adding init_check to every handler (another possibility, not very convenient though) will effectively hang calling processes. So why do I need "working" supervisor with "not quite working" gen_server? Clean implementation should probably include "not_ready" reply for any message during initialization (why not to spawn full initialization from init + send message back to self() when complete, which would reset "not_ready" status), but then "not ready" reply should be properly processed by the caller and this adds a lot of complexity. Just suspending a reply is not a good idea.
io:format calls in common_test modules don't come to the user console, although error_log messages do. I can't figure out where io:format calls DO go, either. Running ack in my repo on relevant strings turns up nothing. Does anyone know where they go?
Define {logdir, "logs"}. in your spec and then the io:format goes to the log. It is done by setting the group_leader via erlang:group_leader/2 to capture the IO output from your tests.
The actual output is underneath the respective test case in the log output of that test case. logs/index.html is your starting point to peruse.
Finally, it is somewhat loosely documented in http://www.erlang.org/doc/apps/common_test/run_test_chapter.html section 6.9.