Deadlock In Time Download [VERIFIED]
Andrew Henson
A wait timeout occurs when the configured timeout period (e.g. innodb_lock_wait_timeoutin the case of InnoDB locks) elapses while a transaction awaits a lock, perhaps because a slow transaction is holding the lock and has not finished executing or perhaps because a number of transactions are queuing for the lock. It's possible (even, likely) that the lock would have become available and have been acquired if the transaction had waited longer, but the timeout exists to avoid applications waiting on the database indefinitely. A wait timeout is analogous to a driver giving up and turning back because of delays.
This is the amount of time to wait on a lock before checking to see if there is a deadlock condition. The check for deadlock is relatively expensive, so the server doesn't run it every time it waits for a lock. We optimistically assume that deadlocks are not common in production applications and just wait on the lock for a while before checking for a deadlock. Increasing this value reduces the amount of time wasted in needless deadlock checks, but slows down reporting of real deadlock errors. If this value is specified without units, it is taken as milliseconds. The default is one second (1s), which is probably about the smallest value you would want in practice. On a heavily loaded server you might want to raise it. Ideally the setting should exceed your typical transaction time, so as to improve the odds that a lock will be released before the waiter decides to check for deadlock. Only superusers and users with the appropriate SET privilege can change this setting.
When log_lock_waits is set, this parameter also determines the amount of time to wait before a log message is issued about the lock wait. If you are trying to investigate locking delays you might want to set a shorter than normal deadlock_timeout.
The shared lock table has space for max_locks_per_transaction objects (e.g., tables) per server process or prepared transaction; hence, no more than this many distinct objects can be locked at any one time. This parameter limits the average number of object locks used by each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have queries that touch many different tables in a single transaction, e.g., query of a parent table with many children. This parameter can only be set at server start.
The shared predicate lock table has space for max_pred_locks_per_transaction objects (e.g., tables) per server process or prepared transaction; hence, no more than this many distinct objects can be locked at any one time. This parameter limits the average number of object locks used by each transaction; individual transactions can lock more objects as long as the locks of all transactions fit in the lock table. This is not the number of rows that can be locked; that value is unlimited. The default, 64, has historically proven sufficient, but you might need to raise this value if you have clients that touch many different tables in a single serializable transaction. This parameter can only be set at server start.
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This article discusses deadlocks in the SQL Server Database Engine in depth. Deadlocks are caused competing, concurrent locks in the database, often in multi-step transactions. For more on transaction locking, see Transaction locking and row versioning guide.
Both transactions in a deadlock will wait forever unless the deadlock is broken by an external process. the SQL Server Database Engine deadlock monitor periodically checks for tasks that are in a deadlock. If the monitor detects a cyclic dependency, it chooses one of the tasks as a victim and terminates its transaction with an error. This allows the other task to complete its transaction. The application with the transaction that terminated with an error can retry the transaction, which usually completes after the other deadlocked transaction has finished.
Deadlocking is often confused with normal blocking. When a transaction requests a lock on a resource locked by another transaction, the requesting transaction waits until the lock is released. By default, SQL Server transactions do not time out, unless LOCK_TIMEOUT is set. The requesting transaction is blocked, not deadlocked, because the requesting transaction has not done anything to block the transaction owning the lock. Eventually, the owning transaction will complete and release the lock, and then the requesting transaction will be granted the lock and proceed. Deadlocks are resolved almost immediately, whereas blocking can, in theory, persist indefinitely. Deadlocks are sometimes called a deadly embrace.
Deadlock is a condition that can occur on any system with multiple threads, not just on a relational database management system, and can occur for resources other than locks on database objects. For example, a thread in a multithreaded operating system might acquire one or more resources, such as blocks of memory. If the resource being acquired is currently owned by another thread, the first thread might have to wait for the owning thread to release the target resource. The waiting thread is said to have a dependency on the owning thread for that particular resource. In an instance of the SQL Server Database Engine, sessions can deadlock when acquiring non-database resources, such as memory or threads.
In the illustration, transaction T1 has a dependency on transaction T2 for the Part table lock resource. Similarly, transaction T2 has a dependency on transaction T1 for the Supplier table lock resource. Because these dependencies form a cycle, there is a deadlock between transactions T1 and T2.
Deadlocks can also occur when a table is partitioned and the LOCK_ESCALATION setting of ALTER TABLE is set to AUTO. When LOCK_ESCALATION is set to AUTO, concurrency increases by allowing the SQL Server Database Engine to lock table partitions at the HoBT level instead of at the table level. However, when separate transactions hold partition locks in a table and want a lock somewhere on the other transactions partition, this causes a deadlock. This type of deadlock can be avoided by setting LOCK_ESCALATION to TABLE; although this setting will reduce concurrency by forcing large updates to a partition to wait for a table lock.
A deadlock occurs when two or more tasks permanently block each other by each task having a lock on a resource that the other tasks are trying to lock. The following graph presents a high level view of a deadlock state where:
The SQL Server Database Engine automatically detects deadlock cycles within SQL Server. the SQL Server Database Engine chooses one of the sessions as a deadlock victim and the current transaction is terminated with an error to break the deadlock.
Each user session might have one or more tasks running on its behalf where each task might acquire or wait to acquire a variety of resources. The following types of resources can cause blocking that could result in a deadlock.
Locks. Waiting to acquire locks on resources, such as objects, pages, rows, metadata, and applications can cause a deadlock. For example, transaction T1 has a shared (S) lock on row r1 and is waiting to get an exclusive (X) lock on r2. Transaction T2 has a shared (S) lock on r2 and is waiting to get an exclusive (X) lock on row r1. This results in a lock cycle in which T1 and T2 wait for each other to release the locked resources.
Worker threads. A queued task waiting for an available worker thread can cause a deadlock. If the queued task owns resources that are blocking all worker threads, a deadlock will result. For example, session S1 starts a transaction and acquires a shared (S) lock on row r1 and then goes to sleep. Active sessions running on all available worker threads are trying to acquire exclusive (X) locks on row r1. Because session S1 cannot acquire a worker thread, it cannot commit the transaction and release the lock on row r1. This results in a deadlock.
Memory. When concurrent requests are waiting for memory grants that cannot be satisfied with the available memory, a deadlock can occur. For example, two concurrent queries, Q1 and Q2, execute as user-defined functions that acquire 10 MB and 20 MB of memory respectively. If each query needs 30 MB and the total available memory is 20 MB, then Q1 and Q2 must wait for each other to release memory, and this results in a deadlock.
Parallel query execution-related resources. Coordinator, producer, or consumer threads associated with an exchange port might block each other causing a deadlock usually when including at least one other process that is not a part of the parallel query. Also, when a parallel query starts execution, SQL Server determines the degree of parallelism, or the number of worker threads, based upon the current workload. If the system workload unexpectedly changes, for example, where new queries start running on the server or the system runs out of worker threads, then a deadlock could occur.
Session mutex. The tasks running in one session are interleaved, meaning that only one task can run under the session at a given time. Before the task can run, it must have exclusive access to the session mutex.
Transaction mutex. All tasks running in one transaction are interleaved, meaning that only one task can run under the transaction at a given time. Before the task can run, it must have exclusive access to the transaction mutex.
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