Complex-Lock report

AIX Complex lock supports recursive locking, where a thread can acquire the lock more than once before releasing it, as well as differentiating between write-locking, which is exclusive, from read-locking, which is not exclusive.

This report begins with [AIX COMPLEX Lock]. Most of the entries are identical to the simple lock report, while some of them are differentiated by read/write/upgrade. For example, the SpinQ and WaitQ statistics include the minimum, maximum, and average number of threads spinning or waiting on the lock. They also include the minimum, maximum, and average number of threads attempting to acquire the lock for reading versus writing. Because an arbitrary number of threads can hold the lock for reading, the report includes the minimum, maximum, and average number of readers in the LockQ that holds the lock.

A thread might hold a lock for writing; this is exclusive and prevents any other thread from securing the lock for reading or for writing. The thread downgrades the lock by simultaneously releasing it for writing and acquiring it for reading; this permits other threads to also acquire the lock for reading. The reverse of this operation is an upgrade; if the thread holds the lock for reading and no other thread holds it as well, the thread simultaneously releases the lock for reading and acquires it for writing. The upgrade operation might require that the thread wait until other threads release their read-locks. The downgrade operation does not.

A thread might acquire the lock to some recursive depth; it must release the lock the same number of times to free it. This is useful in library code where a lock must be secured at each entry-point to the library; a thread will secure the lock once as it enters the library, and internal calls to the library entry-points simply re-secure the lock, and release it when returning from the call. The minimum, maximum, and average recursion depths of any thread holding this lock are reported in the table.

A thread holding a recursive write-lock is not permitted to downgrade it because the downgrade is intended to apply to only the last write-acquisition of the lock, and the prior acquisitions had a real reason to keep the acquisition exclusive. Instead, the lock is marked as being in the downgraded state, which is erased when the this latest acquisition is released or upgraded. A thread holding a recursive read-lock can only upgrade the latest acquisition of the lock, in which case the lock is marked as being upgraded. The thread will have to wait until the lock is released by any other threads holding it for reading. The minimum, maximum, and average recursion-depths of any thread holding this lock in an upgraded or downgraded state are reported in the table.

The Lock Activity report also breaks down the time based on what task the lock is being secured for (reading, writing, or upgrading).

No time is reported to perform a downgrade because this is performed without any contention. The upgrade state is only reported for the case where a recursive read-lock is upgraded. Otherwise, the thread activity is measured as releasing a read-lock and acquiring a write-lock.

The function and thread details also break down the acquisition, spin, and wait counts by whether the lock is to be acquired for reading or writing.