On Mon, Mar 04, 2019 at 08:13:53AM +0100, Markus Armbruster wrote: > Daniel P. Berrangé <berrange@xxxxxxxxxx> writes: > > > On Fri, Mar 01, 2019 at 06:33:28PM +0100, Igor Mammedov wrote: > >> On Fri, 1 Mar 2019 15:49:47 +0000 > >> Daniel P. Berrangé <berrange@xxxxxxxxxx> wrote: > >> > >> > On Fri, Mar 01, 2019 at 04:42:15PM +0100, Igor Mammedov wrote: > >> > > The parameter allows to configure fake NUMA topology where guest > >> > > VM simulates NUMA topology but not actually getting a performance > >> > > benefits from it. The same or better results could be achieved > >> > > using 'memdev' parameter. In light of that any VM that uses NUMA > >> > > to get its benefits should use 'memdev' and to allow transition > >> > > initial RAM to device based model, deprecate 'mem' parameter as > >> > > its ad-hoc partitioning of initial RAM MemoryRegion can't be > >> > > translated to memdev based backend transparently to users and in > >> > > compatible manner (migration wise). > >> > > > >> > > That will also allow to clean up a bit our numa code, leaving only > >> > > 'memdev' impl. in place and several boards that use node_mem > >> > > to generate FDT/ACPI description from it. > >> > > >> > Can you confirm that the 'mem' and 'memdev' parameters to -numa > >> > are 100% live migration compatible in both directions ? Libvirt > >> > would need this to be the case in order to use the 'memdev' syntax > >> > instead. > >> Unfortunately they are not migration compatible in any direction, > >> if it where possible to translate them to each other I'd alias 'mem' > >> to 'memdev' without deprecation. The former sends over only one > >> MemoryRegion to target, while the later sends over several (one per > >> memdev). > > > > If we can't migration from one to the other, then we can not deprecate > > the existing 'mem' syntax. Even if libvirt were to provide a config > > option to let apps opt-in to the new syntax, we need to be able to > > support live migration of existing running VMs indefinitely. Effectively > > this means we need the to keep 'mem' support forever, or at least such > > a long time that it effectively means forever. > > > > So I think this patch has to be dropped & replaced with one that > > simply documents that memdev syntax is preferred. > > We have this habit of postulating absolutes like "can not deprecate" > instead of engaging with the tradeoffs. We need to kick it. > > So let's have an actual look at the tradeoffs. > > We don't actually "support live migration of existing running VMs > indefinitely". > > We support live migration to any newer version of QEMU that still > supports the machine type. > > We support live migration to any older version of QEMU that already > supports the machine type and all the devices the machine uses. > > Aside: "support" is really an honest best effort here. If you rely on > it, use a downstream that puts in the (substantial!) QA work real > support takes. If upstream deletes the feature, then that in turn breaks the downstream unless downstream reverts the upstream change. When we have large overlap between downstream & upstream maintainer, it is not beneficial to delete the feature upstream as any effort saved upstream usually expands into larger effort downstream. > Feature deprecation is not a contract to drop the feature after two > releases, or even five. It's a formal notice that users of the feature > should transition to its replacement in an orderly manner. > > If I understand Igor correctly, all users should transition away from > outdated NUMA configurations at least for new VMs in an orderly manner. > > So, how could this formal notice be served constructively? > > If we reject outdated NUMA configurations starting with machine type T, > we can remove the means to create those configurations along with > machine type T-1. Won't happen anytime soon, will happen eventually, > because in the long run, all machine types are dead (apologies to > Keynes). > > If we deprecate outdated NUMA configurations now, we can start rejecting > them with new machine types after a suitable grace period. How is libvirt going to know what machines it can use with the feature ? We don't have any way to introspect machine type specific logic, since we run all probing with "-machine none", and QEMU can't report anything about machines without instantiating them. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list
