I’m doing some optimization into an application that uses Riak Core, GenStage, and Phoenix PubSub. I’ve set up a load test that takes the application near the pod restart limit (using Kubernetes) and notice a large quantity of busy_dist_port.

After some search, I found the +zdbbl flag in Erlang page with the default value of 1024. Notice that on Riak KV performance page, their default for Distribution Buffer is 32MB, so I’ve updated mine to 32 MB, and 99.9% of the monitor messages related to the busy_dist_port are gone.

The monitor logs that remains are the following:

[info] monitor busy_dist_port <0.7799.0> [{initial_call,{'Elixir.GenStage',init,1}},{almost_current_function,{ets,match,2}},{message_queue_len,0}] {#Port<0.364>,unknown}

Where the almost_current_function can have different values:

- {almost_current_function,{ets,match,2}}
- {almost_current_function,{erts_internal,dsend_continue_trap,1}}
- {almost_current_function,{ets,lookup_element,3}}
- {almost_current_function,{erlang,bif_return_trap,2}}
- {almost_current_function,{pg2,'-group_members/1-lc$^0/1-0-',1}}

Which I think is related to too much load, and probably nothing I can do.

The other one is for

[info] monitor large_heap <0.4933.0> [{name,'Elixir.pubsub_name.Adapter'},{initial_call,{'Elixir.Phoenix.PubSub.PG2',init,1}},{almost_current_function,{erlang,bif_return_trap,2}},{message_queue_len,771}]

With also similar different values in almost_current_function.

- {gen_server,handle_msg,6}
- {'Elixir.Phoenix.PubSub',local_broadcast,4}
- {erlang,bif_return_trap,2}
- {ets,lookup_element,3}

Is there some recommendation in optimization that can be done that I’m missing? These should be the standard monitor messages when the server in under extreme load, right?

EDIT: The pool_size used for PubSub in this project was set to 10 for a 2.5 core machine (2500m) when the documentation of PubSub recommends 1 partition per 4 cores. Change to a proper number (1) the large_heap monitor logs produced disappeared.