Disambiguation / Clarification on Selected Counters
In this section, we provide some clarification on some of the subtleties of the performance counters we report.
| • | CPU Utilization |
| • | Memory Usage |
| • | Disk Read and Write Statistics |
| • | Network IO |
| • | CPU Active/Running (VMware only) |
The percent and absolute CPU utilization of a given device may be collected directly from the device's operating system (through WMI, SSH, or SNMP), and for virtual machines it may be reported by VMware. For details, see Data Sources. When both metrics are present, you may notice a difference in the two. VMware's view of CPU utilization includes the operational overhead of virtualization, so it is often higher than the value reported by the operating system. Conversely, at times of high host resource consumption, VMware may restrict a guest's CPU access in a way that is not visible to the operating system, resulting in VMware reporting a lower CPU utilization than the host does.
Please note that we collect a different representative metric for memory utilization from VMware than we do directly from the OS. For details, see Data Sources. Operating systems report committed memory - the amount of memory currently reserved by processes, and thus unavailable to the OS. VMware, in contrast, reports the amount of memory actively being used by guest processes. Because processes may not be actively using all of the memory they have reserved, the two metrics are not exactly equivalent. However, both metrics play similar roles in our analytics.
Disk Read and Write Statistics
Counters relating to disk IO are generally estimated by sampling the absolute counters provided by the operating system twice, separated by a brief interval. For SSH and some 64-bit SNMP implementations, we will use a longer sampling window to improve convergence. For details, see Sampled vs. Aggregate Data.
Data transmit and receive rates (both in packets and bytes) are estimated by sampling the absolute counters provided by the operating system twice, separated by a brief interval. For SSH and some 64-bit SNMP implementations, we will use a longer sampling window to improve convergence. For details, see Sampled vs. Aggregate Data.
Packets dropped and packets with errors are reported as an average number seen during the sampling window, rather than as a rate. Because 32-bit counters do not provide the same level of coverage for detecting interface errors and discards as 64-bit counters, we recommend against comparing interface errors and discards between different devices. An increase or decrease in this statistic for a given device is worthy of note, and any value above 0 for these counters may be indicative of a problem.
CPU Active/Running (VMware only)
These counters are reported by VMware as a percentage, but please be aware that they are reported as the sum of the percent utilization of each logical core. This means that the total can exceed 100% for systems with multiple logical cpu cores.