SQL基础：MySQL slave 延迟一列 外键检查和自增加锁

MySQL slave 延迟 外键检查和自增加锁

一、现象

延迟大，大事物。

• 表结构

• 无IO

• SQL THREAD占用CPU 100%

二、pscak 采样

采样30个点

• 外键检查 占70%

• 自增锁获取 占30%

三、自增锁获取逻辑

逻辑如下其实也是innodb_autoinc_lock_mode参数的作用

switch (lock_mode) {
case AUTOINC_NO_LOCKING://innodb_autoinc_lock_mode=2
/* Acquire only the AUTOINC mutex. */
dict_table_autoinc_lock(m_prebuilt->table);
break;

case AUTOINC_NEW_STYLE_LOCKING: // innodb_autoinc_lock_mode=1 注意这里没有break 巧妙的完成了逻辑
/* For simple (single/multi) row INSERTs, we fallback to the
old style only if another transaction has already acquired
the AUTOINC lock on behalf of a LOAD FILE or INSERT … SELECT
etc. type of statement. */
if (thd_sql_command(m_user_thd) == SQLCOM_INSERT
|| thd_sql_command(m_user_thd) == SQLCOM_REPLACE) {

dict_table_t* ib_table = m_prebuilt->table;

/* Acquire the AUTOINC mutex. */
dict_table_autoinc_lock(ib_table);

/* We need to check that another transaction isn’t
already holding the AUTOINC lock on the table. */
if (ib_table->n_waiting_or_granted_auto_inc_locks) {
/* Release the mutex to avoid deadlocks. */
dict_table_autoinc_unlock(ib_table);
} else {
break;
}
}
/* Fall through to old style locking. */

case AUTOINC_OLD_STYLE_LOCKING://innodb_autoinc_lock_mode=0 触发
DBUG_EXECUTE_IF(“die_if_autoinc_old_lock_style_used”,
ut_ad(0););
error = row_lock_table_autoinc_for_mysql(m_prebuilt); //这个函数上表上的自增锁

if (error == DB_SUCCESS) {

/* Acquire the AUTOINC mutex. */
dict_table_autoinc_lock(m_prebuilt->table);
}
break;

default:
ut_error;
}