[erlang-questions] Mnesia secondary index: a memory leak or lasy cleanup?

[Dan Gudmundsson]

Yep seems broken for 'bag' types.

I will take at look at that, thanks for the bug report.

/Dan

On Wed, Aug 15, 2018 at 8:38 PM Karolis Petrauskas <k.petrauskas@REDACTED>
wrote:

> Forgot to mention the versions.
> This case is reproducible with "Erlang/OTP 21 [erts-10.0] [source]
> [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [hipe]".
> The same behaviour is observed on Erlang/OTP 19.
>
> Karolis Petrauskas
> On Wed, Aug 15, 2018 at 5:49 PM Karolis Petrauskas
> <k.petrauskas@REDACTED> wrote:
> >
> > Hello,
> >
> > I have strange trend in memory consumption on some servers. The ETS
> > memory constantly increases. The tables that are increasing are
> > mnesia_index tables. It looks like these tables are not cleaned-up
> > after the corresponding records are deleted from the indexed table.
> > Maybe the index is cleaned up later? I was waiting for a month, the
> > mnesia_index table has grown to 1GB approximately.
> >
> > To reproduce the situation I made a simple test case. The code of the
> > tst module is shown at the end of this email.
> >
> > First I create a bag table and add some (1000) records to it:
> >
> >     4> tst:create_table().
> >     ok
> >     10> tst:data_add().
> >     ok
> >     12> ets:i().
> >      id              name              type  size   mem      owner
> >
> ----------------------------------------------------------------------------
> >      ...
> >      45084           mnesia_index      ordered_set 1000   10088
> > mnesia_monitor
> >      ...
> >      t               t                 bag   1000   9298
>  mnesia_monitor
> >     ok
> >
> > The table and the index has same number of records (the data for the
> > primary and the secondary indexes are unique in this case).
> > Then I delete the records:
> >
> >     14> tst:data_del().
> >     ok
> >     16> ets:i().
> >      id              name              type  size   mem      owner
> >
> ----------------------------------------------------------------------------
> >      ...
> >      45084           mnesia_index      ordered_set 1000   10088
> > mnesia_monitor
> >      ...
> >      t               t                 bag   0      298
> mnesia_monitor
> >     ok
> >
> > Here the table has 0 records (as expected), and the index still has
> > 1000 records.
> >
> > The code of the tst module:
> >
> >     -module(tst).
> >     -export([create_table/0, data_add/0, data_del/0]).
> >     -record(t, {a, b, c}).
> >
> >     create_table() ->
> >         {atomic, ok} = mnesia:create_table(t, [{type, bag},
> > {attributes, record_info(fields, t)}]),
> >         {atomic, ok} = mnesia:add_table_index(t, b),
> >         ok.
> >
> >     data_add() ->
> >         mnesia:activity(transaction, fun () ->
> >             lists:foreach(fun (Key) ->
> >                 mnesia:write(#t{a = Key, b = Key})
> >             end, lists:seq(1, 1000))
> >         end).
> >
> >     data_del() ->
> >         mnesia:activity(transaction, fun () ->
> >             lists:foreach(fun (Key) ->
> >                 [R] = mnesia:match_object(#t{a = Key, _ = '_'}),
> >                 mnesia:delete_object(R)
> >             end, lists:seq(1, 1000))
> >         end).
> >
> >
> > Karolis Petrauskas
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