[erlang-questions] ETS and CPU

Philip Clarke <>
Tue Mar 15 17:37:48 CET 2016


Hi Alex,

It would be interesting to see if the cpu is busy with garbage collection
when the process is reading from ETS.

I suspect that when the process is started without the map and then reads
the map from ETS that it has to increase its heap size several times.  In
order to increase heap size a full GC has to be run and I have found this
in the past to be quite expensive.

I suspect that when you create the process with spawn(fun()->
do_something(A) end)  that the process is initialised with the correct heap
size thereby avoiding the GC and hence the better CPU performance.

Perhaps you could rerun the program with a higher default heap size per
process (*+hms)?*
I would be interested to know how it goes.

Regards
Philip

On 15 March 2016 at 15:07, 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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