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<  Erlang  ~  Unexpected beam.smp process

odi
Posted: Fri Sep 09, 2011 11:27 am Reply with quote
Joined: 19 Feb 2009 Posts: 9
Hi,
We've been running an Erlang application on multicore machine and we've noticed that our monitoring tool (atop) sometimes informs that there're multiple instances of beam.smp process (however there should be only one Erlang VM running).

Output from atop:
Quote:

PRG S2 1315467785 2011/09/08 09:43:05 5 32589 (beam.smp) S 0 0 32589 29 0 1315466977 (/home/services/bin/beam.smp -sbt db -P 134217727 -- -root /home/services -progname erl -- -home /root -- -boot ./r) 1 2 26 1 0 0 0 0 0 0 0

PRG S2 1315467785 2011/09/08 09:43:05 5 13225 (beam.smp) D 0 0 13225 1 -2147483648 1315467784 (/home/services/bin/beam.smp -sbt db -P 134217727 -- -root /home/services -progname erl -- -home /root -- -boot ./r) 32589 0 0 1 0 0 0 0 0 0 0

It looks like there're 2 Erlang VMs with the same name, but the second one (PID 13225) is in a D state (uninterruptible sleep) and is a child of the real beam.smp process (PID 32589).

Does anybody know what is the 2nd process? Is it some kind of port or something like that? What else can we check?
P.S. We use OTP R14B01
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rvirding
Posted: Sun Sep 18, 2011 9:46 pm Reply with quote
User Joined: 30 Aug 2006 Posts: 452 Location: Stockholm, Sweden
I am not familiar with atop but could these be separate threads within the same process? The beam normally runs at least one thread pre core.

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Robert Virding, Erlang Solutions Ltd.
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odi
Posted: Mon Sep 19, 2011 7:19 am Reply with quote
Joined: 19 Feb 2009 Posts: 9
These are certainly system processes forked by beam.smp.
You can try simple program:
Code:
-module(spawn).

-export([do/1]).

do(N) ->
    PIDs = spawn_do(N, []),
    ok = start_do(PIDs, self()),
    ok = wait_do(PIDs).

spawn_do(0, PIDs) ->
    PIDs;
spawn_do(N, PIDs) ->
    PID = spawn_link(fun() ->
            receive
                {start, PPID} ->
                    ok
            end,
            os:cmd("ping -c 1 -W 1 127.0.0.1"),
            PPID ! {done, self()}
        end),
    spawn_do(N - 1, [PID | PIDs]).

start_do([], _PPID) ->
    ok;
start_do([PID | PIDs], PPID) ->
    PID ! {start, PPID},
    start_do(PIDs, PPID).

wait_do([]) ->
    io:format("Test finished!~n", []),
    ok;
wait_do([PID | PIDs]) ->
    receive {done, PID} ->
        io:format("~p process is done.~n", [PID])
    end,
    wait_do(PIDs).

By running it with
Quote:
[user@arch workspace]$ erl -sname node
Erlang R14B (erts-5.8.1) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.1 (abort with ^G)
(node@arch)1> spawn:do(1000).
and checking system processes with
Quote:
while [ 1 -eq 1 ]; do ps ax | grep -E "beam|unix" | grep -v "grep"; done | tee spawn_test.log
you can see that there're forked child_setup processes and sometimes there's more than one beam.smp process with '-sname node'.
I suppose that Erlang VM forks child process for the purpose of 'os:cmd/1' and before exec is run, the new beam.smp process is reported by 'ps' with the same name as main Erlang VM process.
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NedFrank
Posted: Wed Dec 21, 2011 3:14 pm Reply with quote
Joined: 21 Dec 2011 Posts: 7 Location: Kansas
odi wrote:
Hi,
We've been running an Erlang application on multicore machine and we've noticed that our monitoring tool (atop) sometimes informs that there're multiple instances of beam.smp process (however there should be only one Erlang VM running).



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