Threads in mysqld can run at four different priorities, defined in mysql_priv.h:
#define INTERRUPT_PRIOR 10 #define CONNECT_PRIOR 9 #define WAIT_PRIOR 8 #define QUERY_PRIOR 6
Some threads try to set their priority; others don't. These calls are passed along to pthread_setschedparam() if the native threading library implements it.
The different threads are:
The main thread. Runs at CONNECT_PRIOR priority. Calls thr_setconcurrency() if it is available at compile time; this call is generally assumed to exist only on Solaris, its value should reflect the number of physical CPUs.
The "bootstrap" thread. See handle_bootstrap() in sql_parse.cc. The mysql_install_db script starts a server with an option telling it to start this thread and read commands in from a file. Used to initialize the grant tables. Runs once and then exits.
The "maintenance" thread. See sql_manager_cc. Like the old "sync" daemon in unix, this thread occasionally flushes MyISAM tables to disk. InnoDB has a separate maintenance thread, but BDB also uses this one to occasionally call berkeley_cleanup_log_files(). Begins at startup and persists until shutdown.
The "handle TCP/IP sockets" thread. See handle_connections_sockets() in mysqld.cc. Loop with a select() function call, to handle incoming connections.
The "handle named pipes" thread. Only on Windows.
The "handle shared memory connections" thread. Only on Windows.
Signal handler ("interrupt") thread. See signal_hand() in mysqld.cc. Runs at INTERRUPT_PRIOR priority. Sets up to receive signals, and then handles them as they come in. Begins at server startup and persists until shutdown.
The "shutdown" thread. See kill_server() in mysqld.cc. Created by the signal handling thread. Closes all connections with close_connections(), the ends.
Active and cached per-connection threads. See handle_one_connection() in sql_parse.cc. These can run at QUERY_PRIOR priority or WAIT_PRIOR priority depending on what they are doing.
The "delayed" thread. See handle_delayed_insert() in sql_insert.cc. Used for MyISAM's delayed inserts.
The two slave threads, in slave.cc. One thread connects to the master and handles network IO. The other reads queries from the relay log and executes them.
In InnoDB, all thread management is handled through os/os0thread.c InnoDB's threads are:
The I/O handler threads, See io_handler_thread().
Two "watchmen" threads: srv_lock_timeout_and_monitor_thread(), and srv_error_monitor_thread().
The master thread "which does purge and other utility operations", See srv_master_thread().
InnoDB's internal os_thread_set_priority() function implements three priorities (Background, normal, and high) but only on windows. The function is a no-op on unix.
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