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Re: [Qemu-ppc] [PATCH 1/5] virtio-balloon: Remove unnecessary MADV_WILLN
Re: [Qemu-ppc] [PATCH 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate
Tue, 5 Mar 2019 11:52:08 +1100
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 <address@hidden>
> > Reviewed-by: David Hildenbrand <address@hidden>
> > Reviewed-by: Michael S. Tsirkin <address@hidden>
> 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_!
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- Re: [Qemu-ppc] [PATCH 1/5] virtio-balloon: Remove unnecessary MADV_WILLNEED on deflate,
David Gibson <=