Re: [PATCH v16.1 0/9] mm / virtio: Provide support for free page reporting

[Alexander Graf]

On 23.01.20 17:26, Alexander Duyck wrote:
> On Thu, 2020-01-23 at 11:20 +0100, Alexander Graf wrote:
> > Hi Alex,
> >
> > On 22.01.20 18:43, Alexander Duyck wrote:
[...]
> > > The overall guest size is kept fairly small to only a few GB while the test
> > > is running. If the host memory were oversubscribed this patch set should
> > > result in a performance improvement as swapping memory in the host can be
> > > avoided.
> >
> > I really like the approach overall. Voluntarily propagating free memory
> > from a guest to the host has been a sore point ever since KVM was
> > around. This solution looks like a very elegant way to do so.
> >
> > The big piece I'm missing is the page cache. Linux will by default try
> > to keep the free list as small as it can in favor of page cache, so most
> > of the benefit of this patch set will be void in real world scenarios.
>
> Agreed. This is a the next piece of this I plan to work on once this is
> accepted. For now the quick and dirty approach is to essentially make use
> of the /proc/sys/vm/drop_caches interface in the guest by either putting
> it in a cronjob somewhere or to have it after memory intensive workloads.
>
> > Traditionally, this was solved by creating pressure from the host
> > through virtio-balloon: Exactly the piece that this patch set gets away
> > with. I never liked "ballooning", because the host has very limited
> > visibility into the actual memory utility of its guests. So leaving the
> > decision on how much memory is actually needed at a given point in time
> > should ideally stay with the guest.
> >
> > What would keep us from applying the page hinting approach to inactive,
> > clean page cache pages? With writeback in place as well, we would slowly
> > propagate pages from
> >
> >     dirty -> clean -> clean, inactive -> free -> host owned
> >
> > which gives a guest a natural path to give up "not important" memory.
>
> I considered something similar. Basically one thought I had was to
> essentially look at putting together some sort of epoch. When the host is
> under memory pressure it would need to somehow notify the guest and then
> the guest would start moving the epoch forward so that we start evicting
> pages out of the page cache when the host is under memory pressure.

I think we want to consider an interface in which the host actively asks 
guests to purge pages to be on the same line as swapping: The last line 
of defense.

In the normal mode of operation, you still want to shrink down 
voluntarily, so that everyone cooperatively tries to make free for new 
guests you could potentially run on the same host.

If you start to apply pressure to guests to find out of they might have 
some pages to spare, we're almost back to the old style ballooning approach.

Btw, have you ever looked at CMM2 [1]? With that, the host can 
essentially just "steal" pages from the guest when it needs any, without 
the need to execute the guest meanwhile. That means inside the host 
swapping path, CMM2 can just evict guest page cache pages as easily as 
we evict host page cache pages. To me, that's even more attractive in 
the swap / emergency case than an interface which requires the guest to 
proactively execute while we are in a low mem situation.

> > The big problem I see is that what I really want from a user's point of
> > view is a tuneable that says "Automatically free clean page cache pages
> > that were not accessed in the last X minutes". Otherwise we may run into
> > the risk of evicting some times in use page cache pages.
> >
> > I have a hard time grasping the mm code to understand how hard that
> > would be to implement that though :).
> >
> >
> > Alex
>
> Yeah, I am not exactly an expert on this either as I have only been
> working int he MM tree for about a year now.
>
> I have submitted this as a topic for LSF/MM summit[1] and I am hoping to
> get some feedback on the best way to apply proactive memory pressure as
> one of the subtopics if it is selected.

That's a great idea! Hannes just mentioned LSF/MM as a good forum to 
discuss this at last night, I'm glad to see you already picked up on it :).


Alex

[1] https://www.kernel.org/doc/ols/2006/ols2006v2-pages-321-336.pdf



Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrung: Christian Schlaeger, Jonathan Weiss
Eingetragen am Amtsgericht Charlottenburg unter HRB 149173 B
Sitz: Berlin
Ust-ID: DE 289 237 879