Locks vs Latches

Separate…User transactionsThreads
Protect…Database ContentsIn-Memory Data Structures
During…Entire TransactionsCritical Sections
Modes…Shared, Exclusive, Update, IntentionRead, Write
DeadlockDetection & ResolutionAvoidance
…by…Waits-for, Timeout, AbortsCoding Discipline
Kept in…Lock ManagerProtected Data Structure

Test and Set Spin Latch (TAS)

A spin latch is essentially a location in memory that threads try to update

  • Example: std::atomic<T>
  • Advantages: Latch/unlatch operations are efficient (single instruction to lock/unlock on x86, CAS).

Blocking OS Mutex

Linux provides the futex (fast user-space mutex), which is comprised of (1) a spin latch in user-space and (2) an OS-level mutex.
If the DBMS can acquire the user-space latch, then the latch is set. It appears as a single latch to the DBMS even though it contains two internal latches. If the DBMS fails to acquire the user-space latch, then it goes down into the kernel and tries to acquire a more expensive mutex. If the DBMS fails to acquire this second mutex, then the thread notifies the OS that it is blocked on the mutex and then it is de-scheduled.

  • Example: std::mutex
  • Advantages: Simple to use and requires no additional coding in DBMS.

Reader Writer Latches

  • Example: std::shared mutex
  • Advantages: Allows for concurrent readers.
  • Disadvantages: The DBMS has to manage read/write queues to avoid starvation. Larger storage overhead than spin Latches due to additional meta-data.