On 05/31/2013 12:38 AM, Peter Lieven wrote: > > > > Am 30.05.2013 um 15:41 schrieb "Paolo Bonzini" <pbonz...@redhat.com>: > >> Il 30/05/2013 11:08, Peter Lieven ha scritto: >>> Am 30.05.2013 10:18, schrieb Alexey Kardashevskiy: >>>> On 05/30/2013 05:49 PM, Paolo Bonzini wrote: >>>>> Il 30/05/2013 09:44, Alexey Kardashevskiy ha scritto: >>>>>> Hi! >>>>>> >>>>>> I found the migration broken on pseries platform, specifically, this >>>>>> patch >>>>>> broke it: >>>>>> >>>>>> f1c72795af573b24a7da5eb52375c9aba8a37972 >>>>>> migration: do not sent zero pages in bulk stage >>>>>> >>>>>> The idea is not to send zero pages to the destination guest which is >>>>>> expected to have 100% empty RAM. >>>>>> >>>>>> However on pseries plaftorm the guest always has some stuff in the RAM >>>>>> as a >>>>>> part of initialization (device tree, system firmware and rtas (?)) so it >>>>>> is >>>>>> not completely empty. As the source guest cannot detect this, it skips >>>>>> some >>>>>> pages during migration and we get a broken destination guest. Bug. >>>>>> >>>>>> While the idea is ok in general, I do not see any easy way to fix it as >>>>>> neither QEMUMachine::init nor QEMUMachine::reset callbacks has >>>>>> information >>>>>> about whether we are about to receive a migration or not (-incoming >>>>>> parameter) and we cannot move device-tree and system firmware >>>>>> initialization anywhere else. >>>>>> >>>>>> ram_bulk_stage is static and cannot be disabled from the platform >>>>>> initialization code. >>>>>> >>>>>> So what would the community suggest? >>>>> Revert the patch. :) >>>> I'll wait for 24 hours (forgot to cc: the author) and then post a revert >>>> patch :) >>> does this problem only occur on pseries emulation? >> >> Probably not. On a PC, it would occur if you had 4K of zeros in the >> source BIOS but not in the destination BIOS. When you reboot, the BIOS >> image is wrong. >> >>> not sending zero pages is not only a performance benefit it also makes >>> overcomitted memory usable. the madv_dontneed seems to kick in >>> asynchronously >>> and memory is not available immediately. >> >> You could also scan the page for nonzero values before writing it. > > i had this in mind, but then choosed the other approach.... turned out to be > a bad idea. > > alexey: i will prepare a patch later today, could you then please verify it > fixes your problem.
Yes I can, where is the patch? :) > paolo: would we still need the madvise or is it enough to not write the > zeroes? > > Peter > >> >> Paolo > -- Alexey
