:memsup and BEAM memory consumption


I am testing an application where I perform some actions depending on the amount of memory the machine has free. To supervise the machine’s memory I am using :memsup.get_memory_data/0 but I am getting some really weird results.


I am have a machine with 8GB of RAM, which should be more than enough for opening iex and doing some tests. However, :memsup always returns absurd results like the following:

iex(9)> {total, allocated, _worse} = :memsup.get_memory_data()
{8235098112, 7862636544, {#PID<0.118.0>, 372944}}
iex(10)> (allocated / total) * 100                             

No matter what I do, I always have :memsup report BEAM is using up to 95% of my machine’s memory. All I have open is a single session of iex that I am using as a playground.

What’s even more weird is that htop returns me a completely different value of memory usage, stating I am using 6.22GB out of 7.67GB, which still doesn’t hold up to 95% usage…

(6.22 / 7.67) * 100


So now I have some questions:

  1. Does :memsup even work?
  2. Why are the values :memsup gives me so different?
  3. Why is BEAM using over 90% of the total memory if all I have is a simple iex shell session open?
  4. Is there a reliable way to know if my machine is struggling with memory?

The function is normally asynchronous in the sense that it does not invoke a memory check, but returns the latest available value. The one exception if is the function is called before a first memory check is finished, in which case it does not return a value until the memory check is finished.

So it is possible that the issue is that your check ran in the past and since then it wasn’t updated. Additionally memsup returns memory usage for whole machine not just BEAM environment.

According to my understanding, :mem_sup checks the memory every minute by default. Indeed, if I close my iex session and return 5 minutes later, I see a different value from total usage, but not one that diverges significantly from the 95% I got before.

And no, I am not using 95% of the entire memory my machine has available, htop and other tools confirm it.

I wonder if the numbers seem off because total reports physical memory while allocated includes(?) cached memory.

memsup seems to be designed to pinpoint individual processes that are heavy memory users - the rest of the functionality seems to just report information reported by the OS.

You may want to look into recon (possibly via one of the Elixir wrappers) - see page 41 of Erlang in Anger


To piggy back off of this, it looks like recon_alloc:memory/1 has the answers.

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