Re: [Qemu-devel] [PATCH RFC kernel] balloon: speed up inflating/deflating process

[Li, Liang Z]

> > > >
> > > > Hi MST,
> > > >
> > > > I have measured the performance when using a 32K page bitmap,
> > >
> > > Just to make sure. Do you mean a 32Kbyte bitmap?
> > > Covering 1Gbyte of memory?
> > Yes.
> >
> > >
> > > > and inflate the balloon to 3GB
> > > > of an idle guest with 4GB RAM.
> > >
> > > Should take 3 requests then, right?
> > >
> >
> > No,  we can't assign the PFN when allocating page in balloon driver,
> > So the PFNs of pages allocated may be across a large range,  we will
> > tell the host once the pfn_max -pfn_min >= 0x40000(1GB range), so the
> > requests count is most likely to be more than 3.
> >
> > > > Now:
> > > > total inflating time: 338ms
> > > > the count of virtio data transmission:  373
> > >
> > > Why was this so high? I would expect 3 transmissions.
> >
> > I follow your suggestion:
> > ----------------------------------------------------------------------
> > -------------- Suggestion to address all above comments:
> >     1. allocate a bunch of pages and link them up,
> >        calculating the min and the max pfn.
> >        if max-min exceeds the allocated bitmap size,
> >        tell host.
> >     2. limit allocated bitmap size to something reasonable.
> >        How about 32Kbytes? This is 256kilo bit in the map, which comes
> >        out to 1Giga bytes of memory in the balloon.
> > ----------------------------------------------------------------------
> > --------------- Because the PFNs of the allocated pages are not linear
> > increased, so 3 transmissions are  impossible.
> >
> >
> > Liang
> 
> Interesting. How about instead of tell host, we do multiple scans, each time
> ignoring pages out of range?
> 
>       for (pfn = min pfn; pfn < max pfn; pfn += 1G) {
>               foreach page
>                       if page pfn < pfn || page pfn >= pfn + 1G
>                               continue
>                       set bit
>               tell host
>       }
> 

That means we have to allocate/free all the requested pages first, and then 
tell the host.
It works fine for inflating, but for deflating, because the page has been 
deleted from the vb-> vb_dev_info->pages,
so, we have to use a struct to save the dequeued pages before calling 
release_pages_balloon(),
 I think a page bitmap is the best struct to save these pages, because it 
consumes less memory.
And that bitmap should be large enough to save pfn 0 to max_pfn.

If the above is true, then we are back to the square one. we really need a 
large page bitmap. Right?

Liang