Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

[Michael S. Tsirkin]

On Thu, Mar 07, 2019 at 08:27:32PM +0100, David Hildenbrand wrote:
> On 07.03.19 19:53, Michael S. Tsirkin wrote:
> > On Thu, Mar 07, 2019 at 10:45:58AM -0800, Alexander Duyck wrote:
> >> To that end what I think w may want to do is instead just walk the LRU
> >> list for a given zone/order in reverse order so that we can try to
> >> identify the pages that are most likely to be cold and unused and
> >> those are the first ones we want to be hinting on rather than the ones
> >> that were just freed. If we can look at doing something like adding a
> >> jiffies value to the page indicating when it was last freed we could
> >> even have a good point for determining when we should stop processing
> >> pages in a given zone/order list.
> >>
> >> In reality the approach wouldn't be too different from what you are
> >> doing now, the only real difference would be that we would just want
> >> to walk the LRU list for the given zone/order rather then pulling
> >> hints on what to free from the calls to free_one_page. In addition we
> >> would need to add a couple bits to indicate if the page has been
> >> hinted on, is in the middle of getting hinted on, and something such
> >> as the jiffies value I mentioned which we could use to determine how
> >> old the page is.
> > 
> > Do we really need bits in the page?
> > Would it be bad to just have a separate hint list?
> > 
> > If you run out of free memory you can check the hint
> > list, if you find stuff there you can spin
> > or kick the hypervisor to hurry up.
> > 
> > Core mm/ changes, so nothing's easy, I know.
> 
> We evaluated the idea of busy spinning on some bit/list entry a while
> ago. While it sounds interesting, it is usually not what we want and has
> other negative performance impacts.
> 
> Talking about "marking" pages, what we actually would want is to rework
> the buddy to skip over these "marked" pages and only really spin in case
> there are no other pages left. Allocation paths should only ever be
> blocked if OOM, not if just some hinting activity is going on on another
> VCPU.
> 
> However as you correctly say: "core mm changes". New page flag?
> Basically impossible.

Well not exactly. page bits are at a premium but only for
*allocated* pages. pages in the buddy are free and there are
some unused bits for these.

> Reuse another one? Can easily get horrbily
> confusing and can easily get rejected upstream. What about the buddy
> wanting to merge pages that are marked (assuming we also want something
> < MAX_ORDER - 1)? This smells like possibly heavy core mm changes.
> 
> Lesson learned: Avoid such heavy changes. Especially in the first shot.
> 
> The interesting thing about Nitesh's aproach right now is that we can
> easily rework these details later on. The host->guest interface will
> stay the same. Instead of temporarily taking pages out of the buddy, we
> could e.g. mark them and make the buddy or other users skip over them.
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb