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What to Look for in vmstat

Oracle Tips by Burleson Consulting

As you can see, each dialect of vmstat reports different information about the current status of the server. Despite these dialect differences, there are only a small number of metrics that are important for server monitoring. These metrics include:

  • r (runqueue) The runqueue value shows the number of tasks executing and waiting for CPU resources. When this number exceeds the number of CPUs on the server, a CPU bottleneck exists, and some tasks are waiting for execution.

  • pi (page in) A page-in operation occurs when the server is experiencing a shortage of RAM memory. While all virtual memory servers will page out to the swap disk, page-in operations show that the servers has exceeded the available RAM storage. Any nonzero value for pi indicates excessive activity as RAM memory contents are read in from the swap disk.

  • us (user CPU) This is the amount of CPU that is servicing user tasks.

  • sy (system CPU) This is the percentage of CPU being used to service system tasks.

  • id (idle) This is the percentage of CPU that is idle.

  • wa (wait?IBM-AIX only) This shows the percentage of CPU that is waiting on external operations such as disk I/O.

Note that all of the CPU metrics are expressed as percentages. Hence, all of the CPU values (us + sy + id + wa) will always sum to 100. Now that we have a high-level understanding of the important vmstat data, let's look into some methods for using vmstat to identify server problems.

Identifying CPU Bottlenecks with vmstat

Waiting CPU resources can be shown in UNIX vmstat command output as the second column under the kthr (kernel thread state change) heading. Tasks may be placed in the wait queue (b) if they are waiting on a resource, while other tasks appear in the run queue (r) column. As we see in Figure 5-1, server tasks are queued for execution by the server.

Figure 5-23: Tasks queuing for service by the CPUs

In short, the server is experiencing a CPU bottleneck when r is greater than the number of CPUs on the server. To see the number of CPUs on the server, you can use one of the following UNIX commands.

Display the Number of CPUs in IBM AIX and HP/UX

In AIX and HP/UX the lsdev command can be used to see the number of CPUs on a server. This is very important, because it shows the number of Parallel Query processes that can be used on that server. That, in turn, limits the value that you can use following the DEGREE keyword in a Parallel Query or DML statement. The following example is taken from an AIX server, and shows that the server has four CPUs:

L 5-6

>lsdev -C|grep Process|wc ?l
4

Display Number of CPUs in Solaris

In Solaris, the prsinfo command can be used to count the number of CPUs on the processor. Here we see that we have two CPUs on this server:

L 5-7

>psrinfo -v|grep "Status of processor"|wc -l
       2

Display Number of CPUs in Linux

To see the number of CPUs on a Linux server, you can cat the /proc/cpuinfo file. In the example here we see that our Linux server has four CPUs:

L 5-8

>cat /proc/cpuinfo|grep processor|wc -l
      4

Remember that we need to know the number of CPUs on our server because the vmstat runqueue value must never exceed the number of CPUs. A runqueue value of 32 is perfectly acceptable for a 36-CPU server, while a value of 32 would be a serious problem for a 24-CPU server.

In the following example, we run the vmstat utility. For our purposes, we are interested in the first two columns: the run queue r, and the kthr wait b column. In the next listing we see that there are an average of about eight new tasks entering the run queue every five seconds (the r column), while there are five other tasks that are waiting on resources (the b column). Also, a nonzero value in the b column may indicate a bottleneck.

L 5-9

vmstat 5 5

kthr     memory             page              faults        cpu    
----- ----------- ------------------------ ------------ -----------
 r  b   avm    fre  re  pi  po  fr   sr  cy  in     sy  cs  us sy id wa
 7  5 220214   141   0   0   0  42   53   0 1724 12381 2206 19 46 28  7
 9  5 220933   195   0   0   1 216  290   0 1952 46118 2712 27 55 13  5
13  5 220646   452   0   0   1  33   54   0 2130 86185 3014 30 59  8  3
 6  5 220228   672   0   0   0   0    0   0 1929 25068 2485 25 49 16 10

The rule for identifying a server with CPU resource problems is quite simple. Whenever the value of the runqueue r column exceeds the number of CPUs on the server, tasks are forced to wait for execution. There are several solutions to managing CPU overload, and these alternatives are presented in their order of desirability:

1.      Add more processors (CPUs) to the server.

2.      Load balance the system tasks by rescheduling large batch tasks to execute      during off-peak hours.

3.      Adjust the dispatching priorities (nice values) of existing tasks.

To understand how dispatching priorities work, we must remember that incoming tasks are placed in the execution queue according to their nice value (see Figure 5-2). Here we see that tasks with a low nice value are scheduled for execution above those tasks with a higher nice value.

Figure 5-24: Tasks queued for execution according to their nice value

We will go into detail about these resolutions later in this chapter. Now that we can see when the CPUs are overloaded, let's look into vmstat further and see how we can tell when the CPUs are running at full capacity.

 

This is an excerpt from "Oracle9i High Performance tuning with STATSPACK" by Oracle Press.
 

If you like Oracle tuning, you may enjoy the new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & 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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