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Allow disabling off-heap messages #1597

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merged 1 commit into from Sep 6, 2018

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nickva commented Sep 6, 2018

Description

Off-heap messages is an Erlang 19 feature:

http://erlang.org/doc/man/erlang.html#process_flag_message_queue_data

It is adviseable to use that setting for processes which expect to receive a
lot of messages. CouchDB sets it for couch_server, couch_log_server and bunch
of others as well.

In some cases the off-heap behavior could alter the timing of message receives
and expose subtle bugs that have been lurking in the code for years. Or could
slightly reduce performance, so a safety measure allow disabling it.

How to test

> erlang:process_info(whereis(couch_server), message_queue_data).
{message_queue_data,off_heap}


> config:set("off_heap_mqd","couch_server","false").
ok

> exit(whereis(couch_server), kill).
true

> erlang:process_info(whereis(couch_server), message_queue_data).
{message_queue_data,on_heap}

Related

#1392

@davisp

If we're gonna make it configurable lets at least have it configurable per module. Something like set_mqd_off_heap(?MODULE) and then config:get_boolean("off_heap_mqd", atom_to_list(Module), true) maybe?

@nickva

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nickva commented Sep 6, 2018

Good idea, @davisp

Allow disabling off-heap messages
Off-heap messages is an Erlang 19 feature:

http://erlang.org/doc/man/erlang.html#process_flag_message_queue_data

It is adviseable to use that setting for processes which expect to receive a
lot of messages. CouchDB sets it for couch_server, couch_log_server and bunch
of others as well.

In some cases the off-heap behavior could alter the timing of message receives
and expose subtle bugs that have been lurking in the code for years. Or could
slightly reduce performance, so a safety measure allow disabling it.

@nickva nickva force-pushed the allow-disabling-off-heap-messages branch from 3acd37e to f29b09c Sep 6, 2018

@davisp

davisp approved these changes Sep 6, 2018

+1

@nickva nickva merged commit ee3c4b5 into master Sep 6, 2018

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@nickva nickva deleted the allow-disabling-off-heap-messages branch Sep 6, 2018

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