Finding the bottleneck

The aspects of the system that you are most interested in measuring are CPU usage and memory usage.

It is possible that even after extensive tuning efforts the CPU is not powerful enough to handle the workload, in which case a CPU upgrade is required. Similarly, if the program is running in an environment in which it does not have enough memory after tuning, you must increase memory size.

Given that any performance problem could be caused by any one of several factors, you must look at several areas to eliminate each one. First, determine which resource is constraining the system:
  • CPU
  • Memory
  • Input/Output (I/O)
To do this, use the vmstat command. The vmstat command produces a compact report that details the activity of these three areas: 
> vmstat 1 10
outputs: 
kthr     memory             page              faults        cpu     
----- ----------- ------------------------ ------------ -----------
 r  b   avm   fre  re  pi  po  fr   sr  cy  in   sy  cs us sy id wa 
 0  0 189898   612   0   0   0   3   11   0 178  606 424  6  1 92  1
 1  0 189898   611   0   1   0   0    0   0 114 4573 122 96  4  0  0
 1  0 189898   611   0   0   0   0    0   0 115  420 102 99  0  0  0
 1  0 189898   611   0   0   0   0    0   0 115  425  91 99  0  0  0
 1  0 189898   611   0   0   0   0    0   0 114  428  90 99  0  0  0
 1  0 189898   610   0   1   0   0    0   0 117  333 102 97  3  0  0
 1  0 189898   610   0   0   0   0    0   0 114  433  91 99  1  0  0
 1  0 189898   610   0   0   0   0    0   0 114  429  94 99  1  0  0
 1  0 189898   610   0   0   0   0    0   0 115  437  94 99  0  0  0
 1  0 189898   609   0   1   0   0    0   0 116  340  99 98  2  0  0

The previous example shows a system that is CPU bound. This can be seen as the user (us) plus system (sy) CPU values either equal or are approaching 100. A system that is memory bound shows significant values of page in (pi) and page out (po). A system that is disk I/O bound will show an I/O wait percentage (wa) exceeding 10%. More details of vmstat can be found in AIX debugging commands.