On 2017年08月15日 22:10, Konrad Rzeszutek Wilk wrote: > On Tue, Aug 15, 2017 at 11:00:04AM +0800, Lan Tianyu wrote: >> On 2017年08月12日 03:35, Konrad Rzeszutek Wilk wrote: >>> On Fri, Aug 11, 2017 at 03:00:20PM +0200, Radim Krčmář wrote: >>>> 2017-08-11 10:11+0200, David Hildenbrand: >>>>> On 11.08.2017 09:49, Lan Tianyu wrote: >>>>>> Hi Konrad: >>>>>> Thanks for your review. >>>>>> >>>>>> On 2017年08月11日 01:50, Konrad Rzeszutek Wilk wrote: >>>>>>> On Thu, Aug 10, 2017 at 06:00:59PM +0800, Lan Tianyu wrote: >>>>>>>> Intel Xeon phi chip will support 352 logical threads. For HPC usage >>>>>>>> case, it will create a huge VM with vcpu number as same as host cpus. This >>>>>>>> patch is to increase max vcpu number to 352. >>>>>>> >>>>>>> Why not 1024 or 4096? >>>>>> >>>>>> This is on demand. We can set a higher number since KVM already has >>>>>> x2apic and vIOMMU interrupt remapping support. >>>>>> >>>>>>> >>>>>>> Are there any issues with increasing the value from 288 to 352 right now? >>>>>> >>>>>> No found. >>>> >>>> Yeah, the only issue until around 2^20 (when we reach the maximum of >>>> logical x2APIC addressing) should be the size of per-VM arrays when only >>>> few VCPUs are going to be used. >>> >>> Migration with 352 CPUs all being busy dirtying memory and also poking >>> at various I/O ports (say all of them dirtying the VGA) is no problem? >> >> This depends on what kind of workload is running during migration. I >> think this may affect service down time since there maybe a lot of dirty >> memory data to transfer after stopping vcpus. This also depends on how >> user sets "migrate_set_downtime" for qemu. But I think increasing vcpus >> will break migration function. > > OK, so let me take a step back. > > I see this nice 'supported' CPU count that is exposed in kvm module. > > Then there is QEMU throwing out a warning if you crank up the CPU count > above that number. > > Red Hat's web-pages talk about CPU count as well. > > And I am assuming all of those are around what has been tested and > what has shown to work. And one of those test-cases surely must > be migration. > Sorry. This is a typo. I originally meant increasing vcpu shouldn't break migration function and just affect service downtime. If there was such issue, we should fix it. > Ergo, if the vCPU count increase will break migration, then it is > a regression. > > Or a fix/work needs to be done to support a higher CPU count for > migrating? > > > Is my understanding incorrect? You are right. > >> >>> >>> >>>> >>>>>>> Also perhaps this should be made in an Kconfig entry? >>>>>> >>>>>> That will be anther option but I find different platforms will define >>>>>> different MAX_VCPU. If we introduce a generic Kconfig entry, different >>>>>> platforms should have different range. >>> >>> >>> By different platforms you mean q35 vs the older one, and such? >> >> I meant x86, arm, sparc and other vendors' code define different max >> vcpu number. > > Right, and? If we introduce a general kconfig of max vcpus for all vendors, it should have different max vcpu range for different vendor. -- Best regards Tianyu Lan
