Resolving lock contention
Performance can be improved using different approaches for dealing with locks.
There are two mechanisms for reducing the rate of lock contention:
- Reducing the time during which the lock is owned when taken. For example, limiting the amount of work done under the lock or in the synchronized block of code.
- Reducing the scope of the lock. For example, using a separate lock for each row in a table instead of a single lock for the whole table.
Reducing the hold time for a lock
A thread must spend as little time holding a lock as possible. The longer a lock is held, the more likely it is that another thread tries to obtain the lock. Reducing the duration that a lock is held reduces the contention on the lock and enables the application to scale further.
When a lock has a long average hold time, examine the source code to see if these conditions apply:
- All the code run while the lock is held is acting on the shared resource. Move any code in a lock that does not act on the shared resource outside the lock so that it can run in parallel with other threads.
- Any code run while the lock is held results in a blocking operation; for example, a connection to another process. Release the lock before any blocking operation is started.
Reducing the scope of a lock
The locking architecture in an application must be granular enough that the level of lock contention is low. The greater the amount of shared resource that is protected by an individual lock, the more likely it is that multiple threads will try to access the resource at the same time. Reducing the scope of the resource protected by a lock reduces the level of lock contention and enables the application to scale further.