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Oracle UNIX Monitoring Goals Administration

Oracle UNIX/Linux Tips by Burleson Consulting

UNIX Monitoring Goals

The monitoring of the UNIX environment involves monitoring disk, RAM, CPU and network components.  For this book, we consider server monitoring to be limited to CPU and RAM memory monitoring.  We have divided the total UNIX monitoring tasks into three general areas:

* Server Monitoring ? Monitor CPU and RAM memory

* Disk Monitoring ? Monitor the I/O subsystem ? Chapter 4

* Network monitoring ? Protocol and packet monitoring ? Chapter 5

Let?s begin with an overview of UNIX CPU monitoring goals.

UNIX CPU Monitoring

CPU consumption on an Oracle server is a simple matter because the server manages all CPU transactions automatically. All servers are configured to use CPU cycles on an as-needed basis, and the Oracle database will use CPU resources freely. The internal machine code will manage the assignment of processors to active tasks and ensure that the maximum amount of processing power is applied to each task.

CPU shortages are evidenced in cases where the CPU run queue is greater than the number of CPUs, as shown in Figure 3-1. In these cases, the only solutions are to increase the number of CPUs on the processor or reduce the CPU demands on Oracle. You can decrease CPU demands on Oracle by turning off Oracle Parallel Query, replacing the standard Oracle listener with the multithreaded server (MTS), and other actions that would reduce the processing demands on the hardware.

Figure 1: CPU overload on an Oracle server with 12 CPUs

Tasks are serviced in UNIX according to their internal dispatching priority. Important tasks such as the UNIX operating system tasks will always have a more favorable dispatching priority because the UNIX system tasks drive the operating system

CPU overload is usually evidenced by high values in the vmstat runqueue column. Whenever the runqueue value exceeds the number of CPUs of the server, some task may be waiting for service. When we see a CPU overload, we have several alternatives:

1. Add additional processors? This is usually the best solution, because an Oracle server that is overloading the CPU will always run faster with additional processors.

2. Reduce server load? If the CPU overload is not constant, task load balancing may be the solution. For example, it is not uncommon to see a server overloaded during peak work hours, and then return to 80-percent idle in the evenings. In these cases, batch tasks can be rescheduled to execute when there are more idle CPU resources available.

3. Alter task dispatching priorities? Most all UNIX operating systems allow the root user to change the dispatching priority for tasks. As a general rule, the online database background tasks are given more priority (a smaller priority value), while less critical batch processes are placed with less priority (a higher priority value). However, altering the default dispatching priorities is not a good long-term solution, and it should only be undertaken in emergency situations.

Upgrading an Entire Server

On mission-critical databases where speed is a primary concern, adding additional processors may not be the best solution. Oracle tuning professionals will sometimes recommend upgrading to a faster server architecture. For example, many of the new 64-bit CPU processors will handle Oracle transactions an order of magnitude faster than their 32-bit predecessors. For example, in the IBM AIX environment, the IBM SP2 processors run on 32 bits. IBM's next generation of processors utilize a 64-bit technology, and these systems can process information far faster than their 32-bit ancestors. The new IBM Blackbird servers will often double the overall processing speed of an Oracle database.

When making recommendations for upgrades of entire servers, many Oracle tuning professionals use the analogy of the performance of a 16-bit PC compared to the performance of 32-bit PC. In general, moving to faster CPU architecture can greatly improve the speed of Oracle applications, and many vendors such as IBM will allow you to actually load your production system onto one of the new processors for speed benchmarks prior to purchasing the new servers.

Adding Additional CPU Processors

Most symmetric multiprocessor (SMP) architectures for Oracle databases servers are expandable, and additional processors can be added at any time. Once added, the processor architecture will immediately make the new CPUs available to the Oracle database.

The problem with adding additional processors is the high cost that can often outweigh the cost of a whole new server. Adding additional processors to an existing server can commonly cost over $100,000, and most managers require a detailed cost-benefit analysis when making the decision to buy more CPUs. Essentially, the cost-benefit analysis compares the lost productivity of the end users (due to the response time latency) with the additional costs of the processors.

Another problem with justifying additional processors is the sporadic nature of CPU overloads. Oracle database servers often experience ?transient? overloads, and there will be times when the processors are heavily burdened and other times when the processors are not at full utilization. Before recommending a processor upgrade, most Oracle tuning professionals will perform a load-balancing analysis to ensure that any batch-oriented tasks are presented to the server at non-peak hours.

Load Balancing of Server Tasks

When CPU overload is experienced, the DBA will generally see periods during the days when the runqueue gets quite long and other periods at night when the processors are mostly idle. (Figure 3-2) A common question asked by a systems administrator is ?The CPU is 40-percent idle for 16 hours a day, so why should we add more processors??

Figure 2: Average Oracle server run queue values before task load balancing

However, there are times when it makes sense to add more processors, even if the processors are idle during off-peak times. For example, if you are working in an online environment, the only response time that matters is the time between 7:00 A.M. and 8:00 P.M. when your online users are active. The fact that the server is largely idle during the middle of the night has no bearing on the decision to upgrade with additional CPUs.

Once we identify the times when the CPU activity is excessive, we need to go to STATSPACK and examine the activity at the times of the overload. Once we have identified the times when the processors are overloaded, we must then see if it is possible to reschedule batch tasks to run at off-peak hours. On an Oracle database server, tasks may be scheduled in many ways:

* The dbms_job utility

* The UNIX cron utility

* A TP monitor such as Tuxedo

* Oracle Concurrent Manager (for Oracle Applications)

* SAPGUI if we are running SAP

Regardless of the method of scheduling Oracle tasks, the idea is to find large batch tasks that may be scheduled during peak processing times. Our task is to find a large regularly scheduled SQL task that runs during these times.

STATSPACK Solutions for Finding High-Impact Tasks

Within STATSPACK, we can examine the stats$sql_summary table, looking for SQL statements that are executed during the peak hours. The high-impact tasks will generally be associated with SQL statements that have a high value for rows processed. Here is an easy STATSPACK table query to find the SQL:

set lines 80;
set pages 999;
set heading off;

   to_char(snap_time,'yyyy-mm-dd hh24'),
   stats$sql_summary a,
   stats$snapshot    sn
   a.snap_id = sn.snap_id
   to_char(snap_time,'hh24') = 10
   to_char(snap_time,'hh24') = 15
order by
   rows_processed desc;

Here is a sample of the output from this report, showing the beginning of all SQL statements, ordered in descending order of rows_processed.  This tells the DBA which SQL statements are causing the most I/O resources.

Yr.  Mo dy Hr SQL_TEXT
------------- -------------------------------------------------
2000-09-20 10 begin :retCode := toc_maint . toc_insert_entry ( :
2000-09-20 10 begin :retCode := toc_maint . toc_insert_entry ( :
2000-09-21 10 select 'ALTER ' || substr(object_type,1,20) || ' '
2000-09-21 10 DECLARE job BINARY_INTEGER := :job; next_date DATE
2000-09-21 10 DECLARE job BINARY_INTEGER := :job; next_date DATE
2000-09-21 10 DECLARE job BINARY_INTEGER := :job; next_date DATE
2000-09-20 15 begin pageimages_curs . page_get_image ( :myisbn:i
2000-09-20 15 BEGIN sys.dbms_ijob.remove(:job); END;
2000-09-20 15 begin pageimages_curs . page_get_image ( :myisbn:i
2000-09-20 15 BEGIN sys.dbms_ijob.remove(:job); END;

If we are diligent, we can locate the online or batch SQL tasks that are overwhelming the server. Once we reschedule the offending tasks to run during off-peak hours, our average runqueue for the server falls below the number of CPUs, as shown in Figure 3-3.

Figure 3: Average Oracle server runqueue values after task load balancing

Of course, load balancing Oracle tasks is often not as trivial is it might appear. The Oracle DBA generally has no control over when members of the end-user community submit batch-oriented work against the Oracle database. Unless you are using scheduling tools such as Oracle's Concurrent Manager, the end users are free to submit large resource-intensive reports against Oracle anytime they feel like it. In shops where end users generate ad hoc SQL statements, there are three Oracle features that can govern end-user query resources:

* The dbms_resource_manager PL/SQL package can be used to govern end-user resources. In the example below, dbms_resource_manager is called to assign CPU, SQL plan, and Parallel Query options for the ADHOC_Group of end users:

    SQL> execute dbms_resource_manager.update_plan_directive(
              plan => 'SINGLE_LEVEL_PLAN',
              group_or_subplan => 'ADHOC_Group',
              new_comment => 'ADHOC day users sessions at level 1',
              new_cpu_p1 => 10,
              new_parallel_degree_limit_p1 => 0);

* Oracle Applications systems have a profile feature to restrict ad hoc queries. For ad hoc users, their Oracle user ID is associated with a group, and the group is linked to a profile that governs the amount of server resources the end users are allowed to use.

* SQL*Plus profilesare for end users who execute queries from SQL*Plus. The product_user_profile table can be used to restrict access.

Hence, it is up to the Oracle professional to become a detective, and hunt through the library cache in order to see when high-resource statements are being added to the Oracle system.

The stats$sql_summary table is one STATSPACK table that has a great deal of information to help you find offending SQL statements. SQL statements within the stats$sql_summary table can be sorted according to a number of resource utilization metrics, including rows processed, buffer gets, disk reads, and executions.

Even though Oracle developers write most SQL, it is still the duty of the Oracle DBA to monitor the behavior of the SQL within the system in order to determine those SQL statements that are creating excessive load upon the server processor. The Oracle DBA must also load balance those intensive SQL statements by requiring the end users to submit them during less active times.


If you like Oracle tuning, see the book "Oracle Tuning: The Definitive Reference", with 950 pages of tuning tips and scripts. 

You can buy it direct from the publisher for 30%-off and get instant access to the code depot of Oracle tuning scripts.


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