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

[David Gibson]

On Mon, Mar 04, 2019 at 09:29:24PM -0500, Michael S. Tsirkin wrote:
> On Tue, Mar 05, 2019 at 11:52:08AM +1100, David Gibson wrote:
> > 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.
> 
> Because of 4K page thing?

For one thing.  I believe David H has bunch of other reasons.

> It's an annoying bug for sure.  There were
> patches to add a feature bit to just switch to plan s/g format, but they
> were abandoned. You are welcome to revive them though.
> Additionally or alternatively, we can easily add a field specifying
> page size.

We could, but I'm pretty disinclined to work on this when virtio-mem
is a better solution in nearly every way.

> > 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?
> 
> Here's one idea - I tried to come up with a worst case scenario here.
> Basically based on idea by Alex Duyck. All credits are his, all bugs are
> mine:

Ok.  I'll try to find time to implement this and test it.

> Setup:
> Memory-15837 MB
> Guest Memory Size-5 GB
> Swap-Disabled
> Test Program-Simple program which allocates 4GB memory via malloc, touches it 
> via memset and exits.
> Use case-Number of guests that can be launched completely including the 
> successful execution of the test program.
> Procedure:
> Setup:
> A first guest is launched and once its console is up,
> test allocation program is executed with 4 GB memory request (Due to
> this the guest occupies almost 4-5 GB of memory in the host)
> Afterwards balloon is inflated by 4Gbyte in the guest.
> We continue launching the guests until a guest gets
> killed due to low memory condition in the host.
> 
> 
> Now repeatedly, in each guest in turn, balloon is deflated and
> test allocation program is executed with 4 GB memory request (Due to
> this the guest occupies almost 4-5 GB of memory in the host)
> After program finishes balloon is inflated by 4GB again.
> 
> Then we switch to another guest.
> 
> Time how many cycles of this we can do.
> 
> 
> Hope this helps.
> 
> 
> 

-- 
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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