[erlang-questions] ETS and CPU

Hynek Vychodil <>
Tue Mar 15 19:13:20 CET 2016


If you start it in this way, your map is copied from a heap of the parent
process to a heap of the new process. The difference could be starting size
of a heap. Sou you could try to start the process as spawn_op(fun()->
do_something(A) end, {}min_heap_size, Size}).
With Size around 2 * erts_debug:flat_size(Map).

On Tue, Mar 15, 2016 at 4:07 PM, Alex Howle <> wrote:

> The map is not being sent as a message. It is passed in to the spawned
> processes by being in scope of the spawned function.
>
> Pseudocode:
>
> A=bigmap,
> spawn(fun()-> do_something(A) end).
> On 15 Mar 2016 13:43, "Alex Howle" <> wrote:
>
>> The map is not being sent as a message. It is passed in to the spawned
>> processes by being in scope of the spawned function.
>>
>> Pseudocode:
>>
>> A=bigmap,
>> spawn(fun()-> do_something(A) end).
>> On 15 Mar 2016 11:32, "Sverker Eriksson" <>
>> wrote:
>>
>>> Each successful ets:lookup call is a copy operation of the entire term
>>> from ETS to the process heap.
>>>
>>> If you are comparing ets:lookup of big map
>>> to sending big map in message then I would expect
>>> ets:lookup to win, as copy_shallow (used by ets:lookup)
>>> is optimized to be faster than copy_struct (used by send).
>>>
>>>
>>> /Sverker, Erlang/OTP
>>>
>>>
>>> On 03/15/2016 09:52 AM, Alex Howle wrote:
>>>
>>> I've been experiencing an issue and was wondering if anyone else has any
>>> experience in this area. I've stripped back the problem to its bare bones
>>> for the purposes of this mail.
>>>
>>>
>>>
>>> I have an Erlang 18.1 application that uses ETS to store an Erlang map
>>> structure. Using erts_debug:flat_size/1 I can approximate the map's size to
>>> be 1MB. Upon the necessary activity trigger the application spawns about 25
>>> short-lived processes to perform the main work of the application. This
>>> activity trigger is fired roughly 9 times a second under normal operating
>>> conditions. Each of these 25 processes performs 1 x ets:lookup/2 calls to
>>> read from the map.
>>>
>>>
>>>
>>> What I've found is that the above implementation has a CPU profile that
>>> is quite "expensive" - each of the CPU cores (40 total comprised of 2
>>> Processors with 10 hyperthreaded cores) frequently runs at 100%. The
>>> machine in question also has 32GB RAM of which about 9GB is used at peak.
>>> There is no swap usage whatsoever. Examination shows that copy_shallow is
>>> performing the most work.
>>>
>>>
>>>
>>> After changing the implementation so that the 25 spawned processes no
>>> longer read from the ETS table to retrieve the map structure and, instead
>>> the map is passed to the processes on spawn, the CPU usage on the server is
>>> considerably lower.
>>>
>>>
>>>
>>> Can anyone offer advice as to why I'm seeing the differing CPU profiles?
>>>
>>>
>>> _______________________________________________
>>> erlang-questions mailing ://erlang.org/mailman/listinfo/erlang-questions
>>>
>>>
>>>
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