[erlang-questions] ETS and CPU

[Hynek Vychodil]

Just to be sure, when you wrote that erts_debug:flat_size/1 approximate the
map's size to be 1MB, you mean something around this value

> erts_debug:flat_size(Map)
131072

Because if you have something like
> erts_debug:flat_size(Map)
1048576

It is 8MB and your memory IO is like 25*9/s*8MB = 1800MB/s. It is still
manageable by your HW but should be considered in your design.

On Wed, Mar 16, 2016 at 10:43 AM, Alex Howle <itshowlertime@REDACTED>
wrote:

> Assuming that when you say "win" you mean that ets:lookup should be more
> efficient (and less CPU intensive) then I'm seeing the opposite.
> On 15 Mar 2016 11:32, "Sverker Eriksson" <sverker.eriksson@REDACTED>
> 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?
>>
>>
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>>
>>
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