Controlling system clocks in virtual machines

1. A computer program product for managing system clocks of a computing environment, said computer program product comprising:

a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising:

obtaining a host system clock value of a host system clock of a host system;

determining a system clock adjustment value, the determining the system clock adjustment value comprising:

determining a loss of performance value using one process time for a process executing in one processing mode and another process time for the process executing in another processing mode;

determining an adjusted system clock slowing down value; and

using the loss of performance value and the adjusted system clock slowing down value to determine the system clock adjustment value;

determining a virtual machine system clock value of a system clock of a virtual machine managed by the host system, wherein the determining comprises using the host system clock value and the system clock adjustment value to determine the virtual machine system clock value; and

adjusting the system clock of the virtual machine using the virtual machine system clock value.

2. The computer program product of claim 1 , wherein the determining the virtual machine system clock value comprises dividing the host system clock value by the system clock adjustment value to determine the virtual machine system clock value.

3. The computer program product of claim 1 , wherein the using the loss of performance value and the adjusted system clock slowing down value to determine the system clock adjustment value comprises performing a calculation.

4. The computer program product of claim 3 , wherein the calculation is a function of the adjusted system clock slowing down value, a time safety factor, the loss of performance value, and the adjusted system clock slowing down value.

5. The computer program product of claim 4 , wherein the calculation comprises: system clock adjustment value=the adjusted system clock slowing down value+a time safety factor*(the loss of performance value−the adjusted system clock slowing down value).

6. The computer program product of claim 1 , wherein the determining the loss of performance value uses one or more functional tests.

7. The computer program product of claim 1 , wherein the determining the adjusted system clock slowing down value uses one or more functional tests.

8. The computer program product of claim 7 , wherein the determining the adjusted system clock slowing down value comprises:

setting the system clock of the virtual machine to one value;

performing a functional test using a selected value;

determining whether a result of the functional test is a particular relationship with respect to a particular value; and

performing an action, based on the determining.

9. The computer program product of claim 8 , wherein the performing the action comprises, based on the result of the functional test not having the particular relationship with respect to the particular value: selecting another selected value and repeating the performing the functional test, the determining whether the result of the functional test is the particular relationship with respect to the particular value, and the performing the action.

10. The computer program product of claim 8 , wherein the performing the action comprises, based on the result of the functional test having the particular relationship with respect to the particular value, setting the adjusted system clock slowing down value to the selected value.

11. A computer system for managing system clocks of a computing environment, said computer system comprising:

a memory; and

a processor in communication with the memory, wherein the computer system is configured to perform a method, said method comprising:

obtaining a host system clock value of a host system clock of a host system;

determining a system clock adjustment value, the determining the system clock adjustment value comprising:

determining a loss of performance value using one process time for a process executing in one processing mode and another process time for the process executing in another processing mode;

determining an adjusted system clock slowing down value; and

using the loss of performance value and the adjusted system clock slowing down value to determine the system clock adjustment value;

determining a virtual machine system clock value of a system clock of a virtual machine managed by the host system, wherein the determining comprises using the host system clock value and the system clock adjustment value to determine the virtual machine system clock value; and

adjusting the system clock of the virtual machine using the virtual machine system clock value.

12. The computer system of claim 11 , wherein the determining the virtual machine system clock value comprises dividing the host system clock value by the system clock adjustment value to determine the virtual machine system clock value.

13. The computer system of claim 11 , wherein the using the loss of performance value and the adjusted system clock slowing down value to determine the system clock adjustment value comprises performing a calculation.

14. The computer system of claim 13 , wherein the calculation is a function of the adjusted system clock slowing down value, a time safety factor, the loss of performance value, and the adjusted system clock slowing down value.

15. The computer system of claim 14 , wherein the calculation comprises: system clock adjustment value=the adjusted system clock slowing down value+a time safety factor*(the loss of performance value—the adjusted system clock slowing down value).

16. The computer system of claim 11 , wherein the determining the loss of performance value uses one or more functional tests.

17. The computer system of claim 11 , wherein the determining the adjusted system clock slowing down value uses one or more functional tests.

18. The computer system of claim 17 , wherein the determining the adjusted system clock slowing down value comprises:

setting the system clock of the virtual machine to one value;

performing a functional test using a selected value;

determining whether a result of the functional test is a particular relationship with respect to a particular value; and

performing an action, based on the determining.

19. The computer system of claim 18 , wherein the performing the action comprises, based on the result of the functional test not having the particular relationship with respect to the particular value: selecting another selected value and repeating the performing the functional test, the determining whether the result of the functional test is the particular relationship with respect to the particular value, and the performing the action.

20. The computer system of claim 18 , wherein the performing the action comprises, based on the result of the functional test having the particular relationship with respect to the particular value, setting the adjusted system clock slowing down value to the selected value.