Re: [Qemu-devel] [RFC 2/2] virtio-balloon: Restore MADV_WILLNEED hint on balloon deflate

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 05, 2019 at 05:10:42PM +0100, David Hildenbrand wrote:
> On 05.03.19 17:03, Michael S. Tsirkin wrote:
> > On Tue, Mar 05, 2019 at 03:15:38PM +0100, David Hildenbrand wrote:
> >> On 05.03.19 06:11, David Gibson wrote:
> >>> Prior to f6deb6d9 "virtio-balloon: Remove unnecessary MADV_WILLNEED on
> >>> deflate", the balloon device issued an madvise() MADV_WILLNEED on
> >>> pages removed from the balloon.  That would hint to the host kernel
> >>> that the pages were likely to be needed by the guest in the near
> >>> future.
> >>>
> >>> It's unclear if this is actually valuable or not, and so f6deb6d9
> >>> removed this, essentially ignoring balloon deflate requests.  However,
> >>> concerns have been raised that this might cause a performance
> >>> regression by causing extra latency for the guest in certain
> >>> configurations.
> >>
> >> I mean, it will mainly create page tables as far as I know. Any write to
> >> a page will have an overhead either way (COW zero page). Reads *might*
> >> be faster.
> >>
> >> As we are working on 4k granularity in the balloon (and doing
> >> MADV_DONTNEED on 4k granularity!), there will most probably be page
> >> tables already either way. A page table could only be zapped if all
> >> pages of that page table are MADV_DONTNEED'ed (or I assume never were
> >> touched), and I am not sure if "random MADV_DONTNEED'ing of 4k pages"
> >> will actually get rid of page tables (my assumption would be: only if a
> >> complete range is zapped at once). I haven't looked into the details,
> >> though (plenty of other stuff to do).
> >>
> >> I am not sure if I share the concerns. Real-time workload should never
> >> use the virtio-balloon in a way that anything like that would be possible.
> >>
> >>>
> >>> So, until we can get actual benchmark data to see if that's the case,
> >>> this restores (by default) the old behaviour, issuing a MADV_WILLNEED
> >>> when a page is removed from the balloon.  A new property on the
> >>> balloon device "hint-on-deflate" can be set to false to remove this
> >>> behaviour for testing.
> >>
> >> This is certainly a good approach for you to finally be able to leave
> >> the ugly land of virtio-balloon :)
> >>
> >> But at least to me, this looks completely useless. I'll be happy to be
> >> proven wrong as always :)
> > 
> > Point is if we don't intend to extend balloon any further then let's
> > not make random untested changes to its behaviour.
> 
> Agreed, but than I'd much rather want to avoid the original change
> instead of adding a new property that most probably nobody will ever use.

OK, I don't mind either way.

> -- 
> 
> Thanks,
> 
> David / dhildenb