[eeps] New EEP: setrlimit(2) analogue for Erlang

[Michael Truog]

An idea which seems like it should be included in this EEP, is simply a function to determine how much real memory is consumed by an erlang variable.  I understand we have erlang:external_size which provides the size based on the external term format.  I also understand that the memory pools make erlang consume more than the size that would be returned.  There is some documentation on memory consumption of erlang data here: http://www.erlang.org/doc/efficiency_guide/advanced.html#id68921 .  I attempted to use an older version of this documentation to write my own function, but ran into problems (created output that seemed incorrect) and taking into account HiPE data is a separate part that would generate different results.

So, these erlang limits are great additions, but it seems like we would need a function for understanding where the limits are broken, for debugging and development (and possibly dynamic runtime changes).  It would be interesting if the function was allowed in guard tests, which might help for embedded development.

Thanks,
Michael

On 02/07/2013 07:27 AM, Björn-Egil Dahlberg wrote:
> I dug out what I wrote a year ago ..
>
> eep-draft:
> https://github.com/psyeugenic/eep/blob/egil/system_limits/eeps/eep-00xx.md
>
> Reference implementation:
> https://github.com/psyeugenic/otp/commits/egil/limits-system-gc/OTP-9856
> Remember, this is a prototype and a reference implementation.
>
> There is a couple of issues not addressed or at least open-ended.
>
> * Should processes be able to set limits on other processes? I think not though my draft argues for it. It introduces unnecessary restraints on erts and hinders performance. 'save_calls' is such an option.
>
> * ets - if your table increases beyond some limit. Who should we punish? The inserter? The owner? What would be the rationale? We cannot just punish the inserter, the ets table is still there taking a lot of memory and no other process could insert into the table. They would be killed as well. Remove the owner and hence the table (and potential heir)? What kind of problems would arise then? Limits should be tied into a supervision strategy and restart the whole thing.
>
> * In my draft and reference implementation I use soft limits. Once a process reaches its limit it will be marked for termination by an exit signal. The trouble here is there is no real guarantee for how long this will take. A process can continue appending a binary for a short while and ending the beam with OOM still. (If I remember it correctly you have to schedule out to terminate a process in SMP thus you need to bump all reduction. But, not all things handle return values from the garbage collector, most notably within the append_binary instruction). There may be other issues as well.
>
> * Message queues. In the current implementation of message queues we have two queues. An inner one which is locked by the receiver process while executing and an outer one which other processes will use and thus not compete for a message queue lock with the executing process. When the inner queue is depleted the receiver process will lock the outer queue and move the entire thing to the inner one. Rinse and repeat. The only guarantee we have to ensure with our implementation is: signal order between two processes. So, in the future we might have several queues to improve performance. If you introduce monitoring of the total number messages in the abstracted queue (all the queues) this will most probable kill any sort of scalability. For instance a sender would not be allowed to check the inner queue for this reason. Would a "fast" counter check in the inner queue be allowed? Perhaps if it is fast enough, but any sort of bookkeeping costs performance. If we introduce even
> more queues for scalability reasons this will cost even more.
>
> * What about other memory users? Drivers? NIFs?
>
> I do believe in increments in development as long it is path to the envisioned goal.
> And to reiterate, i'm not convinced that limits on just processes is the way to go. I think a complete monitoring system should be envisioned, not just for processes.
>
> // Björn-Egil
>
> On 2013-02-06 23:03, Richard O'Keefe wrote:
>> Just today, I saw Matthew Evans'
>>
>> 	This pertains to a feature I would like to see
>> 	in Erlang.  The ability to set an optional
>> 	"memory limit" when a process and ETS table is
>> 	created (and maybe a global optional per-process
>> 	limit when the VM is started).  I've seen a few
>> 	cases where, due to software bugs, a process size
>> 	grows and grows; unfortunately as things stand
>> 	today the result is your entire VM crashing -
>> 	hopefully leaving you with a crash_dump. 
>>
>> 	Having such a limit could cause the process to
>> 	terminate (producing a OOM crash report in
>> 	erlang.log) and the crashing process could be
>> 	handled with supervisor rules.  Even better you
>> 	can envisage setting the limits artificially low
>> 	during testing to catch these types of bugs early on.
>>
>> in my mailbox.  I have seen too many such e-mail messages.
>> Here's a specific proposal.  It's time _something_ was done
>> about this kind of problem.  I don't expect that my EEP is
>> the best way to deal with it, but at least there's going to
>> be something for people to point to.
>>
>>
>>
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