Handle many vCPUs -- backport #315
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With the lifted limit on the maximum number of CPU sockets implemented in commit a4c0858, the maximum possible number of vCPUs increased significantly. But it is not without its price. The increased maximum number of vCPUs reduces memory available to the guest OS and may cause some problems, especially with UEFI firmware. There is already MaxNumOfCpusCoefficient config value limiting the maximum number of vCPUs. But it is not enforced strictly, the maximum number of vCPUs can still be significantly higher, to be consistent with the previous behavior. Let’s be more cautious and clearer and apply MaxNumOfCpusCoefficient strictly, unless the maximum number of vCPUs is not very large, in the sense of the newly introduced ManyVmCpus config value. Bug-Url: https://bugzilla.redhat.com/2075486
Large VMs using UEFI may not start if the TSEG size is not big enough. There is no clear rule when the TSEG size must be increased. We increase it in the following cases: - If the maximum number of VM vCPUs is at least 128. - If the maximum memory is at least 16 GB, i.e. the memory taken from the guest by changing the TSEG size is negligible. We increase the TSEG size by wild-guess values based on various comments from Bugzilla, see the comment in the code for more details. As the TSEG size is taken from the memory available to the guest, we do not want to make it unnecessarily large by default. On the other hand, it’s better to be on the safe side and allocate a bit more than perhaps needed rather than causing the VM not to start. We introduce config values for both the limits mentioned above, in case the selected values don’t work well enough. Bug-Url: https://bugzilla.redhat.com/2075486
mz-pdm
requested review from
emesika,
mwperina,
ahadas,
bennyz,
michalskrivanek,
oliel,
sgratch and
didib
as code owners
ahadas
approved these changes
Apr 26, 2022
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Bug-Url: https://bugzilla.redhat.com/2075486