Why is MySQL running as a single process? I have two MariaDB(InnoDB) linux servers. Even if my.cnf is almost the same they behave very differently. On the bad machine MariaDB is ver 10.0.16-MariaDB-1, on the good machine is 10.0.27-MariaDB-0+deb8u1.

They act on different same-size databases (around 3GB) similar in structure and usage. The bad machine has a few more clients (7 against 4) reading the data. DBs don't perform any calculation: they just provide storage.

Checking the OS processes with top/htop I see that the one that misbehaves has only ONE process, while the good one has 31 distinct PIDs. I could imagine that this is the bottleneck.

MySQLTuner returns a big difference for table cache hit rate: bad machine 6% (400/6K opened), good machine 118% (400/338 opened). 

This is the configuration of the bad one. 
Where can I set the max number of processes? Or is there something else I didn't take in account? 

[client]
port        = 3306
socket      = /var/run/mysqld/mysqld.sock

[mysqld_safe]
socket      = /var/run/mysqld/mysqld.sock
nice        = 0

[mysqld]
#
# * Basic Settings
#
user        = XXXXXX
pid-file    = /var/run/mysqld/mysqld.pid
socket      = /var/run/mysqld/mysqld.sock
port        = 3306
basedir     = /usr
datadir     = /var/lib/mysql
tmpdir      = /tmp
lc-messages-dir = /usr/share/mysql
skip-external-locking
innodb_file_per_table  = 1
innodb_file_format     = barracuda

skip-name-resolve

bind-address        = XX.XX.XX.XX

#
# * Fine Tuning
#
key_buffer      = 16M
max_allowed_packet  = 128M
thread_stack        = 192K
thread_cache_size       = 8
# This replaces the startup script and checks MyISAM tables if needed
# the first time they are touched
myisam-recover         = BACKUP
#max_connections        = 100
#table_cache            = 64
#thread_concurrency     = 10


#
# * Query Cache Configuration
#
query_cache_limit   = 1M
query_cache_size        = 16M

#
# Error log - should be very few entries.
#
log_error = /var/log/mysql/error.log


expire_logs_days    = 10
max_binlog_size         = 100M

#binlog_do_db       = include_database_name
#binlog_ignore_db   = include_database_name


#
# * InnoDB
#
# InnoDB is enabled by default with a 10MB datafile in /var/lib/mysql/.
# Read the manual for more InnoDB related options. There are many!
#
# * Security Features
#
# Read the manual, too, if you want chroot!
# chroot = /var/lib/mysql/
#
# For generating SSL certificates I recommend the OpenSSL GUI "tinyca".
#
# ssl-ca=/etc/mysql/cacert.pem
# ssl-cert=/etc/mysql/server-cert.pem
# ssl-key=/etc/mysql/server-key.pem


[mysqldump]
quick
quote-names
max_allowed_packet  = 16M

[mysql]
#no-auto-rehash # faster start of mysql but no tab completition

[isamchk]
key_buffer      = 16M
innodb_buffer_pool_size should not be set to 0.  It should be at least 100M, up to about 1000M for a 4GB machine that is running only MySQL.

This may be indirectly causing your problem.