mysqldump 5.5.25 use swap instead of memory mysql version 5.5.25, while use mysqldump to dump big tables, it eat lots of swap while there is lots of free memory.

# free -m
             total       used       free     shared    buffers     cached
Mem:         32177      31427        749          0        208      10747
-/+ buffers/cache:      20471      11705
Swap:        16002      12952       3049

# cat /proc/sys/vm/swappiness 
5

#[mysqldump]
quick
max_allowed_packet = 16M
Your problem is, that mysqldump normally caches the results it produces.
You have to use the --quick option, to prevent this.

 --quick, -q


This option is useful for dumping large tables. It forces mysqldump to retrieve rows for a table from the server a row at a time rather than retrieving the entire row set and buffering it in memory before writing it out.