Re: [PATCH v1 6/6] virtio-balloon: Add support for aerating memory via hinting

[Alexander Duyck]

On Thu, Jul 18, 2019 at 1:49 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Thu, Jul 18, 2019 at 01:34:03PM -0700, Alexander Duyck wrote:
> > On Thu, Jul 18, 2019 at 1:24 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> > >
> > > On Thu, Jul 18, 2019 at 08:34:37AM -0700, Alexander Duyck wrote:
> > > > > > > For example we allocate pages until shrinker kicks in.
> > > > > > > Fair enough but in fact many it would be better to
> > > > > > > do the reverse: trigger shrinker and then send as many
> > > > > > > free pages as we can to host.
> > > > > >
> > > > > > I'm not sure I understand this last part.
> > > > >
> > > > > Oh basically what I am saying is this: one of the reasons to use page
> > > > > hinting is when host is short on memory.  In that case, why don't we use
> > > > > shrinker to ask kernel drivers to free up memory? Any memory freed could
> > > > > then be reported to host.
> > > >
> > > > Didn't the balloon driver already have a feature like that where it
> > > > could start shrinking memory if the host was under memory pressure? If
> > > > so how would adding another one add much value.
> > >
> > > Well fundamentally the basic balloon inflate kind of does this, yes :)
> > >
> > > The difference with what I am suggesting is that balloon inflate tries
> > > to aggressively achieve a specific goal of freed memory. We could have a
> > > weaker "free as much as you can" that is still stronger than free page
> > > hint which as you point out below does not try to free at all, just
> > > hints what is already free.
> >
> > Yes, but why wait until the host is low on memory?
>
> It can come about for a variety of reasons, such as
> other VMs being aggressive, or ours aggressively caching
> stuff in memory.
>
> > With my
> > implementation we can perform the hints in the background for a low
> > cost already. So why should we wait to free up memory when we could do
> > it immediately. Why let things get to the state where the host is
> > under memory pressure when the guests can be proactively freeing up
> > the pages and improving performance as a result be reducing swap
> > usage?
>
> You are talking about sending free memory to host.
> Fair enough but if you have drivers that aggressively
> allocate memory then there won't be that much free guest
> memory without invoking a shrinker.

So then what we really need is a way for the host to trigger the
shrinker via a call to drop_slab() on the guest don't we? Then we
could automatically hint the free pages to the host.