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

[David Gibson]

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 :)

Frankly, I agree.  But mst was nervous about us making that change to
the balloon's previous behaviour.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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