Latches

The Latches window provides detailed information about current and historical latch statistics. The information originates from the v$latch view. The latch statistics are shown as stacked charts. All information is also presented in a tabular form.

Viewing options.

There are two main areas in this window - the "top latches" charts and the tabular view of values. There are different measures for latches. Select them in the "Show By" radio button box. The seven available options are: Gets, Misses, Sleeps, Immediate gets, Immediate misses, Waits holding latch, Spin gets. These measures correspond to the seven columns in v$latch view:

• GETS - Number of times the latch was requested in willing-to-wait mode;
• MISSES - Number of times the latch was requested in willing-to-wait mode and the requestor had to wait;
• SLEEPS - Number of times a willing-to-wait latch request resulted in a session sleeping while waiting for the latch;
• IMMEDIATE_GETS - Number of times a latch was requested in no-wait mode;
• IMMEDIATE_MISSES - Number of times a no-wait latch request did not succeed (that is, missed);
• WAITS_HOLDING_LATCH - Number of waits for the latch while the waiter was holding a different latch;
• SPIN_GETS - Willing-to-wait latch requests which missed the first try but succeeded while spinning;

Since all measures in v$latch are cumulative, Lab128 differentiates them in time, providing data that can be viewed and analyzed. The derived information is sorted in descending order so that the top latches can be easily identified.

The latch statistic charts are presented as a stack of charts, each of a different color, with the largest contributor being on top, the second largest below, and so on. The Legend section on the right translates colors to corresponding latches. The number of top latches is limited by the value in the "Top" box in the right upper corner of the window.

Different modes of viewing data in tabular view.

The tabular view mode depends on the type of time selection: unselected, point in time selected, period of time selected. Time selection can be done in the Chart area by clicking over the chart, moving to another point, and releasing the mouse button (see Selecting a time interval and Average value for more information on how to make a selection). The mode of tabular view is shown on top. These modes are:

• No selection in Chart area - Current latch statistic values from the most recent snapshot;
• Point in the past selected - Momentary values calculated from the snapshots surrounding a time point;
• Time period selected - Average values calculated from the beginning and ending snapshots defined by a time period.

To include columns with statistic cumulative values, check the "Show Cumulative" box. The check box "Include Inactive" filters out latches that have cumulative values of 0. Check this box if a list of all Oracle latches is needed, including inactive ones.

Viewing charts of individual latches.

To view a chart for an individual latch, select it in the list of latches, then right-click and choose "Show Chart for selected latch" in the pop-up menu.

Descriptions of the columns in the Latch tabular view.

• Latch# - Latch number (v$latch.LATCH#);
• Latch- Latch name and level (combo of v$latch.NAME and v$latch.LEVEL#);
• gets - Number (cumulative) of times the latch was requested in willing-to-wait mode (v$latch.GETS);
• gets/s - Number of gets per second (delta of v$latch.GETS);
• misses - Number (cumulative) of times the latch was requested in willing-to-wait mode and the requestor had to wait (v$latch.MISSES);
• misses/s - Number of misses per second (delta of v$latch.MISSES);
• sleeps - Number (cumulative) of times a willing-to-wait latch request resulted in a session sleeping while waiting for the latch (v$latch.SLEEPS);
• sleeps/s - Number of sleeps per second (delta of v$latch.);
• im_gets - Number (cumulative) of times a latch was requested in no-wait mode (v$latch.IMMEDIATE_GETS);
• im_gets/s - Number of immediate gets per second (delta of v$latch.IMMEDIATE_GETS);
• im_misses - Number (cumulative) of times a no-wait latch request did not succeed, that is, missed (v$latch.IMMEDIATE_MISSES);
• im_misses/s - Number of immediate misses per second (delta of v$latch.IMMEDIATE_MISSES);
• waits_holding_latch - Number (cumulative) of waits for the latch while the waiter was holding a different latch (v$latch.WAITS_HOLDING_LATCH);
• waits_holding_latch/s - Number of waits holding latch per second (delta of v$latch.WAITS_HOLDING_LATCH);
• spin_gets - Willing-to-wait latch requests (cumulative) which missed the first try but succeeded while spinning (v$latch.SPIN_GETS);
• spin_gets/s - Number of spin gets per second (delta of v$latch.SPIN_GETS);