Troubleshooting
Problem
Recent releases of software have made changes to the way that CPU utilisation is calculated, which may result in increases in reported CPU utilisation. This issue is now corrected in V7.3.0.9
Resolving The Problem
This issue is now corrected in V7.3.0.9
The SVC and Storwize software architecture is such that using traditional CPU measurements do not accurately reflect the utilisation of the system. In order to provide an estimate of CPU utilisation, an artificial metric is used to represent how much work is being completed compared to the maximum that the system is capable of. This metric is reported as the CPU utilisation.
The previous CPU metric used in levels earlier than V7.2 was found to be inaccurate on systems using compression, due to the reallocation of the CPU cores which occurs when compression is enabled. This metric calculation was redesigned in V7.2 and later to improve the accuracy of the metric for systems running with compression.
As a result of this change in calculation of the metric, customers upgrading to V7.2 or higher may see an increase in reported CPU utilisation, even if the work being performed by the system has not increased. In some cases the reported CPU utilisation may be an overestimate of the real utilisation of the system.
The following graph demonstrates that with the V7.2 and higher CPU calculation mechanism, the system is still able to perform an additional 100,000 - 150,000 Read IOPS after the system starts reporting 100% CPU utilisation:
Customers who perform an upgrade to V7.2 or higher should not be concerned if they observe an increase in reported CPU utilisation with no other symptoms.
It should also be noted that every major release will include a number of performance enhancements which can improve the response time for IOs. If IOs are processed more quickly, some applications will be able to perform a greater number of IOs per second, and this will result in an increase in the work being performed by the system, which may result in a genuine increase in CPU utilisation.
A further update to the CPU utilisation calculation mechanism to provide a more accurate estimation of actual CPU utilisation is planned for a future release.
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Document Information
Modified date:
28 March 2023
UID
ssg1S1004943