Background

I have a module that saves data into an ETS without ever releasing it. Obviously, this is is a terrible idea. If you are not sure why, just ask yourself what happens when the machine no longer has memory to store data

Solution

The obvious solution would be to periodically clean the data from the table, but I have orders to not do it, it has been decided another process in another application will do it. This process doesn’t exist yet and may never even exist, depending on who implements the other application.

Since I can’t rely on this, I decided to add a fallback mechanism. Dam’s have a fallback mechanism for excess water to flow, Nuclear Reactors have a mechanism for sea water to enter the reactors and cool them down (and then they release the heated, radioactive water back to the sea), so it only makes sense that if I want to build a reliable system, instead of waiting for the machine to crash catastrophically and restart, I should follow the example of such systems and decide what to do in case the system faces load it can’t cope with.

I decided I want to ditch requests upon a certain memory usage threshold. Let’s say, if the machine has 10GB of memory and BEAM is using 9GB already, I want to stop saving data into my ETS table.

Problem

To implement said solution, I made some research and found :memsup:

So, I rushed to open a iex terminal and test it out. But I can’t.

:memsup.start_link()
{:ok, #PID<0.142.0>}
iex(3)> [os_mon] memory supervisor port (memsup): Erlang has closed

16:17:12.618 [error] GenServer :memsup terminating
** (ArgumentError) argument error
    (stdlib) gen_event.erl:269: :gen_event.send/2
    memsup.erl:994: :memsup.set_alarm/2
    memsup.erl:375: :memsup.handle_info/2
    (stdlib) gen_server.erl:637: :gen_server.try_dispatch/4
    (stdlib) gen_server.erl:711: :gen_server.handle_msg/6
    (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3
Last message: {:collected_sys, {7173283840, 8235118592}}
State: {:state, {:unix, :linux}, true, :undefined, :undefined, false, 60000, 30000, 0.8, 0.05, #PID<0.143.0>, #Reference<0.2483620304.3371171843.48667>, :undefined, [:reg], []}
** (EXIT from #PID<0.136.0>) shell process exited with reason: an exception was raised:
    ** (ArgumentError) argument error
        (stdlib) gen_event.erl:269: :gen_event.send/2
        memsup.erl:994: :memsup.set_alarm/2
        memsup.erl:375: :memsup.handle_info/2
        (stdlib) gen_server.erl:637: :gen_server.try_dispatch/4
        (stdlib) gen_server.erl:711: :gen_server.handle_msg/6
        (stdlib) proc_lib.erl:249: :proc_lib.init_p_do_apply/3

I get this cryptic error when trying to start_link the process so I can use it. What am I doing wrong?

Extra

I know that comparing BEAM’s total memory usage against the machine’s memory usage is not perfect, processes or atoms could be taking 99% of the used memory and I would still punish the ets tables. However, I still think this would make sense, since if that is the problem, I still don’t want my ETS tables to save anything until the issue is solved.