RE: [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

[Li, Liang Z]

> Subject: Re: [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating
> & fast live migration
> 
> On 12/07/2016 05:35 AM, Li, Liang Z wrote:
> >> Am 30.11.2016 um 09:43 schrieb Liang Li:
> >> IOW in real examples, do we have really large consecutive areas or
> >> are all pages just completely distributed over our memory?
> >
> > The buddy system of Linux kernel memory management shows there
> should
> > be quite a lot of consecutive pages as long as there are a portion of
> > free memory in the guest.
> ...
> > If all pages just completely distributed over our memory, it means the
> > memory fragmentation is very serious, the kernel has the mechanism to
> > avoid this happened.
> 
> While it is correct that the kernel has anti-fragmentation mechanisms, I don't
> think it invalidates the question as to whether a bitmap would be too sparse
> to be effective.
> 
> > In the other hand, the inflating should not happen at this time
> > because the guest is almost 'out of memory'.
> 
> I don't think this is correct.  Most systems try to run with relatively little free
> memory all the time, using the bulk of it as page cache.  We have no reason
> to expect that ballooning will only occur when there is lots of actual free
> memory and that it will not occur when that same memory is in use as page
> cache.
> 

Yes.
> In these patches, you're effectively still sending pfns.  You're just sending
> one pfn per high-order page which is giving a really nice speedup.  IMNHO,
> you're avoiding doing a real bitmap because creating a bitmap means either
> have a really big bitmap, or you would have to do some sorting (or multiple
> passes) of the free lists before populating a smaller bitmap.
> 
> Like David, I would still like to see some data on whether the choice between
> bitmaps and pfn lists is ever clearly in favor of bitmaps.  You haven't
> convinced me, at least, that the data isn't even worth collecting.

I will try to get some data with the real workload and share it with your guys.

Thanks!
Liang