Re: [Qemu-devel] [PATCH 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Feb 28, 2019 at 08:36:58AM -0500, Michael S. Tsirkin wrote:
> On Thu, Feb 14, 2019 at 03:39:12PM +1100, David Gibson wrote:
> > When the balloon is inflated, we discard memory place in it using madvise()
> > with MADV_DONTNEED.  And when we deflate it we use MADV_WILLNEED, which
> > sounds like it makes sense but is actually unnecessary.
> > 
> > The misleadingly named MADV_DONTNEED just discards the memory in question,
> > it doesn't set any persistent state on it in-kernel; all that's necessary
> > to bring the memory back is to touch it.  MADV_WILLNEED in contrast
> > specifically says that the memory will be used soon and faults it in.
> > 
> > This patch simplify's the balloon operation by dropping the madvise()
> > on deflate.  This might have an impact on performance - it will move a
> > delay at deflate time until that memory is actually touched, which
> > might be more latency sensitive.  However:
> > 
> >   * Memory that's being given back to the guest by deflating the
> >     balloon *might* be used soon, but it equally could just sit around
> >     in the guest's pools until needed (or even be faulted out again if
> >     the host is under memory pressure).
> > 
> >   * Usually, the timescale over which you'll be adjusting the balloon
> >     is long enough that a few extra faults after deflation aren't
> >     going to make a difference.
> > 
> > Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
> > Reviewed-by: David Hildenbrand <david@redhat.com>
> > Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> 
> I'm having second thoughts about this. It might affect performance but
> probably won't but we have no idea.  Might cause latency jitter after
> deflate where it previously didn't happen.  This kind of patch should
> really be accompanied by benchmarking results, not philosophy.

I guess I see your point, much as it's annoying to spend time
benchmarking a device that's basically broken by design.

That said.. I don't really know how I'd go about benchmarking it.  Any
guesses at a suitable workload which would be most likely to show a
performance degradation here?

-- 
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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