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tiw

This tool allows you to dump all the timers inside the Erlang VM via gdb. Please note that this might not work properly on your OTP version and it currently does not work on OTP >= 18.

Why?

There are two main reasons why you might need this:

  • you assumed that you can distribute load uniformly using timers, but some glitch caused timers to synchronize (all timers are triggered in a short period of time)
  • you like to look under the hood

How?

Since one example is worth a thousand words...

(you can rerun the commands on a Docker container built from docker/Dockerfile, you need to start two terminals in order to attach gdb to beam).

Terminal 1:

root@e9858d91550c:/tiw# erl -smp enable -pa ebin
Erlang/OTP 17 [erts-6.4] [source] [64-bit] [smp:1:1] [async-threads:10]
[kernel-poll:false]

Eshell V6.4  (abort with ^G)
1> os:getpid().
"19"
2> tiw_example:process_timers(100, 10000),
2> timer:sleep(2000),
2> tiw_example:bif_timers(100, 20000),
2> timer:sleep(2000),
2> tiw_example:port_timers(100, 30000).
[<0.236.0>,<0.237.0>,<0.238.0>,<0.239.0>,<0.240.0>,
 <0.241.0>,<0.242.0>,<0.243.0>,<0.244.0>,<0.245.0>,<0.246.0>,
 <0.247.0>,<0.248.0>,<0.249.0>,<0.250.0>,<0.251.0>,<0.252.0>,
 <0.253.0>,<0.254.0>,<0.255.0>,<0.256.0>,<0.257.0>,<0.258.0>,
 <0.259.0>,<0.260.0>,<0.261.0>,<0.262.0>,<0.263.0>,<0.264.0>|...]

Terminal 2:

root@10a857bd690c:/# gdb --pid=19
(gdb) source
~/.kerl/builds/17.5/otp_src_17.5/erts/etc/unix/etp-commands.in
(gdb) source tiw/priv/tiw.in
(gdb) set height 0
(gdb) set logging file timers_smp
(gdb) set logging on
(gdb) tiw
=== tiw start
35696
36083
3
bif_timer_timeout
<0.145.0> ! {msg,10}.

36120
16
ptimer_timeout
#NotPid<0x2a47> | #Port<0.676>

36401
8
ptimer_timeout
<0.88.0> | #NotPort<0x583>
...
35329
9
ptimer_timeout
<0.101.0> | #NotPort<0x653>
=== tiw end

This should've generated a /timers_smp file.

Later you can use some helpers to parse the file (of course it can be done somewhere else):

root@10a857bd690c:/tiw# erl -pa ebin
Erlang/OTP 17 [erts-6.4] [source] [64-bit] [async-threads:10]
[kernel-poll:false]

Eshell V6.4  (abort with ^G)
1> rr("src/tiw.hrl").
[timer]
2> Timers = tiw:load("../timers_smp").
[#timer{expires_in = 1000,callback = aux_work_timeout,
        info = <<"NULL">>},
 #timer{expires_in = 4993,callback = ptimer_timeout,
        info = <<"<0.36.0> | #NotPort<0x243>">>},
 #timer{expires_in = 14993,callback = ptimer_timeout,
        info = <<"<0.37.0> | #NotPort<0x253>">>},
 #timer{expires_in = 16995,callback = bif_timer_timeout,
        info = <<"<0.136.0> ! {msg,1}.">>},
 #timer{expires_in = 24993,callback = ptimer_timeout,
        info = <<"<0.38.0> | #NotPort<0x263>">>},
 #timer{expires_in = 29000,callback = ptimer_timeout,
        info = <<"#NotPid<0x2827> | #Port<0.642>">>},
 #timer{expires_in = 34993,callback = ptimer_timeout,
        info = <<"<0.39.0> | #NotPort<0x273>">>},
 #timer{expires_in = 36995,callback = bif_timer_timeout,
        info = <<"<0.137.0> ! {msg,2}.">>},
 #timer{expires_in = 44993,callback = ptimer_timeout,
        info = <<"<0.40.0> | #NotPort<0x283>">>},
 #timer{expires_in = 54993,callback = ptimer_timeout,
        info = <<"<0.41.0> | #NotPort<0x293>">>},
 #timer{expires_in = 56995,callback = bif_timer_timeout,
        info = <<"<0.138.0> ! {msg,3}.">>},
 #timer{expires_in = 59000,callback = ptimer_timeout,
        info = <<"#NotPid<0x2837> | #Port<0.643>">>},
 #timer{...},
 {...}|...]
3> tiw:buckets(Timers, 1000).
[{{1000,1999},1},
 {{4000,4999},1},
 {{14000,14999},1},
 {{16000,16999},1},
 {{24000,24999},1},
 {{29000,29999},1},
 {{34000,34999},1},
 {{36000,36999},1},
 {{44000,44999},1},
 {{54000,54999},1},
 {{56000,56999},1},
 {{59000,59999},1},
 {{64000,...},1},
 {{...},...},
 {...}|...]

The second command returns number of timers in appropriate seconds (interval is the second argument, in milliseconds).