HW enqueue contention

HW enqueue

When a session needs access to a resource, it requests a lock on that resource in a specific mode. Internally, lock and resource structures are used to control access to a resource. Enqueues, as name suggests, have First In First Out queueing mechanism.

Segments have High Water Mark (HWM) indicating that blocks below that HWM have been formatted. New tables or truncated tables [ that is truncated without reuse storage clause ], have HWM value set to segment header block. Meaning, there are zero blocks below HWM. As new rows inserted or existing rows updated (increasing row length), more blocks are added to the free lists and HWM bumped up to reflect these new blocks. HW enqueues are acquired in Exclusive mode before updating HWM and essentially HW enqueues operate as a serializing mechanism for HWM updates.

Measuring HW enqueue contention

We will use few test cases to see how underlying extent size and table structures are affecting HW enqueue contention. But, before we need to find a way to measure total number of gets on HW enqueue. If total number of gets on HW enqueue is reduced, enqueue contention can be relieved.

Fixed table x$ksqst stores statistics about total number of enqueue gets, success and failures of those gets at instance level. For example, to see total number of gets on HW enqueue, following query can be used. Column ksqstreq indicates total # of gets and ksqstwat shows total # of waits.

SQL> select ksqstreq, ksqstwat from x$ksqst where ksqsttyp='HW';

From Oracle version 10g and above, x$ksqst is externalized as v$enqueue_statistics.