ubuntu - Kernel attempts to kill MySQL with sigkill

I'm running an Ubuntu server for MySQL.

Server info

• Ubuntu 12.10


• MySQL installed via apt



• RAM: 512M


• innodb_buffer_pool_size : 300M


• There is no other memory intensive application running on this box.

Problem

Every morning, at approx. 6:40am something happens to cause a noticeable change in memory:

https://dl.dropbox.com/u/12520837/mem.s.png

At the same time, a systematic "kill" of running processes seems to occur, causing MySQL to restart.

Apr 10 06:43:40 mysql-01 kernel: [1866472.511966] select 1 (init), adj 0, size 41, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.511973] select 385 (dbus-daemon), adj 0, size 44, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.511975] select 389 (rsyslogd), adj 0, size 124, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.511982] select 4578 (snmpd), adj 0, size 160, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.514157] select 1 (init), adj 0, size 41, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.514164] select 385 (dbus-daemon), adj 0, size
44, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.514166] select 389 (rsyslogd), adj 0, size 124, to kill

Apr 10 06:43:40 mysql-01 kernel: [1866472.514171] select 4578 (snmpd), adj 0, size 160, to kill

Apr 10 06:43:44 mysql-01 /etc/mysql/debian-start[21807]: Upgrading MySQL tables if necessary.

Apr 10 06:43:45 mysql-01 /etc/mysql/debian-start[21810]: /usr/bin/mysql_upgrade: the '--basedir' option is always ignored

Apr 10 06:43:45 mysql-01 /etc/mysql/debian-start[21810]: Looking for 'mysql' as: /usr/bin/mysql

Apr 10 06:43:45 mysql-01 /etc/mysql/debian-start[21810]: Looking for 'mysqlcheck' as: /usr/bin/mysqlcheck

Apr 10 06:43:45 mysql-01 /etc/mysql/debian-start[21810]: This installation of MySQL is already upgraded to 5.5.29, use --force if you still need to run mysql_upgrade

Apr 10 06:43:45 mysql-01 /etc/mysql/debian-start[21821]: Checking for insecure root accounts.
Apr 10 06:43:45 mysql-01 /etc/mysql/debian-start[21826]: Triggering myisam-recover for all MyISAM tables

Any help diagnosing this would be much appreciated!