Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

[Li, Liang Z]

> fast (de)inflating & fast live migration
> 
> Hello,
> 
> On Fri, Dec 09, 2016 at 05:35:45AM +0000, Li, Liang Z wrote:
> > > On 12/08/2016 08:45 PM, Li, Liang Z wrote:
> > > > What's the conclusion of your discussion? It seems you want some
> > > > statistic before deciding whether to  ripping the bitmap from the
> > > > ABI, am I right?
> > >
> > > I think Andrea and David feel pretty strongly that we should remove
> > > the bitmap, unless we have some data to support keeping it.  I don't
> > > feel as strongly about it, but I think their critique of it is
> > > pretty valid.  I think the consensus is that the bitmap needs to go.
> > >
> >
> > Thanks for you clarification.
> >
> > > The only real question IMNHO is whether we should do a power-of-2 or
> > > a length.  But, if we have 12 bits, then the argument for doing
> > > length is pretty strong.  We don't need anywhere near 12 bits if doing
> power-of-2.
> > >
> > So each item can max represent 16MB Bytes, seems not big enough, but
> > enough for most case.
> > Things became much more simple without the bitmap, and I like simple
> > solution too. :)
> >
> > I will prepare the v6 and remove all the bitmap related stuffs. Thank you 
> > all!
> 
> Sounds great!
> 
> I suggested to check the statistics, because collecting those stats looked
> simpler and quicker than removing all bitmap related stuff from the patchset.
> However if you prefer to prepare a v6 without the bitmap another perhaps
> more interesting way to evaluate the usefulness of the bitmap is to just run
> the same benchmark and verify that there is no regression compared to the
> bitmap enabled code.
> 
> The other issue with the bitmap is, the best case for the bitmap is ever less
> likely to materialize the more RAM is added to the guest. It won't regress
> linearly because after all there can be some locality bias in the buddy 
> splits,
> but if sync compaction is used in the large order allocations tried before
> reaching order 0, the bitmap payoff will regress close to linearly with the
> increase of RAM.
> 
> So it'd be good to check the stats or the benchmark on large guests, at least
> one hundred gigabytes or so.
> 
> Changing topic but still about the ABI features needed, so it may be relevant
> for this discussion:
> 
> 1) vNUMA locality: i.e. allowing host to specify which vNODEs to take
>    memory from, using alloc_pages_node in guest. So you can ask to
>    take X pages from vnode A, Y pages from vnode B, in one vmenter.
> 
> 2) allowing qemu to tell the guest to stop inflating the balloon and
>    report a fragmentation limit being hit, when sync compaction
>    powered allocations fails at certain power-of-two order granularity
>    passed by qemu to the guest. This order constraint will be passed
>    by default for hugetlbfs guests with 2MB hpage size, while it can
>    be used optionally on THP backed guests. This option with THP
>    guests would allow a highlevel management software to provide a
>    "don't reduce guest performance" while shrinking the memory size of
>    the guest from the GUI. If you deselect the option, you can shrink
>    down to the last freeable 4k guest page, but doing so may have to
>    split THP in the host (you don't know for sure if they were really
>    THP but they could have been), and it may regress
>    performance. Inflating the balloon while passing a minimum
>    granularity "order" of the pages being zapped, will guarantee
>    inflating the balloon cannot decrease guest performance
>    instead. Plus it's needed for hugetlbfs anyway as far as I can
>    tell. hugetlbfs would not be host enforceable even if the idea is
>    not to free memory but only reduce the available memory of the
>    guest (not without major changes that maps a hugetlb page with 4k
>    ptes at least). While for a more cooperative usage of hugetlbfs
>    guests, it's simply not useful to inflate the balloon at anything
>    less than the "HPAGE_SIZE" hugetlbfs granularity.
> 
> We also plan to use userfaultfd to make the balloon driver host enforced (will
> work fine on hugetlbfs 2M and tmpfs too) but that's going to be invisible to
> the ABI so it's not strictly relevant for this discussion.
> 
> On a side note, registering userfaultfd on the ballooned range, will keep
> khugepaged at bay so it won't risk to re-inflating the MADV_DONTNEED
> zapped sub-THP fragments no matter the sysfs tunings.
> 

Thanks for your elaboration!

> Thanks!
> Andrea