PostgreSQL locking, part 3: lightweight locks

LWLocks lightweight locks postgres

PostgreSQL logoPostgreSQL lightweight locks, or LWLocks, control memory access. PostgreSQL uses multi-process architecture and should allow only consistent reads and writes to shared memory structures. LWLocks have two levels of locking: shared and exclusive. It’s also possible to release all acquired LWLocks to simplify clean up. Other databases often call primitives similar to LWLocks “latches”. Because LWLocks is an implementation detail, application developers shouldn’t pay much attention to this kind of locking.

This is the third and final part of a series on PostgreSQL locking, related to latches protecting internal database structures. Here are the previous parts: Row-level locks and table-level locks.

Instrumentation

Starting from PostgreSQL 9.6, LWLocks activity can be investigated with the pg_stat_activity system view. It could be useful under high CPU utilization. There are system settings to help with contention on specific lightweight locks.

Before PostgreSQL 9.5, the LWLocks implementation used spin-locks.  It was a bottleneck. This was fixed in 9.5 with atomic state variable.

Potential heavy contention places

  • WALInsertLock: protects WAL buffers. You can increase the number of wal buffers to get a slight improvement. Incidentally, synchronous_commit=off increases pressure on the lock even more, but it’s not a bad thing. full_page_writes=off reduces contention, but it’s generally not recommended.
  • WALWriteLock: accrued by PostgreSQL processes while WAL records are flushed to disk or during a WAL segments switch. synchronous_commit=off removes the wait for disk flush, full_page_writes=off reduces the amount of data to flush.
  • LockMgrLock: appears in top waits during a read-only workload. It latches relations regardless of its size. It’s not a single lock, but at least 16 partitions. Thus it’s important to use multiple tables during benchmarks and avoid single table anti-pattern in production.
  • ProcArrayLock: Protects the ProcArray structure. Before PostgreSQL 9.0, every transaction acquired this lock exclusively before commit.
  • CLogControlLock: protects CLogControl structure, if it shows on the top of pg_stat_activity, you should check the location of $PGDATA/pg_clog—it should be on a buffered file system.
  • SInvalidReadLock: protects sinval array. Readers using shared lock. SICleanupQueue, and other array-wide updates, requires an exclusive lock. It shows at the top of the pg_stat_activity when the shared buffer pool is under stress. Using a higher number of shared_buffers helps to reduce contention.
  • BufMappingLocks: protects regions of buffers. Sets 128 regions (16 before 9.5) of buffers to handle the whole buffer cache.

Spinlocks

The lowest level for locking is spinlocks. Therefore, it’s implemented within CPU-specific instructions. PostgreSQL is trying to change an atomic variable value in a loop. If the value is changed from zero to one – the process obtained a spinlock. If it’s not possible to get a spinlock immediately, the process will increase its wait delay exponentially.  There is no monitoring on spinlocks and it’s not possible to release all accrued spinlocks at once. Due to the single state change, it’s also an exclusive lock. In order to simplify the porting of PostgreSQL to exotic CPU and OS variants, PostgreSQL uses OS semaphores for its spinlocks implementation. Of course, it’s significantly slower compared to native CPU instructions port.

Summary

  • Use pg_stat_activity to find which queries or LWLocks are causing lock waits
  • Use fresh branches of PostgreSQL, as developers have been working on performance improvements and trying to reduce locking contention on hot mutexes.

References

Locks Photo by Warren Sammut on Unsplash

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