On Wed, Feb 5, 2014 at 5:49 AM, David Boyes <[email protected]> wrote:
> your dispatch timeslice for the whole virtual machine is divided as > equally as possible between the # of virtual CPUs defined, so defining more > virtual CPUs actually DECREASES the amount of processing time available to > each virtual CPU per timeslice That is not really correct. If that was true, you'd never be able to go over 100%. Each virtual CPU is separately dispatchable and competes with all other VCPUs from all other guests for a timeslice on a logical processor. SHARE value is taken into account by dispatcher to determine the relative priority of all VCPUs for that guest. That will determine how often in a given period of time will that guest get a timeslice on any of its VCPUs. If you want to turn single-VCPU guest into a multi-VCPU one, you need to multiply the relative SHARE value by the number of VCPUs that you defined for a guest in hope that each VCPU gets about the same number of timeslices in a given period of time as the single one was getting on a single-VCPU guest. Adding a virtual CPUs to a guest increases the ratio of total number of VCPUs per logical processor and puts unnecessary burden on dispatcher if it (additional VCPU) is not effectively used. There is a finite number of time slices on all logical processors put together and with more VCPUs, the competition for those timeslices is higher. You should add another VCPU to a guest only if the processes running in it can effectively use it. Let's not forget that if you run in a LPAR that uses shared physical processors, then LPAR's logical processors compete for a timeslice (or "running time") on a physical processor with other LPARs that use shared processors. CPU percentages may be misleading in that environment unless you have a good monitoring tool. CPU seconds per minute may be a better gauge. Ivica Brodaric ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
