Multixacts wraparound monitoring

Hello,

How can we determine when an error of approximation multixacts wraparound?

According to the information from pg_class:

select datname,datminmxid from pg_database;

    datname     | datminmxid

----------------+------------

template1      |  347462426

template0      |  347462426

postgres          |  347462426

zabbix             |  467261307

db_3                |  291141939

db_1               |  388282963

db                    |  388282963

But when the vacuum/autovacuum starts up, an error occurs:

WARNING:  oldest multixact is far in the past

HINT:  Close open transactions with multixacts soon to avoid wraparound problems.

If I understand correctly, approaching Multixact member wraparound.

But how to understand when it comes exactly and what to do?

PostgreSQL version – 9.3.10, OS Debian 7.8.

Thank you.

Sorry, if I chose the wrong mailing list.

Kind regards,

Vladimir Pavlov