[erlang-questions] ETS and CPU

[Alex Howle]

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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