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

[Karolis Petrauskas]

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